STUDIES ON THE HOMOGENEOUS LIQUID-LIQUID EXTRACTION … · A schematic representation of solvent extraction. Liquid-liquid distribution plots. The distribution ratio D for two different

STUDIES ON THE HOMOGENEOUS LIQUID-LIQUID

EXTRACTION OF METAL IONS USING THE MIXTURES

OF 2-PROPANOL WITH WATER

By

NGUYEN HUU CHUNG

A Dissertation Submitted to

Department of Energy and Material Science

Graduate School of Science and Engineering

Saga University, Japan

In Partial Fulfillment of the Requirements for the Degree of

Doctor of Science

September 2004

THIS DISSERTATION IS DEDICATED TO MY WIFE BBUUII TTHHAANNHH HHUUOONNGG,

DAUGHTER NNGGUUYYEENN TTHHUUYY TTRRAANNGG, AND TO MY PARENTS

NNGGUUYYEENN HHUUUU KKIINNHH AND NNGGOO TTHHII TTOOTT.

Abstract This thesis studies systematically the phase separation of mixtures of 2-propanol and

water induced by addition of NaCl and CaCl2. We have utilized the unique

physicochemical properties of the salting-out phase separation for the selective

extraction of gold(III), thallium(III) and cobalt(II) in the presence of several precious

metals, other triply charged ions and transition metals into the 2-propanol phase without

using any extracting reagents. The results obtained are as follows:

The mixture of 2-propanol with water has been employed to extract Au(III) along

with other precious metals such as Pd(II) and Pt(IV) by using NaCl in the concentration

range of 2.5-4.0 mol dm-3. Upon the addition of NaCl within this concentration range

(2.5-4.0 mol dm-3) phase separation was attained. Gold(III), which was originally

present in the aqueous phase, at different concentrations of NaCl, was quantitatively

extracted into the 2-propanol-rich phase. The extraction efficiencies of the other metals

such as Pd(II) and Pt(IV) were much lower than for Au(III). Thus a maximal selective

separation of Au(III) from these metals could be attained using the mixture of 2-

propanol with water. A reaction mechanism involving the ion-pair of Na+ and [AuCl4]-

has been proposed to explain this extraction.

Thallium(III) that is present with other trivalent metals such as gallium, indium,

bismuth and antimony in aqueous solution was quantitatively and selectively extracted

into 2-propanol phase of a mixture of 2-propanol and water by addition of NaCl ranging

at 2.5-4.0 mol dm-3. The extraction efficiencies of gallium, indium, bismuth and

antimony were much lower than that of thallium(III). Thus a maximal selective

separation of thallium(III) from these metals could be attained using the mixture of 2-

propanol with water. Thallium(III) was extracted into 2-propanol phase as [TlCl4]- with

Na+. The detailed extraction mechanism involving the chemical species of the metal

ions in the presence of chloride, water content in the organic phase and counter ions is

discussed.

Selective separation of cobalt(II) in the presence of manganese(II), nickel(II) and

copper(II) was studied by using the mixture of 2-propanol with water by addition of

CaCl2 in the concentrations range of 3.0-6.5 mol dm-3. Cobalt(II) was extracted to

extent of 94.4% into the 2-propanol phase at 6.5 mol dm-3of CaCl2. The extraction of

i

the other metal ions such as Mn(II), Ni(II) and Cu(II) were much lower than that of

Co(II), but they were completely stripped to the aqueous phase by using an aqueous

solution containing of CaCl2. Thus, selective separation of cobalt(II) from these

elements was attained using the mixture of 2-propanol with water. Cobalt(II) was

extracted as CoCl42- with Ca2+ from aqueous solution into the organic phase. The

detailed extraction mechanism involving the ion-pair of Ca2+ and CoCl42- has been

proposed to explain this extraction.

Cobalt(II) was extracted into 2-propanol phase after the phase separation from the

mixture of water and 2-propanol by addition of CaCl2. In order to elucidate the

chemical species of cobalt(II) in both organic and aqueous phases, spectrophotometric

titration of cobalt(II) with CaCl2 was carried out in the mixture solvents of 2-propanol

and water. The absorption spectra indicate that only the tetrahedral chloro cobalt(II) is

formed under the experimental conditions. Formation constants of β(CoCl4) were

determined for the mixed solvents at different mole fraction of 2-propanol through non-

liner regression of the spectrophotometric titration data by computer program

SPECFIT/32TM .

ii

Acknowledgements

I would like to express my sincere thanks and gratitude to Professor Masaaki Tabata,

supervisor, Department of Chemistry, Faculty of Science and Engineering, Saga

University, Japan for his guidance, supervision and useful suggestions, which enabled

me to complete this doctoral program.

I am indebted to Dr Toshiyuki Takamuku and Dr Jun Nishimoto for their kind

cooperation, encouragements, advices and suggestions which also contributed a great

deal to the success of my research work in Saga University. I also express my sincere

appreciation to Prof. Tohru Miyajima, laboratory of Environmental Chemistry for his

encouragement. I am grateful to all the members of the Department of Chemistry, Saga

University for their cooperation and friendship.

I am grateful to Asian Youth Fellowship program for providing me scholarship

award to pursue a 14 months preparatory study in Malaysia. My sincere gratitude also

goes to Ministry of Education, Science and Culture of Japan for providing me with the

Monbusho scholarship that enabled me to pursue this doctor program to a successful

end.

Last but not the least, I am very thankful to Faculty of Chemistry, Ha Noi University

of Education, Viet Nam, where I am working for last fourteenth years, for granting me

study leave to pursue doctoral studies at Saga University. Finally, I am particularly

indebted to my dearest parents, brothers and sisters, and loving wife and daughter for

encouraging me to achieve this great academic objective.

God richly bless you all.

Saga University, September 2004

NGUYEN HUU CHUNG

iii

iv

Table of Contents Abstract i

Acknowledgements iii

Table of Contents iv

List of Figures vi

List of Tables x

Chapter 1. Introduction 1

1. 1. Overview of Solvent Extraction 2

1. 2. Literature Review 2

References 7

Chapter 2. Fundamental Principles of Solvent Extraction 11

2.1. Introduction 12

2. 2. Solvation Effects and Nature of Solute-Solvent Interactions 12

2. 3. Basic Principles for the Solvent Extraction 17

2. 4. Salting-out Phase Separation 23

2. 5. Formation Complexes of Metal Ions 26

References 29

Chapter 3. Phase Separation Occurs by the Addition of NaCl 31

to a Mixture of 2-Propanol and Water

3. 1. Introduction 32

3. 2. Experimental 33

3. 3. Results and Discussion 35

3. 4. Conclusions 41

References 43

Chapter 4. Selective Extraction of Gold(III) in the presence of 44

Palladium(II) and Platinum(IV) by Salting-out

of the Mixture of 2-propanol and Water





References 57

(To be continued)

Page

v

Chapter 5. Selective Extraction of Thallium(III) in the Presence of 59

Gallium(III), Indium(III), Bismuth(III) and Antimony(III)





References 71

Chapter 6. Phase Separation Occurs by the Addition of CaCl2 72

to a Mixture of 2-propanol and Water





References 80

Chapter 7. Selective Extraction of Cobalt(II) in the Presence of 81 Manganese(II), Nickel(II) and Copper(II)





References 95

Chapter 8. Determination of Formation Constants of Chloro Complexes 97

of Cobalt(II) in the Mixture of 2-propanol and Water by

Spectrophotometric Titration Method




References 105

Chapter 9. General Conclusions 106

List of Publications 109

Presentation Conferences 110

List of Figures

16

14

Page

Figure 2

Figure 10

Figure 14

Figure 13

Figure 12

Figure 11

Figure 9

Figure 7

Figure 8

Figure 6

Figure 5

Figure 4

Figure 3

Figure 1 Dipole and hydrogen bond interactions.

Major reactions in solution, classified according to the nature of

the interaction.

Major reaction in solution, classified according to the nature of

the products.

A schematic representation of solvent extraction.

Liquid-liquid distribution plots. The distribution ratio D for two

different substances X and Y, plotted against the variable Z of

the aqueous phase. D and Z are both on the logarithmic scale.

Solvent requirement for countercurrent extraction.

The procedure of phase separation experiment.

Phase diagram of the 2-propanol-water-NaCl ternary mixtures

as a function of mole fraction of 2-propanol, water and sodium

chloride.

Phase separation of 2-propanol-water-NaCl ternary mixture (x2-

propanol = 0.2 ) as a function of initial concentrations of NaCl in

aqueous solution.

Changing the density of two phases after phase separation.

Changing the volume of two phases after phase separation.

Distribution of Cl- after phase separation by salting-out of NaCl.

Distribution of H2O after separated by salting-out of NaCl.

The procedure of extraction of precious metal ions.

17

19

18

34

36

23

37

48

40

40

39

38

vi

50

Page

vii

Figure 23

Figure 22

Figure 21

Figure 20

Figure 19

Figure 18

Figure 17

Figure 16

Figure 15 Effect of initial concentrations of sodium chloride on the

extraction of Au(III) ( ●), Pd(II) ( ▲) and Pt(IV) ( ○) from 1:1

(vol / vol) mixture of 2-propanol and aqueous solution

containing the precious metal ions and 0.1 mol dm-3 HCl at

different concentrations of NaCl.

Gold(III) chloro complexes at various chloride concentration.

Effect of H2O concentrations in organic phase on the extraction

of precious metal ions Au(III) ( ●), Pd(II) ( ▲) and Pt(IV) ( ○).

Effect of chloride concentrations in aqueous (a) and organic

phases (b) on the extraction of precious metal ions Au(III) ( ●),

Pd(II) ( ▲) and Pt(IV) ( ○).

Reaction scheme for the extraction of gold(III) in the presence of

sodium chloride.

Distribution of water between aqueous and 2-propanol phases

separated by salting out with sodium chloride.

Effect of sodium chloride concentrations on the extraction of

Tl(III) (●), Ga(III) (▲), In(III) (■), Bi(III) (○) and Sb(III) (□)

from a 1:1 (v / v) mixture of water with 2-propanol in 0.1 mol

dm-3 HCl.

Distribution of metal chloro complexes at various concentration

of chloride.

Effect of hydrochloric acid concentrations on the extraction of

metal ions at 2.5 mol dm-3 NaCl.

53

52

55

54

56

63

64

66

67

Page

69

viii

Figure 33

Figure 32

Figure 31

Figure 30

Figure 29

Figure 28

Figure 27

Figure 26

Figure 25

Figure 24 Effect of water concentrations on the extraction of Tl(III) (●),

Ga(III) (▲), In(III) (■), Bi(III) (○) and Sb(III) .

Effect of sodium chloride concentrations in the organic solution

on the extraction of Tl(III) (●), Ga(III) (▲), In(III) (■), Bi(III)

(○) and Sb(III) (□).

Reaction scheme for the extraction of thallium(III) in the

presence of sodium chloride.

Phase diagram of the 2-propanol-water-CaCl2 ternary mixtures as

a function of mole fractions of 2-propanol, water and CaCl2. The

symbols of (●) and (○) denote the phase separation and

homogeneous solution, respectively.

Changing the volume of two phases after phase separation.

Distribution of H2O between two phases after phase separation

Changing the [Ca]2+ of two phases after phase separation

Changing the density of two phases after phase separation

Effect of initial concentrations of sodium chloride (a) and

calcium chloride (b) on the extraction of Mn(II) ( ○), Co(II) (■),

Ni(II) (●), Cu(II) (▲ ).

Absorption spectra of cobalt(II) in aqueous solution without 2-

propanol in the presence of different concentrations of CaCl2 of

(1) 0.0, (2) 0.1, (3) 0.5, (4) 1.0, (5) 1.5, (6) 2.0, (7) 2.5, (8) 3.0, (9)

3.5, (10) 4.0, (11) 4.5, (12) 5.0, (13) 5.5, (14) 6.0, (15) 6.5 mol dm-3.

69

70

75

86

78

77

77

76

88

Page

90

ix

Figure 37

Figure 40

Absorption spectra of cobalt(II) in the lower phase after the phase

separation. Arrows indicate the change in absorbance with

increasing initial concentrations of CaCl2 in the aqueous

solutions: (1) 3.0, (2) 3.5, (3) 4.0, (4) 4.5 mol dm-3.

Absorption spectra of cobalt(II) in the organic phase. Arrows

indicate the change in absorbance with increasing the initial

concentrations of CaCl2 in the aqueous solution: (1) 3.0, (2) 3.5,

(3) 4.0, (4) 4.5, (5) 5.0, (6) 5.5, (7) 6.0, (8) 6.5 mol dm-3.

[Co2+]initial = 332.2 ppm for (1)-(4) and 66 ppm for (5)-(8).

Effects of water concentration in the 2-propanol separated from

the mixture of 2-propanol and aqueous solution containing NaCl

(a) and CaCl2 (b), on the extraction of Mn(II) (○), Co(II) (■ ),

Ni(II) (● ) and Cu(II) (▲) into 2-propanol.

Reaction scheme for the extraction of cobalt(II) in the presence of

calcium chloride

Changes in absorption spectrum of cobalt(II) (6.976x10-3 mol dm-

3) upon titration of CaCl2 (1.165 mol dm-3) in 2-propanol-water

mixtures of x2pr = 0.494 at 25oC. The ionic strength was kept at I

= 3.5. Concentration of HCl is 0.032 mol dm-3.

Calculated electronic spectra of cobalt(II) complexes in aqueous

and 2-propanol phase: (1), [Co(H2O)6]2+ and (2), [CoCl4]2-.

The extraction (%) and (KD) of Co(II) as a function of CaCl2 for

the salting-out extraction using 2-PrOH and water mixed solvents

Figure 39

Figure 38

Figure 36

Figure 35

Figure 34

91

92

93

94

100

102

104

List of Tables

Page

42

Table 8

Table 7

Table 6

Table 5

Table 4

Table 3

Table 2

Table 1 Composition of the organic and aqueous phases separated in the

presence of NaCl.

Extraction constants of precious metals, water and NaCl by salting-

out NaCl at 4.0 mol dm-3.

Composition of the organic and aqueous phases after salting-out.

Distribution ratios of triply charged ions, water and NaCl into 2-

propanol after salting-out using NaCl at 4.0 mol dm-3.

Composition of the organic and aqueous phases separated in the

presence of calcium chloride.

Separation factors for the extraction of Co2+ from other transition

metal ions and CaCl2 into 2-propanol after salting-out using CaCl2

at 6.5 mol dm-3.

Compositions in mole fraction of tritrant solutions for the tritration.

Formation constants for the cobalt(II) chloro complexes in the

mixture of 2-propanol with water.

49

64

62

79

87

99

103

x

CHAPTER ONE INTRODUCTION

1

1. 1. Overview of Solvent Extraction

Traditional solvent extraction is one of the most useful techniques that are being

used for selective removal and recovery of metal ions from aqueous solutions and it is

largely applied in the purification processes in numerous chemical and metallurgical

industries [1-3].

In liquid-liquid extraction system, water immiscible and miscible solvents are

employed. Cationic or anionic forms of metals are complexed into an organophilic

compounds or an ion-pairs by chelation or using ion-pairing agents. If the water

immiscible solvent and an aqueous solution containing a hydrophobic species are

brought into contact, the chelate or an ion-pair is transferred into the organic phase.

Advantages of this technique are simplicity and rapidity. The solvents are not highly

flammable and easily recoverable. They are stable, transparent to UV, not emulsifying

during extraction and as selective as possible.

Disadvantages of liquid-liquid extraction methods are emulsion formation, different

extraction efficiencies for various compounds with various extracting agents, and low

sensitivity. In these processes, metal ion containing solution contacts with a large

amount of selective solvent. After extraction, stripping follows this process. Solvent

extraction is very difficult for the separation of quantitatively of metal ions because of

low driving force, and then a large amount of solvent is required. These make the

extraction and stripping of desired species very expensive.

1. 2. Literature Review

1. 2. 1. Extraction of Gold(III)

Gold is widely used in jewelry, dentistry, coinage, electrical contact and plating

materials, as well as in various other kinds of materials [4]. In the medical field, gold is

used as the immunogold-silver staining agent in history [5] as well as in nuclear

medicine where 195mAu is employed for angiocardiography [6]. Gold is found in lode

and placer deposits, and occurs chiefly in the metallic state in numerous alloys, usually

associated with quartz. It is also present sometimes as a component of various telluride

ores [7].

Gold is usually separated from alkaline cyanide solution by carbon adsorption, ion

exchange, co precipitation, solvent extraction, or cation exchange after dissolution of

2

the samples with acid mixtures such as HNO3, HCl, HClO4 and H2SO4 [8]. Traditional

solvent extraction has been proven to be a useful technology for selective removal and

recovery of metal ions from aqueous solutions with organic solvent and aqueous

solution as two immiscible phases.

The solvent extraction technique for the recovery of gold(III) from chloride solutions

has received a strong attention from researchers. Gold can be recovered from different

solutions by well-defined techniques such as cementation, carbon adsorption, solvent

extraction, etc. In the case of conventional solvent extraction technique for the recovery

of gold from several studies have been performed using various types of extractants

such as basic extractants like amines, solvating extractants such as neutral

organophosphorus compounds and other extractants containing S as the donor atom [9].

Few studies have been done on the separation of gold ions by solvation reagents, amines

and quaternary ammonium salts [10-17]. The effectiveness of the operation is

demonstrated by its implementation in various industrial processes to recover this

precious metal. In particular case of gold extraction in chloride media and using amines

as extractants, the observed extraction order is: quaternary ammonium salt > tertiary

amine > secondary amine > primary amine [18].

Recently there has been a renewed interest in the application of solvent extraction to

the recovery of gold using various extractants [19-26], it has unique advantages such as

nontoxic, nonflammable and inexpensive components of systems. It seems that little

attention has been paid to the partition of inorganic compounds in aqueous biphasic

systems [27-31]. However, liquid membranes containing a carrier have been emerging

as a potential alternative method to conventional solvent extraction and have different

applications in the field of separation science [32].

1. 2. 2. Extraction of Thallium(III)

Thallium is a ubiquitous element with an abundance of approximately 1 ppm in the

earth’s crust [33]. Thallium does not occur naturally in its elemental form but is found

in trace quantities in a variety of metal ores, coal, and commercial sources of potassium

such as sylvite (KCl) and carnallite (KCl. MgCl2. 6H2O) [34]. Thallium is also used in

low temperature thermometers, photoelectric cells, dye pigments, and to produce

3

various types of cements. Because of the element’s high refractive index it is utilized in

the manufacturing of optical lenses and imitation jewelry.

Thallium is very important because of a possibility to apply its mineral lorandite (Sb-

As-Tl) in obtaining knowledge in fundamental investigations. Thus, in 1976, Freedman

et al. [35] proposed applying of the reaction between 205Tl and solar neutrinos to study

the neutrino flux from the sun. It is also known that some thallium minerals are present

in lead and zinc mines, also in the metallurgical processes of lead and zinc production

thallium because a very important pollution problem concerning the environment [36].

Thallium metal and its compounds are highly toxic materials and are strictly

controlled to prevent a threat to humans and environment. Thallium and its compound

can be absorbed into human body by skin contact, ingestion, or inhalation of dust or

fumes. In recent years, there has been growing concern about the toxic effects of

thallium in the aquatic environment. Many countries, including Viet Nam, are facing

serious ecological and toxicological problems resulting from the discharge of complex

effluents and toxic chemical substances into watersheds. Thallium metal pollutants are

among the most toxic and persistent pollutants in wastewater discharges and receiving

waters [37]. Therefore, it is important to know the distribution of toxic thallium in

pollution sources and receiving waters in order.

The number of laboratory and industrial applications of membrane-assisted

separations is ever growing. Recently, a new radiochemical process for production of

the radio pharmaceutical 201 TlCl, based on non-depressive solvent extraction, has been

developed [38-40]. One of the most suitable techniques for separation of thallium(III)

from lead is solvent extraction using butyl acetate (BuAc). From this solution Tl(III)

will be extracted in the form of HTlCl4 complex [41,42]. A chelating polycalixarene

has been synthesized by introducing the hydroxamate chelating group into the

calixarene is used for the chromatographic separation of Ga(III), In(III) and Tl(III) [43].

In the extraction of thallium from aqueous solution into organic phase by using

various extractants also has been investigated. Such as, the extraction of thallium (III)

by carboxylic acids in kerosene [44], aqueous solution containing hydrochloric by using

tributyl phosphate (TBP) and triotylamine (TOA) in benzene [45], 18-crown-6 or 18-

crown-6 ethers [46], and complexes of thallium(III) with ethylenediamine (en) [47].

1. 2. 3. Extraction of Cobalt(II)

4

Cobalt is rare but widely distributed in nature. Its content is usually low in drinking

water (0.1-5µg / litter) and in ambient air (0.3-2.3 nm / m3). Plants are generally low in

Co but they are the main source for animal. Animals are able to synthesize vitamin B12,

which is the main source of cobalt in animal foods [48]. Cobalt is used in the

manufacture of alloys with a high melting point, and resistance to oxidation, in nuclear

technology, and in the manufacture of hard metal alloys for grinding wheels [49].

The role of cobalt metal in animal and plant biological systems is very important, but

very complex. A lack of these microelements in an organism can cause many diseases

and illnesses. However, their sufficient quantities can induce many harmful

consequences, also. The major source for the production of nickel and cobalt appears to

be from certain raw materials such as oxidic and sulphide ores wastes, dusts, catalysts,

etc [50]. Cobalt also is one of the metal of high economic and strategic importance,

because of its wide range of applications and dwindling supplies.

Several hydrometallurgical processes have been developed and used to extract cobalt

from various sources. A number of monodentate and chelating agents in organic media

have been utilized as extractants of base metals from aqueous solutions [51-54].

Recently, it was reported that extraction of cobalt ions with di(2-ethylhexyl)phosphoric

acid (D2EHPA) from an aqueous medium into kerosene [55], using the sodium salt of

Cyanex 272 as extractant diluted with kerosene with tri-n-butyl phosphate (TBP) [56].

However, hydrometallurgical methods of leaching or dissolution of such materials

under pressure or atmospheric conditions employing chloride, sulphate and chloride–

sulphate systems result in leach liquors containing nickel and cobalt along with some

impurities. It is extremely difficult to obtain pure cobalt from these leach liquors

because of the difficulties in separating cobalt from nickel, which have similar physico-

chemical properties [50].

Solvent extraction of cobalt has been studied by using Cyanex 301 in the presence of

nickel from sulfate solutions by using Cyanex 301 [57-61]. Separation of Ni and Co

from solutions in the presence of metals such as Cu, Zn and Mn has also been reported

[62,63]. Several papers have reported the cobalt-nickel separation factors from sulphate

media using phosphoric, phosphonic and phosphinic acids [64-70]. The separation

ability of cobalt and nickel increases in the order phosphinic > phosphonic > phosphoric

acid due to the increasing stabilisation of tetrahedral coordination compound of cobalt

5

with the extractant in the organic phase, because the tetrahedral compound is more

lipophilic than the octahedral one.

There is a literature review in current research about extractions of gold(III),

thallium(III) and cobalt(II) have been done by common extraction technique using

conventional organic solvents such as chloroform, benzene with various extractant.

However, these solvents have a low dielectric constant and are difficult for the

extraction of highly charged chemical species. Furthermore, these methods cannot

separate quantitatively gold(III), thallium(III) and cobalt(II) from the mixture of number

of other metal ions, where are using purification of gold(III), thallium(III) and

cobalt(III) in chemistry and industry.

In the present study, we have investigated the phase separation that occurs by the

addition of NaCl and CaCl2 to mixtures of 2-propanol and water. We have utilized the

phase separation processes for selective extraction of gold(III), thallium(III) and

cobalt(II). An extraction method based on salting-out upon addition of electrolyte to

mixed solvent of water and water-miscible organic solvent is attractive. The separated

organic solvents always contains a lot of water and salt, resulting in a highly polar

solvents compared to the corresponding pure organic solvent. Thus, the separated

organic solvents can easily extract ion-pairs and highly charged species such as

metalloporphyrins4+, which normally cannot be extracted using conventional organic

solvents such as chloroform [71]. Therefore, the aims of my research are as follows:

(a) Studies on the phase separation that occurs by addition of sodium chloride and

calcium chloride to mixtures of 2-propanol and water.

(b) Selective extraction of gold(III) in the presence of palladium (II) and platinum (IV)

by salting-out phase separation of the mixtures of 2-propanol and water by addition of

NaCl.

( c) Selective extraction of thallium(III) in the presence of gallium(III), indium(III),

bismuth(III) and antimony(III) by salting-out of NaCl.

( d) Selective extraction of cobalt(II) in the presence of manganese(II), nickel(II) and

copper(II) by salting-out phase separation of the mixtures of 2-propanol and water by

addition of CaCl2 .

6

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7

G. R. Choppin (Eds.), Marcel Dekker, New York, 1992.

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Hydrometallurgy 30 (1992) 291.

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[26] M. I. Martín, F. J. Alguacil, Hydrometallurgy 48 (1998) 309.

[27] R. D. Rogers, J. H. Zhang, Ion Exchange and Solvent Extraction, Marcel Dekker,

New York, 13 (1997) 141-193.

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Solvent Extr. Ion Exch. 13 (1995) 665.

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32 (1997) 867.

[32] L. L. Tavlarides, J. H. Bae, C. K. Lee, Sep. Sci. Technol. 22 (1987) 581.

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Goodman , W. J. Childs, Science 193 (1976) 1117.

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J. Sep. Sci. 7 (2001) 519.

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9

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10

CHAPTER TWO FUNDAMENTAL PRINCIPLES OF SOLVENT EXTRACTION

11

2. 1. Introduction

In its simplest form, extraction refers to the transfer of a solute from one liquid phase

to another. The most common case is the extraction of an aqueous solution with an

organic solvent. Diethyl ether, benzene, and other hydrocarbons are common solvent

that are less dense than water and form a phase that sits on top of the aqueous phase.

Chloroform, dichloromethane, and carbon tetrachloride are common solvents that are

immiscible with and denser than water [1]. In a two phases mixture, some of each

solvent is found in both phases, but one phase is predominantly water and the other

phase is predominantly organic. The volumes of each phase after mixing are not

exactly equal to the volumes that were mixed.

Solvent extraction is another name for liquid-liquid distribution, that is distribution

of a solute between two liquids that must not be completely mutually miscible. This

method makes use of an organic compound capable of extracting the metal ion of

interest, or a complex of it, from the aqueous phase into an immiscible organic solution.

It is a mature technique in that extensive experience has led to a good understanding of

the fundamental chemical reaction. Solvent extractions commonly take place with an

aqueous solution as one liquid and an organic solvent as the other. It consists in

separation of one or several substances (solute) present in a solid or a liquid phase by

contact with another liquid phase (solvent) [2]. The theory of liquid-liquid distribution

for solvent extraction contains three essential elements [3]:

(i) Principles of solute-solvent interaction, which gives a neutral species certain

solubility in an organic solvent.

(ii) Interactions in water between metal cations and anions by which neutral complexes,

either hydrophilic or hydrophobic, are formed.

(iii) Equations that explain the extraction data, that is, relate the measured solvent

distribution ratio (D) of a compound to the concentration of the species in the two

phases. The modeling chemical of solvent extraction processes, particularly for metal

complexes, and how these models can be tested and used to obtain complex formation

and distribution constant.

2. 2. Solvation Effects and Nature of Solute-Solvent Interactions

12

2. 2. 1. Definition of Solvation

In the bulk of a solution (diluted), every solute (molecule or ion) is surrounded by a

very large number of solvent molecules (designated by S). Solvation corresponds to the

energetic interactions that take place between the solute and the whole of the solvent

molecules that surround it (solute-solvent interactions). One defines the solvation

energy as the energy required by the operation of transfer of a molecule of a solute ion,

from an isolated state in vacuum to the bulk of the solvent. This energy can take on the

basis of thermodynamics, notably the Gibbs free energy G, which will characterize

solvation [4].

2. 2. 2. Classification of Solvents

The properties of solvents have obviously a strong bearing on their applicability for

various purposes. These aspects of the behaviors can be achieved by the proper blend

of the chemical properties of structured ness of solvents, polarity, electron pair and

hydrogen bond donation and acceptance ability, softness, acidity and basicity,

hydrophilicity, and redox properties. Thus, numerous solvents are used in solvent

extraction can be divided into different classes [5,6].

Class 1: Solvent capable of forming three-dimensional networks of strong hydrogen

bond (e.g., water, poly and amino alcohol acid).

Class 2: Other solvent that have both active hydrogen atoms and donor atom (O, N, F),

but that do not form three-dimensional network (e.g., primary alcohol, acids, primary

and secondary amines, nitro compounds with α-positioned hydrogen, ammoniac.

Class 3: Solvent composed of molecules containing donor atoms, but no active

hydrogen atoms (e.g., ethers, ketones, aldehydes, esters, tertial amines, nitro compounds

without α –hydrogen, phosphoryls.

Class 4: Solvents composed of molecules containing active hydrogen atoms, but no

donor atoms (e.g., CHCl3 and other aliphatic halides.

Class 5: Solvent with no hydrogen bond-forming capability and no donor atom (e.g.,

pure hydrocarbons, CS2, CCl4).

This diversity in solvent properties results in large differences in distribution ratios

of extracted solutes. Some solvents, particularly those of class 3, easily react directly

with inorganic compounds and extract them without need for any additional extractant,

13

whereas others (classes 4 and 5) do not dissolve salts without the aid of other

extractants. The class 1 solvent is very soluble in water and is useless for extraction of

metal species, although they find use in separations in biochemical.

2. 2. 3. Cohesive Forces and Electrostatic Interactions in Solvents

When in polar liquids the electric dipoles are able to arrange themselves in a “head-

to-tail” configuration, that is, when their positive end is on the average more in the

vicinity of the negative ends of neighboring molecules (Figs. 1a, c), attractive forces

result. However, the structure of the molecules may be such that they prevent a head-

to-tail configuration (Fig. 1b), and the resulting head-to-head configuration cause

repulsion between the polar molecules [7,8]. Some of the liquids that are used in

solvent extraction, especially water, interact by mean of hydrogen bonding. Their

molecules have a hydrogen atom attached to a very electronegative element (mainly

oxygen and, less effectively, nitrogen), and this hydrogen atom can be bound to the

electronegative atom (O, N, or F) of a neighboring molecule, forming a hydrogen bridge.

This bond is of considerably greater strength than dispersion and dipole-dipole

interactions. If the molecules of a substance can both donate and accept a hydrogen

bond, a cyclic dimmer may result that is considerably less polar than the monomers of

this substance (Fig. 1d) [9].

Figure 1. Dipole and hydrogen bond interactions. A schematic representation of (a)

“head-to-tail” dipole-dipole attractive interactions; (b) “head-to-head” dipole-dipole

repulsive interactions, caused by steric hindrance; (c) chainlike dipole-dipole

interactions; (d) a cyclic, hydrogen-bonded, dimmer.

(a)

(d)

(b)

(c)

14

Many liquids used in solvent extraction are polarity. Their polarity is manifested by

a permanent electric dipole in their molecular, since their atoms have differing

electronegative. Oxygen and nitrogen atoms, for instance, generally confer such

dipolarity on a molecule, acting as the negative pole relative to carbon or hydrogen

atoms bonded to them [10,11]. The dipole moment µ characterized such polar

molecules and ranges from 1.15 D (debye unit = 3.336 * 10-30 C.m) for diethyl ether or

chloroform, to 4.03 D for nitrobenzene, and 5.54 D for hexamethyl phosphoric triamide.

Substances that do not have a permanent dipole moment (i.e., µ =o) are called nonpolar.

When nonpolar liquids are placed in an electric force, resulting in some atomic

polarization [12,13].

2. 2. 4. Specific Interactions

Other interactions, not taken account of in the electrostatic model and playing a part

in the case of ionic solutes as well as in the case of non-ionic solutes, are called specific

interaction. They are related to the particular chemical nature of the solutes and of the

solvent in order to distinguish them. These interactions can be considered as chemical

bonds established between the solute and molecules S [14]. They are contact interaction

of primary at short distance, different from electrostatic interactions, which are

interactions at long distance.

In this category are included hydrogen bonds, which are formed between protic

solvent molecules (water, alcohols, amines, non-N-disubstituted amides, carboxylic

acids) and solutes that are electron pair donors, anions notably. The halide anions, for

example, are more strongly solvated in protic dipolar solvents than cations of unit

charge of the same size, whilst this difference does not exist in protic dipolar solvents

(THF, DMSO, propylene carbonate, acetone, ect.). More than hydrogen bonds are

encountered interactions of the electron pair donor-acceptor type (Lewis acid-base),

corresponding to the formation of coordination bonds between donor solvent molecules

and acceptor solutes (metallic cations notably), or between acceptor solvent molecules

and donor solute (anions notably) [15].

15

2. 2. 5. Solvent-Solute Interactions

When a solute particle is introduced into a liquid, it interacts with the solvent particle

in its environment. The totality of these interactions is called the solvation of the solute

in the particular solvent. When the solvent happens to be water, the term used is

hydration. The apparent molar properties of the solute ascribe to the solute itself the

entire change in the properties of the system that occur when 1 mol of solute is added to

an infinite amount of solution of specified composition. In an aqueous solution the

solute is often ionized and refer it as being an electrolyte solution [16]. However, the

solute ions are generally not “bare” ions, but exit as the products of specific interaction,

which according to their physical nature may be classified as ion-ion, ion-dipole, or

covalent interactions (Fig. 2). Another distinction is based on the chemical nature of the

interactions, and classifies them as ion association, hydration or solvation, and

complexation (Fig. 3). The two classifications are not synonymous. A complex is

generally not the product of a purely covalent interaction. Indeed, complex of ionic

contribution may be significant or even exceed the covalent contribution [17].

Ion-Dipole Interaction

Solute

Ion-Ion Interaction

Covalent Interaction

Figure 2. Major reactions in solution, classified according to the nature of the interaction.

16

Hydration or Solvation

Solute

Ion Association

Complex Formation

Figure 3. Major reaction in solution, classified according to the nature of the products.

2. 3. Basic Principles for the Solvent Extraction

2. 3. 1. General of Principles

Solvent extraction is used for the separation of organic as well as inorganic

compounds. Organic compounds are usually lipophilic (i.e., they dissolve easily in

organic solvents) and hydrophobic (i.e., they dislike water). For inorganic substances,

and particularly for metals, typically the situation is the opposite. They are lipophobic

and hydrophilic [18].

The solvent extraction procedure utilizes non-uniform distribution of substances

between two immiscible liquid phases. Enrichment of the substance in one of the

phases is dependent on many factors, such as pH, metal concentration, salt

concentration, reagent concentration, time and temperature. Under suitable conditions a

substance of interest can be transferred to the one phase while unwanted substances are

retained in the other. The development and optimization of a solvent extraction process

involves considerable experimental effort in determining the most suitable conditions.

The principle of the solvent extraction procedure is illustrated in Figures 4 and 5. In an

extraction step two substances are furnished at the concentration X and Y, and distribute

themselves in different ways between an organic solution containing a reagent (the light

phase) and a water solution (the heavy phase) [19].

17

D

(A): Heavy phase (aqueous solution)

(B): Feed containing X and Y

(C): Light phase (organic solution)

(D): Light phase containing X1and Y1

(E): Heavy phase containing X2 and Y2

Distribution of X: Dx = X1/X2

Distribution of Y: Dy = Y1/Y2

Separation factor X from Y: F = Dx / Dy

The percentage: %E = 100D / (1+D)

Shaking

B C A

E

Figure 4. A schematic representation of solvent extraction.

This is because the substances have different propensities to enter into chemical

combination with the reagent in the organic solution. The distribution of a substance

between the two liquid phases is described by a distribution factor, which is the quotient

of the concentration of the substance in the organic phase (X1 and Y1) and the

concentration of the sub-stance in the aqueous phase (X2 and Y2). The distribution

factors and thus the separation factors, which are defined in figure 4, depend on the

physical and chemical conditions represented in figure 5 by the variable Z.

18

E

0 1 2 3

(b)

50

100%D

X

Y

0

(a)

3 2 1

10 X

Y

1

0.1

ZZ

Figure 5. Liquid-liquid distribution plots. (a) The distribution ratio D for two different

substances X and Y, plotted against the variable Z of the aqueous phase. D and Z are

both on the logarithmic scale. Z may represent pH, concentration of extractant in

organic phase, free ligand ion concentration in the aqueous phase, aqueous salt

concentration. (b) Same systems showing percentage extraction % E as a function of Z.

2. 3. 2. Parameter Characterizing of the Solute

The transfer of solute from one liquid phase to another involves extraction reactions,

which permit the establishment of liquid-liquid distribution equilibria. The distribution

of a solute A, equilibrated between an aqueous phase and an organic solvent may be

described by an equilibrium equation [20]:

A(aq) ⇔ A(org) (1)

Thus, when this distribution reaches equilibrium, the distribution ratio (D) of the solute

concentrations between the two phases is:

D = [A]org / [A]aq (2)

For a metal species M, it can be written

[M]t, org

[M]t, aqDM

Conc. of all species containing M in organic phase

Conc. of all species containing M in aqueous phase (3)

19

The percentage (% E) of metal extraction was therefore calculated with Eq. (4).

where M may be present the total complexed forms in the aqueous phase, as well as in

different forms in the organic phase, [M]t refers to the sum of the concentrations of all

M species in a given phase (index t for “total”). An important extraction (transfer of the

major part of the substance into the organic phase) is characterized by a high value of

(D >> 1), whilst a very small value of this distribution ratio (D<< 1) on the contrary

characterizes a very feeble extraction. The extraction yield is between 0 for zero

extraction and 100 for total extraction. In order to influence the extraction yield, one

has available two sorts of factors:

% E 100 D

(1 + D) (4)

(i). Chemical factors modifying the distribution equilibria, thus modifying the values of

the distribution coefficients.

(ii). A physical factor, the ratio V = Vorg / Vaq of the volumes of the two phases brought

into contact.

2. 3. 3. System for the Extraction of Ions

The extraction of an ionic species involves systems that are more complex and more

varied than in the case of a molecule. In effect, for the reasons of electrical charge-each

phase have to remain electrically neutral an ion cannot be transferred alone from one

phase to another. Systems involving the coextraction of a cation and an anion, both

present initially in the aqueous solution and transferred together into organic phase [21].

A+aq + B-

aq ⇔ A+org + B+

org (5) (5)

The solvent used for the extraction possesses a low dielectric constant, the extracted

ions are associate in order to form electrically neutral ion pairs, when two ions of the

same absolute charge. It has been shown that this phenomenon is related to the low

dielectric constant (ε) of the solvent. Thus, two ions of opposite charge find themselves

subjected, in a medium of low value of the dielectric constant. The predominant

extraction system is:

A+aq + B-

aq ⇔ (A+B-)org (6)

The equilibrium distribution constant can be calculated by Eq. (7).

20

[A+B-]org

[A+]aq[B-]aq (7)Kex

The distribution coefficient of the ion A+ was calculated by the formula (8). It is related

to the concentration of the counter-ion in aqueous solution, distribution coefficient (D)

increase with increasing concentration of the counter-ion in aqueous solution (CB+).

DA = Kex [B-]aq (8)

2. 3. 4. Extraction With Chelating Reagents

The term chelate effect refers to the enhanced stability of a complex system

containing chelate rings as compared to the stability of a system that is as similar as

possible but contains none or fewer rings. From the analytical viewpoint the most

important type of extraction is that of uncharged molecules of chelate MLn, which

undergo no polymerization in the organic phase. The separations of metal ions from

each other is to selectively complex one ion using an organic ligand and extract it into

an organic solvent. Each ligand can be represented as a weak acid, HL, which loses one

proton when it binds to a metal ion through the atom. These ligands can react with

many different metal ions, but some selectivity is achieved by controlling the pH. Most

complexes that can be extracted into organic solvents must be neutral [22].

[H+]aq [L-]aq

[HL]aqKAL-

(aq) + H+(aq)

⇔ HL(aq) ; (9) (9)

It is assumed that the predominant form of the metal in the aqueous phase is Mn+ and

the predominant form of the metal in the organic phase is MLn. We define the partition

coefficient for ligand and complex as follows:

[MLn]aq

[Mn+]aq[L-]aq β nL-

(aq) + Mn+(aq) ⇔ MLn (aq) ; (10) (10)

HL(aq) ⇔ HL(org) ; KL = [HL]org / [HL]aq (11)

MLn(aq) ⇔ MLn(org) ; KM = [MLn]org / [MLn]aq (12)

From (Eqs. 10 and 12), we can write

[MLn]org = KM [MLn]aq = KM β [Mn+]aq [L-]naq (13)

21

Using the value of [L-]org from (Eq.9) given

[MLn]org = KM β [Mn+]aq KnA [HL]n

aq / [H+]naq (14)

D ≈ [MLn]org / [Mn+]aq ≈ KM β KnA [HL]n

aq / [H+]naq (15)

Since most of the HL is usually in the organic phase, we can use (Eq.11) to rearrange

(Eq.15) to its most useful form:

KM β KAn [HL]n

org

KLn [H+]n

aqD ≈ (16)(16)

(Eq. 16) says that the distribution coefficient for metal ion extraction depends on the pH

and the ligand concentration. Since the various equilibrium constant are different for

each metal, it is often possible to select a pH where D is large for one metal and small

for another.

2. 3. 5. Countercurrent Extractions

By the simple expedient of equilibration of a separated aqueous phase with fresh

portions of organic phase, a powerful technique for selective separations is available.

The method for carrying out such multiple liquid-liquid extractions is countercurrent

extraction, which permits the separation of substances with different distribution

coefficient. Countercurrent distribution is a serial extraction process devised by L.C.

Craig in 1949 [23]. The object of countercurrent distribution is to separate two or more

solute from each other by series of partition between two liquid phases. A necessary

condition for separation is that the distribution coefficient for the two solutes be

different. The scheme is shown the countercurrent operation in Figure 6. It consists of

a series of glass tubes so arrange that the lighter liquid is transferred from one tube to

the next. After each extraction, transfer the upper phase to the next tube and add fresh

lighter solvent to the original one.

In the beginning the tube 0 contains the mixture of substances to be separated in the

heavier solvent and all the other tubes contain equal volumes of the same solvent. The

lighter solvent is added to tube 0, extraction equilibrium takes place and the phases are

allowed to separate. The upper phase of tube 0 is then transferred to tube 1 and fresh

solvent is added to tube 0, and equilibrium is reached again. The upper layers of tubes 0

22

and 1 are simultaneously transferred to tubes 1 and 2 respectively, and the cycle is

repeated and so on [24]. Obviously, substances with higher distribution ratio move

faster than those with a lower distribution ratio. The greater difference of the

distribution ratio of various substances is the better for separation between each other.

n 1

Y3

2

X2X1

Y2Y1

F, Xf

Figure 6. Solvent requirement for countercurrent extraction.

2. 4. Salting-Out Phase Separation

2. 4. 1. Effect of Salts

The problem of the influence of salts on the activity coefficient of nonelctrolytes in

aqueous solutions is of both fundamental and applied interest. Salt effect studies can

provide considerable information of theoretical importance as to the complex

interactions of ions and neutral molecules and as to the unique nature of water as a

solvent. The data also have application to such related problems as kinetic salt effects

and mechanism of reactions, and they have a practical bearing on the separation of

nonelectrolytes from water solutions by salting-out processes.

The effect of salts on solution of nonelectrolytes is a very complex phenomenon.

For example, the decidedly varying influence of different salts on the activity

coefficient of benzene in water. Most electrolytes salt out benzene, although in

markedly varying degrees, but there are some which actually salt in this inert solute

[25,26]. The phrases “salting-out” and “salting-in” are now generally used to denote,

respective, an increase and a decrease in the activity coefficient of the nonelectrolyte

with increasing concentration of electrolyte.

It was suggested by Kruyt and Robinson [27] that the variations in the specific effect

of salts on different nonelectrolytes might arise from the fact that the water dipoles are

oriented in the hydration shell around an ion. These author pointed out that if there is

23

also a preferred orientation of water molecules toward a polar nonelectrolyte, ions of

one sign should have a tendency to promote its solubility while those of opposite sign,

which orient water molecules unfavorably, should have an increased salting-out effect.

It pointed out that local solvent structure should play a significant role [28-30].

The salting-out results from the effective removal of water molecules from their

solvent role due to hydration of the ions were discussed by Eucken and Hertzberg [31].

Although salting-out must largely be due to a preferential attraction between ions and

water molecules, which can loosely be referred to as “hydration” [32]. The variation

with salts concentration of the distribution of a nonelctrolyte between aqueous solutions

and an immiscible nonaqueous reference phase gives a simple method for determining fi

(fi is molar activity coefficient of i in salt solution). If for two experiments, one

involving pure water and the other a salt solution, the concentration of nonelectrolyte in

the reference phase is constant.

The chief advantages of this method are: it is simple experimentally, equilibrium is

established rapidly, one can always arrange the experiment to have a low concentration

of nonelectrolyte in the aqueous phase, and it can be used with nonelectrolytes which

are miscible with water.

The chief disadvantage is that it is frequently difficult to find a reference solvent,

which is sufficiently immiscible with water, and the distribution ratio of the

nonelectrolyte is such as to give adequate accuracy in the determination of fi.

2. 4. 1. Phase Separation by Salting-Out Agents

Phase separation of homogeneous mixed solvents can be achieved by addition of

salts or application of changing temperatures to organic solvents. For example, phase

separation occurs by addition of (NH4)2SO4 to polyethylene glycol [33], or by raising

the temperature to 30oC in the diethyl ether-water system [20]. The salt-induced phase

separation between acetonitrile and aqueous solution was observed by addition of a

variety of inorganic and organic electrolytes [34]. When inorganic salt is added to a

mixture of water with organic solvent, phase separation occurs. This phenomenon is

referred to a salting-out and can be explained by the following mechanism: Electrolytes

are hydrated but the organic solvent molecules are hard to be hydrated. Thus, the

solubility of organic solvent molecular decreases in the aqueous solution [35].

24

A theoretical investigation on salting-out phase separation in aqueous solution by

using Kirkwoo-Buff solution theory was suggested that micro heterogeneities in local

structure of the mixture are in favor of salting-out phase separation. The mechanism of

salting-out phase separation based on preferential solvation of the salt ions by water and

the inherent micro heterogeneities in the mixed solvent. It was found that beyond large

differences in affinities of the salt ions to the organic components and water molecules,

micro heterogeneities in local structures of the mixed solvent are also in favor of

salting-out phase separation in a hydro-organic mixed solvent [36].

Preferential solvation is observed in almost every chemical process taken place in

mixed solvent and it is an additive result of solute-solvent and solvent-solvent

interactions. The solvation in aqueous solution of several water-miscible organic

solvents including methanol, ethanol, 1-propanol, 2-propanol, acetone, and acetonitrile

have been studied by using 4-(N,N-dimethylamino) benzonitrile (DMABN); 2,6-

diphenyl-4-(2,4,6-triphenylpyridium)-1-phenolate (ET-30); and pyrene(Py) as

solvatochromic indicators [37].

2. 4. 2. Salting-out Extraction

Phase separation occurs in aqueous solution of some water-miscible organic solvents

by addition of electrolytes. This phenomenon, known as salting-out phase separation, is

useful for extraction or concentrations of metal-chelates, ion-pairs, and organic

materials, which cannot be extracted by conventional oil-water extraction method

[38,39]. The salting-out effects have been interpreted on the basis of changes in the

activity coefficient of the uranyl nitrate [40]. Groenewald [41] recently illustrated the

influence of these effects on the liquid-liquid extractions, using ethyl ether and benzene

as organic solvents. The complications due to salt activity coefficients are generally

assumed to be unity.

A phase separation occurs from a mixed solvent of water and water-miscible organic

solvent like acetonitrile upon addition of electrolyte to the mixed solvents, i.e., salting-

out, due to decreased solubility of the organic solvent in aqueous solution [42]. The

separated organic solvent contains water and salts, resulting in large donor and acceptor

abilities compared to those of the corresponding pure organic solvent [43]. Thus, the

solvent can easily extract ion-pair complexes such as tris (2,2’-bipyridine) cobalt(II)

25

chloride [44] and cadmium(II) iodide [45]. A further advantage is a possibility of high

polarity, leading to extract hydrophilic ions that are not extracted into conventional

organic solvents like chloroform [35 ].

The salting-out technique has long been used for extraction of metal-chelates, ion-

pair, or organic material, high-performance liquid chromatography [46,47],

polarography [45], and absorption spectrophotometry [48]. Application of the salting-

out to solvent extraction of ionic species will become easier when we understand the

chemical properties of the phase separated solvent, because the solvents will have high

polarity resulting from dissolution of water and electrolyte into the solvents by salting-

out, in addition to water-miscible solvents themselves.

2. 5. Formation Complexes of Metal Ions in Aqueous Solution

Metal ion complexation in the aqueous phase is an essential factor in solvent

extraction of metals. Such complexation can provide a sufficient difference in

extractability to permit separation of the metals. An understanding of how these factor

work for different metals and different ligands can be major value in choosing new

extraction systems for possible improvement in the separation of metals. The extent of

metal ion complexation for any metal-ligand system is defined by the equilibrium

constant, which is termed the formation constant for metal-ligand interaction. Since

most ligand bind to the metal ion in a regular sequence, equilibria are established for the

formation difference ratio between metal and ligand. The simple complexes can serve

to illustrate the principles and correlations of metal ion complexation [49]. Defining M

as the metal and L as the ligand (without indication of charge for simplicity). In only

considering complexes that are mononuclear, a single central metallic cation associated

with one or several ions or molecules of a ligand L, the successive complexation

reaction can be written as:

M + L ⇔ ML (17)

ML + L ⇔ ML2 (18)

or, generally

MLn-1 + L ⇔ MLn (19)

26

The equilibrium constants for these stepwise reactions are expressed by successive

formation constant as Ki. In this way, the number n, relating to the highest complex that

can be formed, depends on the coordination number of the metallic ion and the number

of the ligand can give, either by series of constants Ki or by that of the constants βi.

(with 1 ≤ i ≤ n). The cumulative combination of those equilibrium and that of their

constant leads to a consideration also of the overall formation constants.

[M Li]

[Mli-1] [L] (12) Ki (20)

[M Li]

[M] [L]i= K1 K2…..Kiβi = (21) (13)

Sometimes the complexation reaction (Eq.9) is written as occurring between M and

an acidic ligand HL.

M + HL ⇔ ML + H (22)

In which case the protonated stepwise formation constant is:

[M L] [H]

[M] [HL] (15) Ki (23)

For more complicated complexes, it is common to use an overall stability constant.

According to international rule (IUPAC 1987) [50], such constants may be written in

several ways, the important thing being that it is always defined in the text. Thus, the

formation of the complexes MmLn(OH)p is actually written in three ways in the present:

mM + nL + pOH ⇔ MmLn(OH)p ; βmnp (24)

Because hydrolysis reactions often occur in acidic solutions, the protonated overall

formation constant may be preferred.

mM + nL + pH2O ⇔ MmLn(OH)p + pH ; βmnp (25)

A quite general and simplified way of writing a reaction is

pM + qH + rL ⇔ MpHqLr ; βpqr (26)

For hydrolysis, a negative q is used to refer to the hydroxo species. This symbolism for

the reaction to form ML.

M + L ⇔ ML ; β101 = [ML] / [M] [L] = K1 (27)

27

However, the formation of multiligand complexes is more complicated. For example:

M + 3L ⇔ ML3 ; β103 = [ML3] / [M] [L]3 = K1.K2.K3 (28)

This can be generalized as:

n

∏i

Ki=1 M + nL ⇔ MLn ; β10n = [MLn] / [M] [L]n = (29)

Some ligands retain an ionizable proton. For example, depending on the pH of the

solution, metals complex with HSO4-, SO4

2- or both. In the formation of MHSO4, the

stability constant may be written as:

M + HSO4 ⇔ MHSO4 ; K1 = [MHSO4] / [M] [HSO4] (30)

or M + H + SO4 ⇔ MHSO4 ; β111 = [MHSO4] / [M] [H] [SO4] (31)

Here β111 = K1 / Ka2

where Ka2 is the dissociation acid constant for HSO4-. Alternately, some complexes are

hydrolyzed and have one or more hydroxo ligand.

28

References

[1] Y. Marcus, The Properties of Solvents, John Wiley & Sons, New York. 1998.

[2] Proceeding to be published through the Chemical Society of Japan, Kyoto. 1990.

[3] T. Sekine, Y. Hasegawa, Solvent Extraction Chemistry, Marcel Dekker,

New York. 1997.

[4] A. Ben-Naim, Y. Marcus, J. Chem. Phys. 70 (1984) 2016.

[5] E. Hecker, Verteilungsverfahren in Laboratorium, Verlag Chemie, Weinheim

Bergstrasse, 1955.

[6] R. E. Treybal, Liquid Extraction, McGraw-Hill, New York, 1963.

[7] Y. Marcus, J. Solution Chem. 21 (1992) 1217.



[10] M. J. Kamlet, J. L. Abboud, R. W. Taft, J. Am. Chem. Soc. 99 (1977) 6027.

[11] J. L. Abboud, M. J. Kamlet, R. W. Taft, J. Am. Chem. Soc. 99 (1977) 8325.

[12] Ch. Reichardt, Solvents and Solvent Effects in Organic Chemistry,

VCH, Weinheim, 2nd ed. 1988.

[13] M. Chastrette, Tetrahedron 35 (1979) 1441.

[14] L. S. Lias, J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin, W. G.

Mallard, J. Phys. Chem. Ref. Data 17, Suppl. 1 (1988).

[15] J. A. Riddick, W. B. Bunger, T. K. Sakano, Organic Solvents, Wiley-Interscience,

New York, 4th ed. 1986.


[17] Y. Marcus, Ion Solvation, John Wiley & Son, Chichester. 1985.

[18] H. Stephen, T. Stephen (eds.), Solubilities of Inorganic and Oganic Compounds.

Pergamon Press, London, 1963.

[19] J. Wisniak, A. Tamir, Liquid-Liquid Equilibrium and Extraction.

Elsevier, Amserdam. 1980.

[20] J. Rydberg, C. Musikas, G. R. Choppin, (Eds.), Principle and Practices of Solvent

Extraction. Marcel Dekker, New York. 1992.

[21] B. Trémillon, Reactions in Solution, John Wiley & Sons, New York. 1993.

[22] J. Stary, The Solvent Extraction of Metal Chelates, Pergamon. 1964.

[23] L. C. Craig, O. Post, Anal. Chem. 21 (1949) 500.

29

[24] G. M. Ritcey, A. W. Ashbrook, Solvent Extraction-Principles and Applications to

Process Metallurgy, Elsrvier, Amsterdam. 1979.

[25] J. H. Saylor, A. I. Whitten, I. Claiborne, P. M. Gross, J. Am. Chem. Soc.

74 (1952) 1778.

[26] W. F. McDevit, F. A. Long, J. Am. Chem. Soc. 74 (1952) 1090.

[27] H. R. Kruyt, C. Robinson, Proc. Acard. Sci. Amsterdam 29 (1926) 1244.

[28] D. Eley, Trans. Faraday Soc. 35 (1939) 1281.

[29] D. Eley, M. G. Evans, Trans. Faraday Soc. 34 (1938) 1093.

[30] H. S. Frank, M. W. Evans, J. Chem. Phys. 13 (1954) 507.

[31] A. Eucken, G. Z. Hertzberg, Physik. Chem. 195 (1950) 1.

[32] O.Ya. Samoilov, V. I. Tikhomirov, Ekstraksiya 2 (1962) 34.

[33] W.J. Ray, C. E. Bracker, Cryst. Growth 76 (1986) 562.

[34] Y. Nagaosa, K. Sakata, Talanta 46 (1998) 647.

[35] M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Chem. 68 (1996) 758.

[36] W. F. Furter, R. A. Cokk, Int. J. Heat Transfer 10 (1967) 23.

[37] Y. G. WU, M. Tabata, T. Takamuku, A. Yamaguchi, T. Kawaguchi, N. H. Chung,

Fluid Phase Equilibria 192 (2001) 1.

[38] C. E. Matkovich, G.D. Christian, Anal. Chem. 45 (1973) 1915.

[39] B. J. Mueller, R. J. Lovett, Anal. Chem. 59 (1987) 1405.

[40] I. L. Jenkins, H. A. C. Mckay, Trans. Faraday Soc. 50 (1954) 107.

[41] T. Groenewald, Anal. Chem. 43 (1971) 1678.

[42] J. Z. Setchenov, Phys. Chem. 4 (1889) 117.

[43] M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Sci. 10 (1994) 383.

[44] Y. Nagosa, Anal. Chim. Acta 120 (1980) 279.

[45] T. Fujinaga, Y. Nagosa, Bull. Chem. Soc. Jpn. 53 (1980) 416.

[46] B. J. Mueller, R. J. Lovett, Anal. Chem. 59 (1987) 1405.

[47] D. C. Leggett, T. F. Jenkins, P. H. Miyares, Anal. Chem. 62 (1990) 1355.

[48] H. Kawamoto, H. Akaiwa, Chem. Lett. 21 (1973) 259.

[49] F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Wiley,

New York. 1976.

[50] H. Freiser, G. H. Nancollas, Compendium of Analytical Nomenclature. Definitive

Rules. IUPAC. Blackwell Scientific Publications, Oxford. 1987.

30

CHAPTER THREE PHASE SEPARATION OCCURS BY THE ADDITION OF SODIUM

CHLORIDE TO A MIXTURE OF 2-PROPANOL AND WATER

31

3. 1. Introduction

Salting-out phase separation is well-known phenomenon that is observed in aqueous

solution of some water-miscible organic solvent with addition of electrolyte. A

investigation on salting-out phase separation has been an important subject in discussion

of physico-chemical properties of microheterogeneity by Kirkwood-Buff parameter for

a long period [1].

Several mechanisms have been proposed for the effect of electrolytes on the salting-

out of water-miscible solvent. For example, some electrolytes have been classified as

having either salting-out or salting-in effect and ranked according to their salting

strength. Most of theories concerned with the salting-out effect have used salting-out

coefficient defined as ks = 1 / m (log S0 / S), where S0 and S are the solubilities of the

organic solvent in water and in an electrolyte solution of molality, respectively [2-4].

The McDevit-Long equation is useful in explaining the salting-out of polar solvents

[5]. According to the McDevit-Long theory, the internal pressure of water increases

when electrolytes are solvated preferentially with water, because of its high dielectric

constant; as a result, the hydrated ion layer excludes a polar solvent molecule. Because

of this salting-out effect [6 ], the solubility of the polar solvent in the aqueous solution

decreases, leading to phase separation. For example, water-miscible polar solvents

such as acetonitrile, 1-methyl-2-pyrrolidone and hexamethylphosphoramide can be

separated from their aqueous solution by salting-out [7]. It has been reported from a

Rayleigh light scattering experiment on 2-propanol-water mixtures that a 2-propanol

molecule is hydrated by 20-30 water molecules at χ 2-propanol = 0.05, whereas 2-propanol

clusters gradually appear in the mixtures with increasing χ 2-propanol [8]. 2-Propanol is

miscible with water at any composition under ambient condition and its aqueous

solution has been widely used as mobile phase in reversed phase liquid chromatography

(RPLC), capillary electrophotoresis (CE), and other separation techniques [9-12]. For

example, Lin et al [13] found that addition of 20% (v/v) 2-propanol in mobile phase

gives best resolution in separation of some atropisomeric polychlorinated biphenyls by

cyclodextrin-modified micellar electrokinetic chromatography (MEKC). Kiss et al. [14]

reported that the presence of 25% (v/v) 2-propanol in a cartridge conditioning solution

was mostly appropriate of concentrating 3-6 ring polycyclic aromatic hydrocarbons on a

32

Sep-Pak C18 stationary phase. Seals et al. [11] found that the plate number N of

analyses retained weakly and intermediately in MEKC were only slightly affected by

addition of 2-propanol, whereas those of the strongly retained analytes decreased with

increasing 2-propanol in the mobile phase.

In this chapter, we describe the phase separation that occurs by the addition of

sodium chloride to a mixture of 2-propanol and water. We then have utilized the

interested in the salt-induced phase separation phenomena of this system for the

extraction of metal ions. The detailed mechanism and analytical method will be

described.

3. 2. Experimental

3. 2. 1. Apparatus

The volume of solution was measured using a volume-calibrated graduated tube. The

concentration of Na+ in the upper 2-propanol phase was determined by atomic

absorption spectrophotometry (Perkin-Elmer ANALYST 100), and the concentration of

Cl- in the lower water phase was determined by argentometry using potassium chromate

as an indicator. The concentration of Na+ in the lower phase was stoichiometrically

calculated from that of Cl- in the lower phase. The concentration of water in the upper

phase was determined by Karl-Fisher titration method using an automatic titrator

(Kyoto Electronics, MKL-200). The concentration of 2-propanol in aqueous phase was

determined by a gas chromatograph (Hewlett Packard, 5890 series II). The density of

solution after phase separation was measured with a densimeter (ANTON Paar K. G.

DMA 60).

3. 2. 2. Reagents

2-Propanol (Wako Pure Chemicals) was purified by drying over 4 Å molecular

sieves. NaCl (Wako Pure Chemicals) was dried in an electric oven at 400o C for 4 hours.

Doubly distilled water was used throughout the experiment and the organic solvent was

2-propanol.

33

2-Propanol

+

NaClaq

Precipitation

Phase separation

and precipitation

Phase separation

Homogeneous

Shaking

Figure 7. The procedure of phase separation experiment

34

3. 2. 3. Phase Separation Procedure

Figure 7 shows the procedure of phase separation of 2-propanol-water-NaCl was

examined as a function of mole fraction of 2-propanol, water and NaCl. First, aqueous

NaCl solutions of various NaCl concentrations were prepared by dissolving dried NaCl

into distilled water. Then, the aqueous NaCl solutions and 2-propanol were mixed to

required mole fractions of 2-propanol in a graduated tube with a stopper. Direct

dissolution of NaCl crystal into 2-propanol-water mixture was not successful because

NaCl did not quickly dissolve. The mixed solution in a tube was vigorously shaken

about 10 minutes and left aside at 298.2±0.3 K for 24 h to reach a complete equilibrium.

3. 3. Results and Discussion

3. 3. 1. Phase Separation of the Mixture of Water and 2-Propanol

In this investigation, the phase separation behavior of 2-propanol-water-NaCl

mixtures was examined at 298 K as a function of the mole fraction of 2-propanol, water

and NaCl. To obtain the phase diagram of the system, the state of the mixed solution

was visually classified into four types: (1) homogeneous solution of 2-propanol-water-

NaCl, (2) phase separation and precipitation of NaCl, (3) precipitation of NaCl, (4)

separation into 2-propanol (upper) and water (lower) phases. In Fig. 8, it can be seen

that the phase separation takes place at 2-propanol mole fraction (x2-propanol ) less than 0.6,

and the NaCl concentration was ranged from 2.0-4.0 mol dm-3. However, the NaCl salt

concentration required for the phase separation depends on the mole fraction of 2-

propanol.

We propose that two factors mainly contribute to the NaCl induced phase separation

of 2-propanol-water mixtures; preferential hydration to ions and microheterogeneity. In

2-propanol-water-NaCl mixtures both Na+ and Cl- are most likely to be preferentially

solvated by water molecules, because both Gutmann’s donor number (DN = 18.0) and

Mayer-Gutmann’s acceptor number AN = 54.8 ) [15] are larger for water than those (AN

=33.5) for 2-propanol [16]. The chemical analysis of both 2-propanol and aqueous

phases after phase separation indicated that most of added NaCl was found in the

aqueous phase, while the NaCl content in the 2-propanol phase was very small. Thus,

the increased Debye correlation lenths (LD) values by increasing salt concentration

35

should be caused by aggregartion of water molecules around preferentially hydrated

Na+ and Cl- [17].

Two theories have been used to explained salting-out phenomena. Long and

Mcdevit [2] have considered the salting-out as arising from electrostriction of the

solvent cause by addition of electrolyte, where the Setschenow constant (ks) has been

expressed by the partial molar volumes electrolyte and organic solvent and the

compressibility of water [2-4]. Another quantitative description has been made based

on Scaled-Particle theory [18,19]. The parameter used for the electrolyte in this theory

are the diameter and polarizabilities of the cation and anion, and the apparent molar

volume of the electrolyte at initial dilution in water. The scaled-particle theory is useful

in understanding solute-solvent interaction, but quantitative description by the theory is

not possible for the relatively high concentrations used in this study.

Figure 8. Phase diagram of the 2-propanol-water-NaCl ternary mixture as a function of

mole fractions of 2-propanol, water and NaCl. The symbols of ○, •, ⊗ and x represent

(1) homogeneous mixture, (2) phase separation, (3) phase separation with precipitation

of NaCl, and (4) precipitation of NaCl, respectively. The solid line represents the

border between homogeneous mixture and phase separation.

36

3. 3. 2. Composition of the Aqueous and 2-Propanol Phase After Phase Separation

3. 3. 2. 1. Changing the 2-Propanol of Two Phases

The chemical compositions of the 2-propanol-water-NaCl mixtures, after shaking a

volume of 5 cm3 of aqueous NaCl solutions of various NaCl concentration was mixed

with 5 cm3 of pure 2-propanol in a graduated tube, was determined in the two phases

immediately after separation and after allowing the phases to sit together for 24 hr. The

results indicate that equilibrium was achieved immediately after mixing. The

compositions of the aqueous and 2-propanol phases after salting-out are given in Table1.

The mixtures of 2-propanol-water-NaCl ternary solution before phase separation and

after phase separation are shown in the Fig. 9. The phase separation of the mixtures at

x2-propanol = 0.20 occurs when the NaCl concentration exceeds 2.0 mol dm-3. These

results are illustrated in Fig. 9 as a function of mole fraction of 2-propanol and initial

Figure 9. Phase separation of 2-propanol-w

concentrations of NaCl in aqueous solution.

ater-NaCl ternary mixture (x2-propanol = 0.2 )

as a function of initial concentrations of NaCl in aqueous solution.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

0.1

0.2

0.3

0.4

0.5

0.6

[NaCl]initial / mol dm -3

Mol

e fr

actio

n of

2-p

ropa

nol Phase separationHomogeneous

Homogeneous

Organic phase

Aqueous phase

37

3. 3. 2. 2. Changing the Density and Volume of Two Phases

Changing the density and volume of two phases after phase separation are given in

ng volume after phase Figs.10 and 11, respectively. From Fig.11 shows the changi

separation, it can be seen that the volume of the organic phase recovered by salting-out

was larger than the initial volume of 2-propanol. This indicates that 2-propanol

interacts strongly with water molecules through hydrogen bonding. This is because the

hydrophobic interaction among the alkyl groups, such as the iso-propyl and ethyl groups,

makes aggregation of 2-propanol molecule easy in aqueous mixtures. On the other

hand, a low frequency Raman spectroscopic investigation on aqueous mixtures of

several aliphatic alcohols showed that microheterogeneity occurs in 2-propanol-water

and ethanol-water mixtures [20].

2 2.5 3 3.5 4 4.50.8

0.9

1

1.1

1.2

Organic phase

Aqueous phase

[NaCl] initial / mol dm -3

Den

sity

/ g

cm-3

Figure 10. Changing the density of two phases after phase separation.

38

2 2.5 3 3.5 4 4.53

4

5

6

7

Organic phase

Aqueous phase


Vol

ume

/ cm

3

Figure 11. Changing the volume of two phases after phase separation.

3. 3. 2. 3. Distribution of Cloride and Water of Two Phases After Phase Separation

Distribution of Cl- and H2O between aqueous and 2-propanol phases separated by

solvent

parated contains water and chloride ions to a great extent. The concentration of

2. 3. Distribution of Cloride and Water of Two Phases After Phase Separation

Distribution of Cl- and H2O between aqueous and 2-propanol phases separated by

solvent

parated contains water and chloride ions to a great extent. The concentration of

salting-out of NaCl are given in Figs 12 and 13, respectively. The organicsalting-out of NaCl are given in Figs 12 and 13, respectively. The organic

sese

chloride ion in the organic phase increases with the concentration of water in the chloride ion in the organic phase increases with the concentration of water in the

organic phase. The high content of water and NaCl makes the 2-propanol phase highly

polar compared to pure 2-propanol: ET values are 220 kJ mol-1 and 203 kJ mol-1 for the

2-propanol phase in aqueous solution and pure 2-propanol, respectively [21]. On the

other hand, as the alkyl group is hydrophobic, the aqueous 2-propanol solution easily

salted-out upon the addition of sodium chloride which leads to a large volume of the

separated organic phase containing a lot of water. The separated organic solvent always

contains water and salts, resulting in a highly polar solvent compared to the

corresponding pure organic solvent. Thus, organic solvents separated by salting-out can

easily extract ion-pair complexes such as tris(2,2’-bipyridine)cobalt(II) chloride [22]

and cadmium(II) iodide [23] and other highly charged chemical species with the charge

of 4+ or 4- for instance, which normally cannot be extracted into conventional organic

organic phase. The high content of water and NaCl makes the 2-propanol phase highly

polar compared to pure 2-propanol: ET values are 220 kJ mol-1 and 203 kJ mol-1 for the

2-propanol phase in aqueous solution and pure 2-propanol, respectively [21]. On the

other hand, as the alkyl group is hydrophobic, the aqueous 2-propanol solution easily

salted-out upon the addition of sodium chloride which leads to a large volume of the

separated organic phase containing a lot of water. The separated organic solvent always

contains water and salts, resulting in a highly polar solvent compared to the

corresponding pure organic solvent. Thus, organic solvents separated by salting-out can

easily extract ion-pair complexes such as tris(2,2’-bipyridine)cobalt(II) chloride [22]

and cadmium(II) iodide [23] and other highly charged chemical species with the charge

of 4+ or 4- for instance, which normally cannot be extracted into conventional organic

39

solvents, such as chloroform [24]. Therefore, the selectivity in the extraction of ionic

species is attained by the control of water content, charge of chemical species and ion-

pair formation constants.

Figure 12. Distribution of Cl- after phase separation by salting-out of NaCl.

2 2.5 3 3.5 4 4.5

-1.2

-0.9

-0.6

-0.3


logD

(Cl )-

2 2.5 3 3.5 4 4.5

-0.8

-0.6

-0.4

-0.2


logD

(H O

)2

Figure13. Distribution of H2O after separated by salting-out of NaCl.

40

3. 4. Conclusions

The above results indicate that phase separation occurs at 0.1-0.4 in mole fractions of

2-propanol and 2.0-4.0 mol dm-3 of sodium chloride. Compared to the phase diagram of

acetonitrile [25,26], the phase separation occurs in a small concentration range of NaCl

and 2-propanol. This suggests strong interaction of 2-propanol with water compared to

acetonitrile. The water content of 2-propanol is higher (0.45 in mol fraction) than that

of acetonitrile (0.19 in mol fraction) [27]. Hence 2-propanol is suitable for the

extraction of ionic species by salting-out.

41

42

a. Table 1. Composition of the organic and aqueous phases separated in the presence of NaCl

[NaCl]initial / mol dm-3

Volume / cm3

Organic Aqueous

Water / mol dm-3

Organic Aqueous

2-Propanol /mol dm-3

Organic Aqueous

NaCl /mol dm-3

Organic Aqueous

Density / g

Organic Aqueous

cm-3

2.5 6.28 3.54 22.939 37.36 8.548 3.871 0.724 2.225 0.914 1.036

3.0 5.52 4.25 15.073 45.35 9.621 1.528 0.411 2.995 0.874 1.084

3.5 5.44 4.39 12.021 47.73 10.350 0.732 0.287 3.588 0.855 1.114

4.0 5.36 4.50 9.408 49.64 10.992 0.059 0.227 4.113 0.843 1.138

a Five cubic centimeters of aqueous NaCl solutions of various NaCl concentration was mixed with 5cm3 of pure 2-propanol .

43

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[23] T. Fujinaga, Y. Nagao, Bull. Chem. Soc. Jpn. 53 (1980) 416.


[25] T. Takamuku, A. Yamaguchi, D. Matsuo, M. Tabata, T. Yamaguchi, T. Otomo,

T. Adachi, J. Phys. Chem. B 105 (2001) 10101.

[26] K. R. Harris, P. J. Newitt, J. Phys. Chem. B 103 (1999) 7015.

[27] Y. G. Wu, M. Tabata, M. Takamuku, A. Yamaguchi, T. Kawaguchi, N. H. Chung,

References

[15] V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum

Fluid Phase Equilibria, 192 (2001) 1.

CHAPTER FOUR SELECTIVE EXTRACTION OF GOLD(III) IN THE PRESENCE

OF PALLADIUM(II) AND PLATINUM(IV) BY SALTING-OUT OF THE

MIXTURE OF 2-PROPANOL AND WATER

44

4. 1. Introduction

Gold has been treasured since ancient times for its beauty and permanence. It is also

used in a wide range of important commercial and industrial, and its unique

combination of properties makes it vital to some of our most advanced technologies.

Gold is sometimes found free in nature but it is usually found in conjunction with silver,

quartz (SiO2), calcite (CaCO3), lead, tellurium, zinc or copper.

Gold exists in the leach solution as AuCl4-, and all the processes rely on the

extraction of this ion or the parent acid by ion pair or solvating extractants. The early

studies by Morris and Khan [1] led to the adoption of dibutylcarbitol by INCO in a gold

refinery several years before the complete solvent extraction refining flow sheet was

produced.

Gold is usually separated from alkaline cyanide solution by carbon adsorption or ion

exchange. Traditional solvent extraction has been proven to be a useful technology for

selective removal and recovery of metal ions from aqueous solutions with organic

solvent and aqueous solution as two immiscible phases. During last few years, there

has been a renewed interest in the application of solvent extraction to the recovery of

gold using various extractants [2-7].

Solvent extraction is a very effective method for the separation of metal ions in

solution and a lot of extraction systems have been developed for the separation and

extraction of precious metals including gold, palladium and platinum [8,9].

Conventional solvent extraction has involved the use of solvents such as chloroform,

benzene, etc. One of the advantages of using these conventional solvents is that most of

them dissolve very little in water and can be used to quantitatively extract metals with

extracting reagents. For example gold(III) has been extracted into CHCl3 by using

tertiary polyamines and quaternary polyammonium salts [10-12]. A disadvantage of

these solvents, however, is that they have a low dielectric constant and are not always

suitable solvents for the extraction of highly charged chemical species except

nitrobenzene [13].

It is well known that phase separation occurs from mixed solvents of water and

water-miscible organic solvents upon the addition of electrolytes to the mixed solvents.

The salting-out is due to the decreased solubility of organic solvents in water in the

presence of electrolytes [14]. The separated organic solvent always contains water and

45

salts, resulting in a highly polar solvent compared to the corresponding pure organic

solvent. Thus, the separated organic solvent can easily extract ion-pair complexes and

highly charged species such as metalloporphyrins, which normally cannot be extracted

into conventional organic solvents, such as chloroform [15].

In this study, we have utilized the phase separation which occurs by the addition of

NaCl to a mixture of 2-propanol and water for the selective extraction of gold(III) in the

presence of palladium(II) and platinum(IV) into the 2-propanol phase without extracting

reagents. The detailed mechanism and analytical method will be described.

4. 2. Experimental

4. 2. 1. Apparatus

The volume of solution was measured using a volume-calibrated graduated tube.

The concentration of Na+ in the upper 2-propanol phase was determined by atomic



as an indicator. The concentration of Na+ in the lower phase was stoichiometrically

calculated from that of Cl- in the lower phase. The concentration of water in the upper

phase was determined by Karl-Fisher titration method using an automatic titrator

(Kyoto Electronics, MKL-200). The concentration of 2-propanol in aqueous phase was

determined by a gas chromatograph (Hewlett Packard, 5890 series II). The density of

solution after phase separation was measured with a densimeter (ANTON Paar K. G.,

DMA 60).

4. 2. 2. Reagents


sieves. NaCl (Wako Pure Chemicals) was dried in an electric oven at 400o C for 4 hours.

Aqueous solutions of metal ions were prepared by dilution of the standard metal

chloride solutions (HAuCl4, PdCl2 and H2PtCl6 ) in 0.1 mol dm-3 HCl. The initial

concentration of metal ions was varied from 0.030-0.25 mmol dm-3. The concentration

of hydrochloric acid was always maintained at 0.1 mol dm-3 and the concentration range

of NaCl was 2.5-4.0 mol dm-3 under the salting-out conditions. Doubly distilled water

was used throughout the experiment and the organic solvent was 2-propanol.

46

where [M]aq, tot and [M]org, tot represent the total concentrations of metal ion that has

equilibrated in the organic and aqueous phases, respectively. The percentage of metal

extraction was therefore calculated with Eq. (33).

The metal distribution coefficient was calculated according to Eq. (32)

The extraction of metal ions from aqueous solutions containing 0.1 mol dm-3

hydrochloric acid in the presence of different concentrations of sodium chloride and 2-

propanol was carried out by mechanical shaking of the solution containing the

appropriate organic and aqueous solutions in a centrifuge tube (Figure 14). The mixture

was then centrifuged and the concentrations of gold(III) distributed between the

aqueous and organic phases were determined by atomic absorption spectrophotometry.

where Vaq and Vorg represent the aqueous and organic volume phases, respectively.

47

4. 2. 3. Procedure for Extraction of Gold(III)

Metal extraction (%) =

[M]Org, tot

[M]Aq, tot

(32)D =

gor

aqD100

V

VD +

(33)

Concentrations of [H+]

Concentrations of [Cl-]

Phase separation

NaClAq (2.5 – 4.0 mol dm-3)

Au3+, Pd2+and Pt4+ (0.03 – 0.25 mmol dm-3)

HCl (0.1 mol dm-3)

Shaking

Organic phase

Aqueous phase

Karl-Fischer

Measurement

ICP and AAS

Argentometry

Sodium hydroxide

[Au3+], [Pd2+] and [Pt4+]

Concentrations of [H2O]

Volume of two phases

Figure 14. The procedure of extraction of precious metal ions.

2-propanol

48


4. 3. 1. Distribution Equilibria in the Presence of NaCl

Figure 15 shows the extraction percent of gold(III). It can be observed that gold(III)

as high as 99% could be extracted into the 2-propanol phase. Gold(III) was

quantitatively extracted over the whole concentration range of NaCl, while the

extraction percent of palladium(II) and platinum (IV) were very poor in the presence of

a high chloride concentration. The extraction of palladium(II) and platinum(IV) was

not observed above NaCl concentration of 4.0 mol dm-3, though only about 24% of

Pd(II) and 33% of Pt(IV) could be extracted at a decreased NaCl concentration of 2.5

mol dm-3 (Table 2).

The above results can be explained by the formation of different charged species at

the high concentration of Cl- in the aqueous phase. Palladium(II) and platinum(IV)

under these experimental conditions exist predominantly as [PdCl4]2- and [PtCl6]2- [16],

and they are hardly extracted into the organic phase. Whiles, the main chemical species

of gold(III) in aqueous solution is [AuCl4]- [16-18] and gold(III) is extracted

quantitatively into the 2-propanol phase. A similar trend was observed in the extraction

of gold(III) from palladium(II) and platinum(IV) at 0.01-10 mol dm-3 HCl into

chloroform by using the amide derivative of calix[4]arene [19], where calix[4]arene

amide works as a counter ion to [AuCl4]- rather than as a coordinating ligand to Au(III).

The difference in charge between the chemical species is an important factor in the

extraction of the ion-pair complexes.

Table 2. Extraction constants of precious metals, water and NaCl by salting-out of

NaCl at 4.0 mol dm-3.

logKD

[AuCl4]- 1.14

[PdCl4]2- -1.28

[PtCl6]2- -1.25

H2O -0.72

NaCl

-1.3

49

2 2.5 3 3.5 4 4.50

20

40

60

80

100


PdCl42-

Ext

ract

ion

perc

enta

geAuCl 4

-

PtCl62-

Figure 15. Effect of initial concentrations of sodium chloride on the extraction of

Au(III) ( ●), Pd(II) ( ▲) and Pt(IV) ( ○) from 1:1 (vol / vol) mixture of 2-propanol and

aqueous solution containing the precious metal ions and 0.1 mol dm-3 HCl at different

concentrations of NaCl.

The negatively charged species, [AuCl4]-, is therefore expected to transfer to the 2-

propanol phase by the formation of ion-pair complexes with Na+ or H+. At HCl

concentrations of 0.1-0.2 mol dm-3 the distribution ratio (D) of gold(III) into the organic

phase was constant at a fixed concentration of NaCl (4.0 mol dm-3). This means that the

main ion-pair formation occurs between [AuCl4]- and Na+. In addition, above HCl

concentration greater than 0.2 mol dm-3, Na+ concentration in the aqueous phase is so

high that NaCl precipitates, and the concentrations of Na+ in the organic phase also is

higher than that of H+ (Table 3). Therefore, the charge of [AuCl4]- is neutralized by Na+.

Furthermore, when the precious metals concentrations were maintained in the range

of 0.030-0.25 mmol dm-3, the extraction of Au(III) was almost constant at different

chloride concentrations, but the extraction of Pd(II) and Pt(IV) decreased with the

increase of NaCl (Fig.15). This indicates that the ion-pair between Na+ ion and [AuCl4]-

50

is also formed more easily in the organic phase. Hence the extractability of gold is very

high and those of platinum and palladium are very low at 4.0 mol dm-3 NaCl.

4. 3. 2. Distribution of Gold(III) Chloro Complexes

In the presence of high chloride concentration the main chemical species of the

precious metals in aqueous solution are [AuCl4]-, [PdCl4]2- and [PtCl6]2-. [AuCl4]- is

very stable in high concentration of hydrochloric acid media and hence is the

predominant species. Gold as high as 99% is extracted in the region of Cl-

concentration greater than 2.5 mol dm-3. The species distribution of Au(III) as a

function of [Cl-] is shown in Fig 16. In the calculation, we used the following values as

the stability constants of the chloro complexes of gold(III): logß1 = 9.26, logß2 = 17.57,

logß3 = 24.89 and logß4 = 31.05 [20]. It can be seen from the distribution curves that

Au(III)-Cl- complexes in the aqueous phase are mainly [AuCl4]- at chloride ion

concentrations higher than 10-5 mol dm-3. The extraction order of chloro complexes of

precious metals by liquid anion-exchangers is [MCl4]- > [MCl6]2- > [MCl4]2- > [MCl6]3-

[21]. Total charge and size of these chloro complexes affect the difference in the

extraction behavior of [MCl6]2-, [MCl4]2- and [MCl4]-. This is a reason why [AuCl4]-

has a high extractability compared to [PdCl4]2- and [PtCl6]2-.

51

-10 -9 -8 -7 -6 -5 -4 -3 -20

0.2

0.4

0.6

0.8

1

log[Cl -] / mol dm -3

Mol

e fr

actio

n

AuCl4-

AuCl3AuCl2+

Au3+

AuCl2+

Figure 16. Gold(III) chloro complexes at various chloride concentration.

4. 3. 3. Effect of Water Concentration

The effects of water in the organic phase on the extraction of gold(III), Pd(II) and

Pt(IV) are shown in Fig. 17. It can be seen that the extraction of the precious metals

increases with increasing concentration of water in the organic phase. This can be

explained by the two important roles of water dissolved in the organic phase. One is the

increase in the hydration ability to charged chemical species and the other is the change

in chemical properties of the organic solvents. The high concentrations of water in the

organic phase (2-propanol) increase the polarity of the 2-propanol phase. The extracted

ion-pair complexes can dissociate into ions in the mixture due to the two effects of

water involved in the 2-propanol phase with increasing water content. However,

considering the higher extraction of [AuCl4]- than [PdCl4]2- and [PtCl6]2-, the polarity of

the 2-propanol phase is much operative in the extraction. Furthermore, the increased

involvement of water in the 2-propanol phase leads to the formation of solvent clusters

of water and of 2-propanol [22-24]. The ion pair complexes are preferentially solvated

52

by the solvent clusters [25], resulting in the increased extraction of the ionic species in

the mixtures of water-soluble organic solvents and water.

9 12 15 18 21 24-1.5

-1

-0.5

0

0.5

1

1.5

log

D

[H 2O]Org / mol dm -3

Figure 17. Effect of water concentrations in organic phase on the extraction of precious

metal ions Au(III) ( ●), Pd(II) ( ▲) and Pt(IV) ( ○).

4. 3. 4. Effect of Chloride Ion Concentration

The effects of chloride concentration in both aqueous and organic phases on the

extraction of precious metals are shown in Figs. 18a and 18b. The relationship between

the chloride ion concentration in both phases and the distribution ratio of Au(III), Pd(II)

and Pt(IV) shows a straight line: the distribution ratio of the precious metals increases

with increasing the chloride concentration in the aqueous phase but decreases with

increasing chloride concentration in the organic phase. The chemical species of

Au(IIII), Pd(II) and Pt(IV) are [AuCl4]-, [PdCl4]2- and [PtCl6]2- respectively, and are

independent of chloride concentration in aqueous phase at higher concentration of Cl-.

Thus, the effects of chloride ions in organic and aqueous phases are ascribed to the

53

concentration of water in the organic phase at different concentrations of Cl-. Water

molecules solvate to 2-propanol and Na+ and Cl-. The increased Na+ and Cl-

concentrations in the aqueous phase lead to the decrease in the distribution of water

molecules to the organic phase, resulting in the decreased distribution of the charged

chemical species into organic phase. Another effect of the concentration of NaCl in the

organic phase is to neutralize the charged anionic species of [AuCl4]-.

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65-1.5

-1

-0.5

0

0.5

1

1.5

log[Cl -]Aq / mol dm -3

logD

(a)

54

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1-1.5

-1

-0.5

0

0.5

1

1.5

log[Cl -]Org / mol dm -3

logD

(b)

Figure 18. Effect of chloride concentrations in aqueous (a) and organic phases (b) on

the extraction of precious metal ions Au(III) ( ●), Pd(II) ( ▲) and Pt(IV) ( ○).

4. 3. 5. Mechanism of Extraction of Gold (III)

Figure 19 shows an equilibrium scheme involving Na+, Cl- and [AuCl4]- in the

present system. Gold(III) reacts with chloride ion to form [AuCl4]- at higher

concentration of NaCl. Since the organic phase contains a lot of water, Na+ and Cl-

distribute more into the organic phase. Hence, [AuCl4]- is extracted into the organic

phase with Na+ and dissociates to [AuCl4]- and Na+ in the organic phase. NaCl works

for the following roles: (1) the phase separation from the mixed aqueous solution of

water miscible-organic solvents, (2) the formation of [AuCl4]- and (3) the neutralization

of [AuCl4]- resulting in the extraction of gold(III) into the organic phase.

the

present system. Gold(III) reacts with chloride ion to form [AuCl4]- at higher

concentration of NaCl. Since the organic phase contains a lot of water, Na+ and Cl-

distribute more into the organic phase. Hence, [AuCl4]- is extracted into the organic

phase with Na+ and dissociates to [AuCl4]- and Na+ in the organic phase. NaCl works

for the following roles: (1) the phase separation from the mixed aqueous solution of

water miscible-organic solvents, (2) the formation of [AuCl4]- and (3) the neutralization

of [AuCl4]- resulting in the extraction of gold(III) into the organic phase.

55

H O2

2

Cl

Cl

-

-][AuClNa

-+

4 4

+- 3+

[AuCl H O

Na+4 Cl Au

]

+

Na +

Na ] [AuCl 4

Organic phase

Aqueous phase

Figure 19. Reaction scheme for the extraction of gold(III) in the presence of NaCl.

4. 4. Conclusions

The mixture of 2-propanol with water has been employed to extract Au(III) along

with other precious metals such as Pd(II) and Pt(IV) by using NaCl in the concentration

range of 2.5-4.0 mol dm-3. Gold(III) in aqueous phase was quantitatively extracted into

the 2-propanol phase at 2.5-4.0 mol dm-3 of NaCl. The extraction of the other metals

such as Pd(II) and Pt(IV) were much lower than for that of Au(III). Thus, the

interesting points are that (1) gold(III) is the only metal ion that can be quantitatively

extracted into the organic phase by salting-out process from several precious metals and

(2) gold(III) is extracted into 2-propanol phase without extracting reagents from an

aqueous NaCl solution below 4 mol dm-3.

The extraction method using 2-propanol and sodium chloride has a unique advantage

for the separation of chemical species that are not extracted in chloroform or 1,2-

dichloroethane. The water soluble solvents separated by salting-out can be through of a

“new solvents” promising extraction and separation of charge chemical species like

tetrachloro metal complexes.

56

References

[1] D. F. C. Morris, M. A. Khan, Talanta 15 (1968) 1301.

[2] M. B. Mooiman, J. D. Miller , In: Proc. Int. Solvent Extr. Conf. ISEC '

AICHE, New York 83 (1983) 530.

[3] M. B. Mooiman, J. D. Miller, Hydrometallurgy 27 (1991) 29.

[4] F. J. Alguacil, C. Caravaca, Hydrometallurgy 42 (1996) 197.

[5] F. J. Alguacil, C. Caravaca, S. Martinez, A. Cobo, Hydrometallurgy

36 (1994) 369.

[6] F. J. Alguacil, C. Caravaca, J. Mochon, A. Sastre, Hydrometallurgy 44 (1997) 359.

[7] G. Ma, W. F. Yan, T. D. Hu, J. Chen, C. H. Yan, H. C. Gao, J. G. Wu, G. X. Xu,

Phys. Chem. Chem. Phys. 1 (1999) 5215.

[8] J. Gao, B. Peng, H. Fan, J. Kang, X. Wang, Talanta 44 (1997) 837.

[9] Y. Marcus, A.S. Kertes, Ion Exchange and Solvent Extraction of Metal

Complexes, John Wiley & Sons, London, 1969.

[10] S. Martinez, A. M. Satre, F. J. Aguacil, Hydrometallurgy 52 (1999) 67.

[11] I. Villaescusa, V. Savado, J. de Pablo, Reactive & Fundamental Polymers

32 (1997) 125.

[12] A. M. Sastre, A. Madi, F. J. Aguacil, Hydrometallurgy 54 (2001) 171.

[13] M. Cox, in: J. Rydberg, C. Musikas, G. R. Choppin (Eds), Principles and

Practices of Solvent Extraction, Marcel Dekker, New York, 1992.



[16] C. F. Baes Jr, R. E. Mesmer, The Hydrolysis of Cations, Krieger Publishing

Company, Florida, 1986.

[17] T. Ohara, S. Matsumoto, H. Yamamoto, J. Shibata, Y. Baba, Solv. Extr. Ion Exch.

3 (1996) 213.

[18] R. Oleschuk, A. Chow, Talanta 44 (1997) 1371.

[19] K. Ohto, H. Yamaga, E. Murakami, K. Inoue, Talanta 44 (1997) 1123.

[20] L. G. Sillen, A. E. Martell, Stability Constants of Metal-Ion Complexes, The

Chemical Society, Burlingson House, London, 1964.

[21] R. A. Grant, C. S. Smith, Solvent Extraction, Melbourne Autralia ISEC,

57

2 (1996) 433.

[22] T. Takamuku, A. Yamaguchi, D. Matsuo, M. Tabata, T. Yamaguchi, T. Otomo,

T. Adachi, J. Phys. Chem. B 105 (2001) 10101.

[23] Y. G. Wu, M. Tabata, M. Takamuku, Talanta 54 (2001) 69.

[24] K. Yoshida, M. Misawa, K. Maruyama, M. Imai, M. Furusaka, J. Chem. Phys. 113

(2000) 2343.

[25] Y. G. Wu, M. Tabata, M. Takamuku, Solution Chemistry 30 (2002) 381.

58

CHAPTER FIVE SELECTIVE EXTRACTION OF THALLIUM(III) IN THE PRESENCE

OF GALLIUM(III), INDIUM(III), BISMUTH(III) AND ANTIMONY(III)

59

5. 1. Introduction

Thallium is an element that is toxic for plants, animals, microorganisms and humans.

The toxicity of the element is greater than that of mercury, cadmium, lead or copper.

Surface water usually exhibits a thallium concentration of the order of 10-100 ng L-1.

Generally the range of thallium concentration in non-polluted soil is of 0.08-1.5 µg g-1.

Therefore, the mobility of thallium in soil samples is a crucial factor for the toxic effect

of element. Thallium(III) compounds are highly toxic and are strictly controlled to

prevent their pollution to humans and environment. Thallium(III) is usually present in

lead(II), cadmium(II), indium(III) or zinc(II) compounds as a trace constituent [1,2].

Also thallium(III) readily forms amalgams with a number of metals such as silver, lead

and antimony. Therefore the separation of thallium(III) from other metal ions has been

a subject of great analytical interest.

The separation of thallium(I) has been intensively studied by the formation of ion-

pair complexes with basic triphenylmethane dyes [3], cryptand 2,2,2 with erythrosine

[4], benzo-15-crown-5 [5], 18-crown-6 [6] and dibenzo-24-crown-8 [7] or 12-crown-4

[8] with picrate. However, antimony(III), lead(II), indium(III) and mercury(II) usually

interfered and these reagents were not effective for the extraction of thallium(III).

There are a few papers on the separation of thallium(III). Thallium(III) has been

separated in the presence of gallium(III) and indium(III) from hydrochloric acid

solution at low acidity by using TBP, TOPO and TOA [9-11]. Amberlite XAD Chelex

[12] and poly(dibenzo-18-crown-6) [13], efficiently adsorbed thallium(III), iron(III),

gold(III) and antimony(V) from hydrochloric acid solutions. However, these methods

cannot separate quantitatively thallium(III) from its mixture with gallium(III),

indium(III), bismuth(III) and antimony(III).

Thallium(III) strongly binds to unidentate ligands such as Cl- and I- to form charged

anionic species of TlCl4- or TlI4

- [14,15]. Therefore, if the ionic species can be extracted

into the organic phase, thallium(III) could be separated from the above trivalent metals.

Extraction method based on salting-out occurred upon addition of electrolytes to

mixed solvents of water and water-miscible organic solvents is an attractive technique

[16]. The separated organic solvents always contain a lot of water and salt, resulting in

the high polar solvents compared to the corresponding pure organic solvents [17]. Thus,

the separated organic solvents can easily extract ion-pair complexes and highly charged

60

species such as metalloporphyrins4+, which normally cannot be extracted using

conventional solvents, such as chloroform [18]. Therefore, it is expected that the

organic phase separated by salting-out from aqueous-organic solvent mixture can

extract thallium(III).

In the present study, we report on a selective extraction method of thallium(III) in the

presence of gallium, indium, bismuth and antimony into 2-propanol phase by the

salting-out method using sodium chloride that causes phase separation. The detailed

extraction mechanism and analytical method will be described.

5. 2. Experimental

5. 2. 1. Apparatus

The volumes of the aqueous and organic phases after phase separation were

measured using a volume-calibrated graduated tube. The concentration of metal ions in

the two phases was determined by ICP atomic absorption spectrophotometry (Perkin-

Elmer Optima 3100 RL), whereas the concentration of Cl- in the lower water phase was

determined by argentometry using potassium chromate as indicator [19]. The

concentration of H+ was determined by sodium hydroxide. The concentration of water

in the upper 2-propanol phase was determined by Karl-Fisher titration method using an

automatic titrator (Kyoto Electronics, MKL-200). The water content initially present in

fresh 2-propanol (99.97%) is a little and negligible compared to the water content in the

2-popanol phase after the phase separation by addition of NaCl. This is because the

water content in 2-propanol phase is very high as 9.408-22.939 mol dm-3.

5. 2. 2. Reagents

NaCl (Wako Pure Chemicals) was dried in an electric oven at 400oC for 4 hours.

Aqueous solutions of metal ions were prepared by dissolving an appropriate amount of

metal chlorides (TlCl3, GaCl3, InCl3, BiCl3 and SbCl3) in hydrochloric acid solution.

The initial concentrations of the metal ions were varied at 2.94 x 10-5 - 14.34 x 10-4 mol

dm-3. While hydrochloric acid concentration was maintained at 0.1 mol dm-3 and NaCl

concentration range was 2.5-4.0 mol dm-3. Organic solvent was 2-propanol (99.97%

Wako Pure Chemicals) and was purified by drying over 4 Å molecular sieves. Double

distilled water was used throughout the experiment.

61

5. 2. 3. General Procedure

Extraction of metal ions was carried out in a similar way to that used in a chapter 4

by mechanical shaking an aqueous solution (5cm3) containing metal ions and

hydrochloric acid of 0.1 mol dm-3 with 2-propanol (5cm3) in a tube in the presence of

different concentrations of sodium chloride ranging at 2.5-4.0 mol dm-3. After shaking

for a ten minutes, the mixture was then centrifuged and the organic and aqueous phases

were allowed to stand for a few minutes. The concentrations of metal ions distributed

between the two phases were analytically determined by ICP. The distribution

coefficient and extraction percent of metals were calculated from the concentrations

determined [20,21]. The salting-out data are summarized in Table 3.

Table 3. Composition of the organic and aqueous phases after salting-out.

[NaCl]initial

mol dm-3

Volume /cm3

Org Aq

[H2O]org

mol dm-3

[Cl-]aq

mol dm-3

[Na+]org

mol dm-3

[H+] / mol dm-3

Org Aq

2.5 6.28 3.54 22.939 2.225 0.724 0.041 0.071

3.0 5.53 4.26 15.073 2.995 0.411 0.028 0.086

3.5 5.44 4.40 12.021 3.588 0.287 0.022 0.089

4.0 5.36 4.51 9.408 4.113 0.228 0.012 0.095


5. 3. 1. Effect of Initial Sodium Chloride Concentrations in Aqueous Solution

Sodium chloride plays mainly three roles in the present system. The first is to phase

separation of the mixture of water and 2-propanol, leading to a change in water

concentration in the organic phase (Fig. 20). The second is the formation of chloro

complexes with trivalent metal ions, and the third is to provide counter ions as Na+ to

the extracted ionic species.

62

2 2.5 3 3.5 4 4.5

-0.8

-0.6

-0.4

-0.2


logD

(H O

)2

Figure 20. Distribution of water between aqueous and 2-propanol phases separated by

salting-out with sodium chloride.

Figure 21 shows the dependence of extraction percent of metal ions on the initial

sodium chloride concentration in aqueous solution. It can be observed that thallium(III)

as high as 99% could be extracted into the 2-propanol phase. Thallium(III) was

quantitatively extracted into the organic phase over the whole concentration range of

sodium chloride, while the extraction of gallium(III), indium(III), bismuth(III) and

antimony(III) were very poor in the presence of a high sodium chloride concentration.

Gallium(III), indium(III), bismuth(III) and antimony(III) were not extracted above NaCl

concentration of 4.0 mol dm-3, though about 40.2% of In(III) and 53.7% of Ga(III),

37.2% of Bi(III) and 34.4% of Sb(III) could be extracted at 2.5 mol dm-3 NaCl. The

distribution ratios (D) of the metal ions at NaCl of 4.0 mol dm-3 are summarized in

Table 4 in the logarithm form.

When the metal concentrations were maintained in the range of 2.94 x 10-5–14.34 x 10-4

mol dm-3, the distribution ratios of Tl(III), Ga(III), In(III), Bi(III) and Sb(III) into

organic phase did not change with the alterations in initial concentrations of these metal

ions. This indicates that the extracted chemical species are monomeric form.

63

Figure 21. Effect of sodium chloride concentrations on the extraction of Tl(III) (●),

Ga(III) (▲), In(III) (■), Bi(III) (○) and Sb(III) (□) from a 1:1 (v/v) mixture of water

with 2-propanol in 0.1 mol dm-3 HCl. The concentration ranges in mol dm-3 are: (1)

Tl(III), 2.94x10-5- 4.98x19-4; (2) Ga(III), 8,61x10-5- 14.34x10-4; (3) In(III), 5.23x10-5-

8.71x10-4; (4) Bi(III), 2.88x10-5- 4.81x10-4; (5) Sb(III), 4.93x10-5- 8.21x10-4.

Table 4. Distribution ratios of triply charged ions, water and NaCl into 2-propanol after

salting-out using NaCl at 4.0 mol dm-3.

log D

TlCl4- 1.52

GaCl2+ + GaCl2+ + Ga3+ -1.09

InCl4- + InCl3 + InCl2

+ -1.31

BiCl63- + BiCl5

2- + BiCl4- + BiCl3 -1.46

SbCl63- + SbCl5

2- + SbCl4- + SbCl3 -1.15

H2O -0.72

NaCl -1.3

2.5 3 3.5 40

20

40

60

80

100


Ext

ract

ion

perc

enta

ge (%

)

64

5. 3. 2. Chemical Species of Metal Ions in the Presence of Chloride

The above results can be explained by the formation of different charged species of

metal ions at high concentrations of Cl- in the aqueous phase. Many stability constants

of the chloro complexes of trivalent thallium(III), gallium(III), indium(III), bismuth(III)

and antimony(III) have been determined in the past by various authors. For example,

the equilibrium constants of TlCl4- are shown in the books of “Stability Constants” as

follows: log β4 (TlCl4-) = 16.3 ± 0.1 (I = 0.5); 18.3 (I = 3); 18.0 ± 0.3 (I = 1.0); 18.3 (I =

0) and 19.4 (I = 0) [14,22], and log β4 (TlCl4-) = 14.94 by Sato [11]. All these data

indicate clearly that the main chemical species of thallium(III) in aqueous solution is

TlCl4- at concentrations of Cl- more than 1.0 mol dm-3. The detailed species distribution

of Tl(III), Ga(III), In(III), Bi(III) ans Sb(III) are shown in Fig. 22 as a function of [Cl-],

where the following stability constants of the chloro complexes of thallium(III) were

used: log ß1 = 7.18, log ß2 = 12.94, log ß3 = 16.09, log ß4 = 18.31 (I = 3.0) [22].

The chemical species of gallium(III), indium(III), bismuth(III) and antimony(III),

consist of mixtures of many ionic species at high concentrations of Cl-: Ga3+, GaCl2+

and GaCl2+; InCl2

+, InCl3 and InCl4-; BiCl4

-, BiCl52- and BiCl6

3-; SbCl4-, SbCl5

2- and

SbCl63- [14,11]. Therefore it is thallium(III) which was preferentially and highly

extracted into the 2-propanol phase at higher concentrations of NaCl while gallium(III),

indium(III), bismuth(III) and antimony(III) were hardly extracted as shown in Fig. 21.

The structure of TlCl4- is tetrahedral and the coordination sites of Tl(III) are fully

occupied by chloride [23], so the water molecules do not bind to TlCl4-. On the other

hand, the mixture of many charged species of Ga(III), In(III), Bi(III) and Sb(III)

resulted in the low extraction of their metal ions due to hydration and difficulty in

charge neutralization for these highly charged species. Hence the extractability of

thallium(III) is very high as compared to gallium(III), indium(III), bismuth(III) and

antimony(III) at 4.0 mol dm-3 NaCl concentration.

65

-8 -7 -6 -5 -4 -3 -2 -1 0

log[Cl -] / mol dm -3

Mol

e fr

actio

n

TlCl4-

TlCl3

TlCl2+

TlCl2+Tl3+

Ga3+

GaCl2+

GaCl2+

In3+

InCl2+

InCl2+

InCl3

InCl4-

Bi3+

BiCl2+

BiCl2+

BiCl3 BiCl4- BiCl6

3-

BiCl52-

Sb3+

SbCl2+

SbCl2+

SbCl3

SbCl4-

SbCl52- SbCl6

3-

Figure 22. Distribution of metal chloro complexes at various concentration of Cl-.

66

5. 3. 3. Effect of Hydrochloric Acid Concentrations

Figure 23 shows the extraction of metal ions from aqueous solution containing

hydrochloric acid concentrations varied over a range of 0.1-0.96 mol dm-3 at the

constant concentrations of metal ions and NaCl. The distribution coefficient of metal

ions in the organic phase is almost constant and independent of HCl under these

experimental conditions. This means that the main ion-pair formation occurs between

TlCl4- and positively charged Na+ ion not H+. In addition, the concentrations of Na+ in

the organic phase is higher than that of H+ (Table 3). Therefore, the charge of TlCl4- is

neutralized by Na+.

A similar extraction was observed for Tl(III) in the presence of potassium iodide

(2.0 mol dm-3) and 18-crown-6 (0.05 mol dm-3). Here, TlI4- has been extracted into

Figure 23. Effect of hydrochloric acid concentrations on the e

dichloromethane with K-18-crown-6+ as the counter ion [15].

xtraction of metal ions at

12.32x10-5.

0 0.2 0.4 0.6 0.8 1

-1

0

1

2

[HCl] initial / mol dm -3

logD

2.5 mol dm-3 NaCl. The concentrations of ions in mol dm-3 are: Tl(III) (●), 7.34x10-5;

Ga(III) (▲), 21.51x10-5; In(III) (■), 13.06x10-5; Bi(III) (○), 7.21x10-5; and Sb(III) (□),

67

5. 3. 4. Effect of Water Concentration on the Extraction of Metal Ions

The influence of water in the organic phase on the extraction of metal ions is shown

creasing the

ic Phase

The effect of sodium chloride in the organic phase on the extraction of the trivalent

Fig. 25. The

in Fig. 24. It can be seen that the extraction of metal ions increases with in

concentration of water in the organic phase. The water dissolved in the 2-propanol

phase does have a large effect on the chemical properties of the solvent. The high

concentration of water in the organic phase increases the polarity of 2-propanol [16].

Thus, the extracted ion-pair complexes can dissociate into their ions in the 2-propanol

phase as the case observed in the aqueous mixture of acetonitrile [18]. Another effect of

water in the 2-propanol phase is to enhance the formation of solvent clusters of 2-

propanol that preferentially solvates to the ion-pair complexes [24], resulting in the

increased extraction of the ionic species in the mixtures of water-soluble organic

solvents and water. As shown in Table 3 the concentration of H+ in organic phase is

higher than 0.01 mol dm-3 and Cl- concentration is higher than 0.22 mol dm-3, so the

hydrolysis of thallium(III) is negligible in the organic phase.

5. 3. 5. Effect of Sodium Chloride Concentrations in Organ

metal ions, thallium, gallium, indium, bismuth, and antimony is shown in

distribution ratio of thallium(III), gallium(III), indium(III), bismuth(III) and

antimony(III) increase with increasing the concentration of NaCl in the organic phase.

In the aqueous phase the main chemical species of thallium(III) is TlCl4-, but those

of gallium(III), indium(III), bismuth(III) and antimony(III) respectively are a mixture of

Ga3+, GaCl2+ and GaCl2+, InCl2

+, InCl3 and InCl4-, BiCl6

3- and BiCl52-, and SbCl6

3-,

SbCl52- and SbCl4

- . Thus, TlCl4- is extracted quantitatively as the ion-pair complex of

Na+[TlCl4]- into organic phase at different concentrations of NaCl in the 2-propanol

phase. However, the extraction of Ga(III), In(III), Bi(III) and Sb(III) is small, but their

distribution ratios increase with chloride concentrations in the organic phase due to high

coordination of chloride to these metal ions.

68

5 10 15 20 25-2

-1

0

1

2

[H 2O]org / mol dm -3

logD

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2

-1

0

1

2

[NaCl] Org / mol dm -3

logD

▲),

-5--4; (4) Bi(III), 2.88x10-5- 4.81x10-4; (5) Sb(III), 4.93x10-5- 8.21x10-4.

Figure 25. Effect of sodium chloride concentrations in the organic phase on the

extraction of Tl(III) (●), Ga(III) (▲), In(III) (III) (■), Bi(III) (○) and Sb(III) (□).

Figure 24. Effect of water concentrations on the extraction of Tl(III) (●), Ga(III) (

In(III) (■), Bi(III) (○) and Sb(III) (□). The concentration ranges in mol dm-3 are: (1)

Tl(III), 2.94x10-5- 4.98x19-4; (2) Ga(III), 8,61x10-5- 14.34x10-4; (3) In(III), 5.23x10

8.71x10

69

5. 3. 6. Mechanism of Extraction of Thallium (III) by 2-Propanol

Figure 26 shows an equilibrium scheme involving Na+, Cl- and TlCl4- in the present

system. First, thallium(III) reacts with chloride ion to form TlCl4- in the presence of

aCl. The organic phase contains a lot of water, Na+ and Cl-. TlCl4- is extracted into

e organic phase with Na+. NaCl works for the following roles: (1) the phase separation

om the mixed aqueous solution of 2-propanol, (2) the formation of TlCl4- in both

queous and organic phases and (3) the charge-neutralization of TlCl4- by Na+, resulting

the extraction of thallium(III) into the organic phase.

N

th

fr

a

in

Aqueous phase

Organic phase

4[TlCl ] Na

+ Na

+

]

Tl 4 Cl +Na

H O[TlCl

3+ - +

44

+ -Na [TlCl]

-

-

Cl

Cl

2

2H O

igure 26. Reaction scheme for the extraction of thallium(III) in the presence of NaCl.

extracted into 2-

F

5. 4. Conclusions

The above results indicate that thallium(III) was quantitatively

propanol phase from the mixture solvents of 2-propanol and water by addition of NaCl

in presence of other trivalent metals such as gallium, indium, bismuth and antimony in

aqueous solution. The extraction efficiencies of gallium(III), indium(III), bismuth(III)

and antimony(III) were very poor compared with thallium(III) at concentration of NaCl

higher than 4.0 mol dm-3 . Thus, selectivity separation of thallium(III) from these

metals could be attained using the mixture solvents of 2-propanol and water without

using any reagents.

70

References

[1] S. Kalyanaraman, S. M. Khopkar, Anal. Chim. Acta 97 (1978) 181.

[2] M. J. Sienko, R. A. Plane, Chemical Principle and Properties, London, 1974.

[3] R. E. V. Aman, J. H. Kanzelmeyer, Anal. Chem. 33 (1961) 1128.

[4] M. N. Gandhi, S. M. Khopkar, Anal. Chim. Acta 270 (1992) 87.

[6] Y. Takeda, H Goto, J. Bull. Chem. Soc. 52 (1979) 1920.

Bull. Chem. Soc. 52 (1979) 2501.

. M. Smith, Critical Stability Constants, Vol. 4, Plenum Press,

[15] R. G. Vibhute, S. M. Khopkar, Anal. Chim. Acta 222 (1989) 215.

6] M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Sci. 10 (1994) 383.

7] S. Haiping, J. Nishimoto, M. Tabata, Anal. Sci. 13 (1997) 119.

oto, J. Nishimoto, Anal. Chem. 68 (1996) 758.

[19] G. H. Jeffery, J. Bassett, J. Mendham, R. C. Denney, Textbook of Quantitative

al Analysis, London, 1989.

0] B. Tremillon, Reactions in Solution, John Wiley& Sons, New York, 1993.

olvent

lexes, The

& Sons

emistry 30 (2002) 381.

[5] H. Tamura, K. Kimura, T. Shono, J. Electroanal. Chem 115 (1980) 115.

[7] Y. Takeda, J.

[8] Y. Takeda, M. Nemoto, S Fujiwara, J. Bull. Chem. Soc. 55 (1982) 3438.

[9] R. Kellner, J. M. Mermet, M. Otto, H. M. Widmer, Analytical Chemistry,

Wiley-vch, New York, 1997.

[10] T. Sato, K. Sato, Y. Noguchi, I. Ishikawa, J. Shigen-to-Sozai. 113 (1997) 185.

[11] T. Sato, J. Shigen-to-Sozai. 112 (1996) 123.

[12] H. Koshima, J. Anal. Sci. 2 (1986) 255.

[13] H. Koshima, H. Onishi, J. Anal. Sci. 3 (1987) 417.

[14] A. E. Martell, R

New York, 1976.

[1

[1

[18] M. Tabata, M. Kumam

Chemic

[2

[21] J. Rydberg, C. Musikas, G. R. Choppin, Principles and Practices of S

Extraction, Marcel Dekker, New York, 1992.

[22] L. G. Sillen, A. E. Martell, Stability Constants of Metal-Ion Comp

Chemical Society, Burling House, London, 1964.

[23] F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Jonh Wiley

New York, 1976.

[24] Y. G. Wu, M. Tabata, M. Takamuku, Solution Ch

71

CHAPTER SIX PHASE SEPARATION OCCURS BY THE ADDITION OF CALCIUM

CHLORIDE TO A MIXTURE OF 2-PROPANOL AND WATER

72

6. 1. Introduction

When a water-miscible solvent is salted-out from aqueous solution, phase separation

results from the decreased solubility of the solvent. Water-miscible polar solvents such

as acetonitrile, 1-methyl-2pyrrolidone and hexamethylphosphoramide have been used

for phase separation from their aqueous solution by salting-out [1]. This salting-out

phenomenon can be valuable in increasing the extractability of metal complexes in

liquid-liquid distribution [2,3]. Furinaga and Nagaosa [4] have shown that acetonitrile

is a very suitable solvent for use in polarogaphic methods after salting-out extraction.

This salting-out technique would seem promising in general extraction chemistry, as

certain ion-pairs could be extracted into polar water-miscible solvents, analogously to

extraction into nitrobenzene [5] and propylene carbonate [6].

In the chapter 3, we reported the phase separation of homogeneous mixture of 2-

propanol and water upon the addition of NaCl. It was found that the phase separation of

the mixture of 2-propanol with water take place at mole fraction of 2-propanol (x2-

propanol ) less than < 0.5, and the NaCl concentration was ranged from 2.0-4.0 mol dm-3.

Furthermore, we also pointed out that the formation different charged species of metal

ions at high concentrations of Cl- in aqueous solution. This led to quantitatively

extracted gold(III) and thallium(III) into the 2-propanol phase.

In this chapter, we report about the phase separation occurs by addition of CaCl2 to a

mixture of 2-propanol and water. We also have expected that an organic phase

separated by salting-out method using CaCl2 can be extracted cobalt(II).

6. 2. Experimental

6. 2. 1. Apparatus

The volume of solution was measured using a volume-calibrated graduated tube. The

concentration of Ca2+ in the upper 2-propanol phase was determined by atomic



as an indicator. The concentration of water in the organic phase was determined by

Karl-Fisher titration method using an automatic titrator (Kyoto Electronics, MKL-200).

The density of solution after phase separation was measured with a densimeter

(ANTON Paar K. G., DMA 60).

73

6. 2. 2. Reagents


sieves. CaCl2 (Wako Pure Chemicals) was dried in an electric oven at 300o C for 4

hours. Doubly distilled water was used throughout the experiment and the organic

solvent was 2-propanol.

6. 2. 3. Phase Separation Procedure

The procedure of phase separation of 2-propanol-water-CaCl2 was carried out in a

similar way in the case of 2-propanol-water-NaCl system (chapter 3) as a function of

mole fraction of 2-propanol, water and CaCl2. First, aqueous CaCl2 solutions of various

CaCl2 concentrations were prepared by dissolving dried CaCl2 into distilled water.

Then, the aqueous CaCl2 solutions and 2-propanol were mixed to required mole

fractions of 2-propanol in a graduated tube with a stopper. The mixed solution in a tube

was vigorously shaken about 10 minutes and left aside at 298.2±0.3 K for 24 h to reach

a complete equilibrium.


6. 3. 1. Phase Separation Diagram of 2-Propanol-Water-CaCl2 Mixtures

Phase separation of 2-propanol-water-CaCl2 mixtures was examined as a function of

mole fractions of 2-propanol, water and CaCl2 as follows. First, aqueous CaCl2

solutions were prepared by dissolving dried CaCl2 into distilled water. Then, the

aqueous CaCl2 solutions were mixed with 2-propanol to a required mole fraction of 2-

propanol in a graduated tube. The mixed solution in the tube was vigorously shaken for

5 min and left for 24 h to reach a complete equilibrium. After 24 h standing the

equilibrium state of the mixed solution are illustrated in Fig. 27 as a function of mole

fractions of 2-propanol (χ2-propanol), water (χwater) and CaCl2 (χCaCl2).

The state of the mixed solutions was classified to two types: (1) homogeneous

solution of 2-propanol-water-CaCl2, (2) separation into 2-propanol (upper) and water

(lower) phases. The precipitation of CaCl2 was not observed at its high concentration.

It can be seen that the phase separation take place at 2-propanol mole fraction (χ2-propanol)

less than 0.6 and CaCl2 mole fraction higher than 0.03.

74

CaCl2 χ

2-Propanolχ

χ

Figure 27. Phase diagram of the 2-propanol-water-CaCl2 ternary mixtures as a function

of mole fractions of 2-propanol, water and CaCl2. The symbols of (●) and (○) denote

the phase separation and homogeneous solution, respectively. The solid line represents

the border between homogeneous ternary- mixture and phase separation.

6. 3.2. Composition of the Aqueous and 2-Propanol Phases After Salting-out Phase

Separation of the Mixture of 2-Propanol and Water by Addition of CaCl2

The compositions of the 2-propanol-water-CaCl2 ternary mixtures were determined

in a similar way to that used in a chapter 3 by shaking a volume of 5 cm3 of aqueous

CaCl2 solutions containing various CaCl2 concentrations with 5 cm3 of pure 2-propanol

in a graduated tube. The compositions of the aqueous and 2-propanol phases after

salting-out are given in Table 5. Changing the compositions of volume, water, [Ca]2+

and density of the aqueous and 2-propanol phase after salting-out are shown in figs. 28,

29, 30 and 31, respectively.

From Figure 28 it can be seen that the change in the volume of the organic and the

aqueous phases is small over the whole CaCl2 concentrations range from 3.0-6.5 mol

From Figure 28 it can be seen that the change in the volume of the organic and the

aqueous phases is small over the whole CaCl2 concentrations range from 3.0-6.5 mol

75

dm-3 in the aqueous solution compared to NaCl obtained previously [7]. This could be

ascribed to strong hydration of Ca2+, resulting in a small change in the water structure.

The volume of the organic phase separated by salting-out was larger than the initial

volume of 2-propanol. This indicates that 2-propanol interacts strongly with water

molecules through hydrogen-bonding. However, as the alkyl group is hydrophobic, the

aqueous 2-propanol solution easily separates into two phases upon the addition of

calcium chloride, which gives a large volume of organic phase containing a lot of water

and salt as shown in Figs 29 and 30, resulting in a highly polar solvent compared to

pure 2-propanol [8]. Thus, organic solvents separated by salting-out are suitable for the

extraction of ion-pair complexes such as tris(2,2’-bipyridine)cobalt(II) chloride [9] and

cadmium(II) iodide [10] and other highly charged species such as a metalloporphyrin4+,

which normally cannot be extracted into conventional solvents such

s chloroform [8,11].

Figure 28. Changing the volume of two phases after phase separation

2.5 3 3.5 4 4.5 5 5.5 6 6.5 73.5

4

4.5

5

5.5

6

6.5

[CaCl 2]initial / mol dm -3

Vol

ume

/ cm

3

Organic phase

Aqueous phase

76

3 4 5 6 7-0.8

-0.75

-0.7

-0.65

-0.6

[C aC l 2] initial / m ol dm -3

logD

(H O

)2

Figure 29. Distribution of H2O between two phases separated by salting-out of CaCl2

2.5 3 3.5 4 4.5 5 5.5 6 6.5 7

1

2

3

4

5

6

7

[C aC l 2] initial / m ol dm -3

Organic phase

[Ca]

2+ /

mol

dm

-3

Aqueous phase

Figure 30. Changing the [Ca]2+ of two phases after phase separation

77

Fig

ure

31.

Ch

ang

ing

the

den

sity

of

two

3 4 5 6 70.75

0.9

1.05

1.2

1.35

1.5


Organic phaseDen

sity

/ cm

-3 Aqueous phase

phases after phase separation .

6. 4. Conclusions

The above results indicate that phase separation occurs at 0.1-0.5 mole fractions of 2-

propanol and 0.03-0.12 mole fractions of CaCl2. Compared to the phase separation of

the aqueous mixture with 2-propanol by addition of NaCl (2.0-4.0 mol dm-3) [7], the

phase separation occurs at higher concentration range of CaCl2 (2.5-6.5 mol dm-3).

78

Table 5

Composition of the organic and aqueous phases separated in the presence of CaCl2. a

[CaCl2]initial(mol dm-3)

Volume(cm3)

Org Aq

[H2O]Org (mol dm-3)

[Ca2+](mol dm-3)

Org Aq

[Cl-](mol dm-3)

Org Aq

[H+] (mol dm-3)

Org Aq

Density (g cm-3)

Org Aq

3.0 5.70 4.16 8.236 0.896 2.379 1.819 4.841 0.027 0.083 0.910 1.181

4.0

5.85 4.09 8.779 0.941 3.555 1.911 7.192 0.029 0.082 0.918 1.276

4.5 5.95 3.95 9.171 0.943 4.276 1.916 8.633 0.030 0.081 0.919 1.317

5.0 6.05 3.85 9.312 1.013 4.916 2.056 9.912 0.030 0.080 0.929 1.357

5.5 6.00 3.85 9.441 1.090 5.444 2.211 10.967 0.031 0.079 0.935 1.393

6.0 6.05 3.82 9.609 1.165 5.892 2.362 11.863 0.032 0.079 0.941 1.427

6.5 6.00 3.86 9.789 1.228 6.382 2.490 12.842 0.034 0.078 0.947 1.461

a 5 cm3 of aqueous solution containing various concentrations of calcium chloride and 0.1 mol dm-3 HCl was mixed with 5 cm3

of 2-propanol.

79

References


[2] Y. Marcus, A. S. Kertes, Ion Exchange and Solvent Extraction of Metal

Complexes, Wiley-Interscience, London, 1969.

[3] G. H. Morrison , H. Freiser, Solvent Extraction in Analytical Chemistry, Wiley-

Interscience, New York, 1966.

[4] F. Fulinaga, Y. Nagaosa, Bull. Chem. Soc. Jpn. 53 (1980) 416.

[5] P. f. Collins, H. Diehl, G. F. Smith, Anal. Chem. 31 (1959) 1862.

[6] B. G. Stephens, H. A. Suddeth, Anal. Chem. 39 (1967) 1478.

[7] N. H. Chung, M. Tabata, Talanta 58 (2002) 927.

[8] M. Tabata, M. Kumamoto, J. Nishimoto, J. Anal. Sci. 10 (1994) 383.


[10] T. Fujinaga, Y. Nagaosa, Bull. Chem. Soc. Jpn. 53 (1980) 416.


80

CHAPTER SEVEN SELECTIVE EXTRACTION OF COBALT(II) IN THE PRESENCE

OF MANGANESE(II), NICKEL(II), AND COPPER(II)

81

7. 1. Introduction

The solvent extraction is one of the most popular methods used for separation of

metal ions from industrial and waste solutions, which are frequently required in

hydrometallurgical processing. The separation of cobalt from nickel is of major interest,

and a number of works have been conducted on the solvent extraction in a

hydrometallurgical field [1]. The major source for the production of cobalt appears to

be from certain raw material such as oxide and sulfide ores, wastes, dust, etc.

Hydrometallurgical methods of dissolution of such material using hydrochloric acid

result in solution containing cobalt along with some impurities. Also cobalt readily

form alloys with other metal ions such as chromium, nickel, copper and tungsten which

have special properties used for cutting stools as drill bits for high-speed machine [2].

Cobalt is recovered from the alloys by treatment with acids in order to dissolve cobalt,

with other metals. Therefore, simple and selective separation of cobalt from other metal

ions in acidic solutions has been interested to hydrometallurgists.

Extraction of cobalt(II) have been carried out by using several complexing reagents.

For example, cobalt(II) can be separated from nickel(II) by using pyridinecarboxylate

esters [3], di(2-ethylhexyl) phosphoric acid (D2EHPA), 2-ethylhexyl phosphonic acid

(PC88A) and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272) [4-7], 1-(2-

thiazolylazo)-2-naphthol (TAN) [8], polyoxyethylene nonyl phenyl ether with 10

ethylene oxide units (PONE10) and 2-ethylhexyl phophonic acid mono 2-ethylhexyl

ester (EHPNA) [9]. Other complexation reactions between metals and ligands have

been used to extract Co(II) by using PC88A [10], sodium di(2-ethylhexyl) phosphate

(D2ENa) and tributyl phosphate (TBP) into cyclohexane and n-dodecane [11,12], N-

Phenyl-N’-(2-butylthiophenyl)thiourea (PBT) into chlorobenzene [13]. Cobalt(II) also

has been extracted as ion-pair complexes together with other transition metal ions by

using N,N’-bis(2-pyridylmethylidene)-1,2-diiminoethane(BPIE), N,N’-bis[1-(2-

pyridyl)ethylidene]-1,2-diiminoethane (BPEE) and N,N’-bis(2-pyridylmethylidene)-

trans-1,2-diiminocyclohexane (BPIC) into nitrobenzene [14], N,N’-bis(2-

hydroxyphenylmethyl)-N,N’-bis(2-pyridylmethyl)-1,2-ethanediamine (BBPEN) into

chloroform [15], N,N-dibutyl-N’-benzoylthiourea (DBBT) into paraffin [16], mixture of

four trialkyl phosphine oxides (cyanex 923) into toluene [17], and trioctylphosphine

oxide (TOPO) into chloroform [18]. Most of above mentioned extractions have been

82

carried out at pH 4-10. Since the extraction ratio of cobalt(II) is not enough high in

acidic medium (pH < 2) that cobalt(II) can be separated by one step extraction, some

extraction stages have been carried out to extract cobalt(II) completely.

Phase separation of homogeneous mixed solvents has been performed by addition of

salts or changing temperature to organic solvents. For example, phase separation occurs

by organic salt of (NH4)2SO4 added to a polyethylene glycol [19], or induced

temperature raised to 30oC in diethylether-water system [1]. In previous studies, we

investigated phase separation of homogeneous mixtures of 2-propanol and water upon

the addition of sodium chloride. It was found that different charged species of metal

ions were formed at high concentration of Cl- in aqueous solution. This led to selective

extraction of specific chemical species such as AuCl4- and TlCl4

- as their ion-pair

complexes with Na+ into the 2-propanol phase [20,21].

In the present study, we have investigated the phase separation that occurred by the

addition of CaCl2 to the mixtures of 2-propanol and water. We have utilized the phase

separation processes for selective extraction of cobalt(II) in the presence of

manganese(II), nickel(II) and copper(II) into 2-propanol phase without using any other

extracting reagents. The detailed analytical method and extraction mechanism will be

described.

7. 2. Experimental

7. 2. 1. Apparatus

The concentrations of metal ions in the two phases were determined by inductively

coupled plasma atomic absorption spectrometry (ICP-AAS, Perkin-Elmer Optima 3100

RL). The concentration of water in the organic phase was determined by Karl-Fischer

titration method using an automatic titrator (Kyoto Electronics, MKL-200). The

densities of aqueous and organic solutions after phase separation were measured with a

densimeter (ANTON Paar K. G., DMA 60). Absorption spectra of cobalt(II) chloro

complexes were determined by a Shimadzu UV-Vis spectrophotometer (model UV-

2100, Japan).

7. 2. 2. Reagents

83

The organic solvent used was 2-propanol (99.97%, Wako Pure Chemicals) and was

purified by drying over 4Å molecular sieve. NaCl (Wako Pure Chemicals) was dried in

an electric oven at 400oC for 4h. Aqueous solution of CaCl2 (Wako Pure Chemicals)

was prepared by taking a given amount of dried CaCl2 and the concentration was

confirmed by EDTA titration [22]. Aqueous solution of relevant ions were prepared by

dilution of the standard metal chloride solution of 1000 ppm for Mn(II), Co(II), Ni(II)

and Cu(II) in 0.1 mol dm-3 HCl. The initial concentrations of the transition metals were

varied from 5-30 ppm, and the concentration of hydrochloric acid was maintained at 0.1

mol dm-3 and the CaCl2 concentration range was 3.0-6.5 mol dm-3. Double distilled

water was used throughout the experiment. All experiments were carried out at room

temperature and water jacket cell (298.2±0.5 K). In most cases, the precision of

extraction and ICP-AAS measurements indicated errors smaller than ± 2% and ±1%,

respectively.

7. 2. 3. Extraction of Cobalt(II) Procedure

Extraction was carried out in a similar way to that used in the case of extractions of

gold(III) and thallium(III) by mechanical shaking of an aqueous solution (5 cm3)

containing Mn(II), Co(II), Ni(II) and Cu(II) ions and 0.1 mol dm-3 hydrochloric acid

with 2-propanol (5 cm3) in a tube in the presence of different concentrations of calcium

chloride ranging from 3.0-6.5 mol dm-3. After shaking for 15 min, the mixture was

centrifuged and the organic and aqueous phases were allowed to stand for a few minutes.

The concentrations of metal ions distributed between the two phases were determined

by ICP-AAS. The distribution coefficient and extraction percent were calculated from

the concentrations determined [23,1]. The separation factor (β = D1 / D2) between the

two metals was calculated as the ratio of the distribution ratios of two metals D1 and D2.

7. 3. Result and Discussion

7. 3. 1. Distribution Equilibria in the Presence of NaCl and CaCl2

Figure 32a shows the dependence of extraction percent of manganese(II), cobalt(II),

nickel(II) and copper(II) on the initial sodium chloride concentration in aqueous

solution. It shows that the extraction of Mn(II), Co(II), Ni(II) and Cu(II) are very low in

the presence of a high sodium chloride concentration. These ions were not extracted at

84

NaCl > 4.0 mol dm-3, although about 48.2% of Mn(II), 50.4% of Co(II), 46.6% of Ni(II)

and 44.4% of Cu(II) were extracted at 2.5 mol dm-3 of NaCl.

However, Fig. 32b shows that the extraction percent of Co(II) increases with

increasing initial calcium chloride concentration in aqueous solution. Cobalt(II) was

extracted to extent of 95.4% into the organic phase at 6.5 mol dm-3 of CaCl2 , while the

extraction of Mn(II), Ni(II) and Cu(II) were very poor. This indicates that calcium ion

plays significant roles for the high extractability of Co(II) in the organic solution.

Calcium chloride plays three important roles in the present system: the first is to cause

phase separation of the mixture of water and 2-propanol. The second is the formation of

chloro complexes with Co(II), and the third is to provide a counter ion as Ca2+ to extract

the ionic species of Co(II).

The extraction of Mn(II), Co(II), Ni(II) and Cu(II) from mixtures of the transition

metal ions in aqueous solution into 2-propanol phase was carried out for their three

different initial concentrations of these test ions (5, 15 and 30 ppm) at various initial

concentrations of CaCl2 (3.0, 4.0, 4.5, 5.0, 5.5, 6.0 and 6.5 mol dm-3). For example, at

6.5 mol dm-3 CaCl2 and 0.1 mol dm-3 HCl in aqueous solution, the distribution ratios

(D) of the test ions at their initial concentrations of 5, 15 and 30 ppm in aqueous

solutions into 2-propanol phase were 0.048, 0.049 and 0.049 for Mn(II), 12.81, 12.79

and 12.84 for Co(II), 0.026, 0.027 and 0.025 for Ni(II), 0.067, 0.065 and 0.066 for

Cu(II), respectively. The distribution ratios of the test ions were independent on the

initial concentrations of these test ions. The independence was also observed at

different concentrations of CaCl2. The metals of 100 ppm beyond the concentration

range of 5-30 ppm were also extracted into 2-propanol phase in the same manner.

85

2 2.5 3 3.5 4 4.50

20

40

60

80

100


Ext

ract

ion

perc

enta

ge(a)

3 4 5 6 70

20

40

60

80

100


Ext

ract

ion

perc

enta

ge

(b)

Figure 32. Effect of initial concentrations of sodium chloride (a) and calcium chloride

(b) on the extraction of Mn(II) ( ○), Co(II) (●), Ni(II) (■), Cu(II) (▲ ) from a 1:1 (v/v)

mixture of 2-propanol and aqueous solution containing the test ions and 0.1 mol dm-3

HCl at different concentrations of NaCl and CaCl2. The initial concentrations of test

ions are 5, 15 and 30 ppm.

86

6. 3. 2. Chemical Species of Cobalt(II) Chloro Complexes in the Aqueous and the

Organic Phases

Cobalt(II) was extracted to extent of 95.4 % into 2-propanol phase at the

concentration of 6.5 mol dm-3of CaCl2, but Mn(II), Ni(II) and Cu(II) were extracted

only about 6.7%, 3.9% and 7.8%, respectively. Further purification method of

cobalt(II) from Mn(II), Ni(II) and Cu(II) is described in section 6. 3. 4. In Table 6, the

separation factors (β = D1 / D2) for the extraction of Co(II) from other transition metal

ions and CaCl2 into 2-propanol after salting-out using CaCl2 of 6.5 mol dm-3 are shown,

where D1 and D2 denote the extraction constants of cobalt(II) and other metal ions,

respectively. The results can be explained by the formation of different charged species

of these test ions at high concentration of Cl- in the aqueous phase.

Table 6 Separation factors for the extraction of Co2+ from other transition metal ions

and CaCl2 into 2-propanol after salting-out using CaCl2 at 6.5 mol dm-3.

Separation factor (β =D1 / D2)a

β(Co-Mn) 2.6 x 102

β(Co-Ni) 4.8 x 102

β(Co-Cu) 1.9 x 102

β(Co-CaCl2) 66.6

a D1 and D2 denote the distribution ratios of cobalt(II) and other metal ions, respectively.

Figures 33 shows the absorption shift of cobalt(II) with changing CaCl2

concentrations in aqueous solution without 2-propanol. It is clear that with increasing

CaCl2 concentrations in aqueous solution, the color of the cobalt(II) changes from pink

to blue, indicating the change in cobalt(II) structure from octahedral species of

CoClx(H2O)6-x to the tetrahedral species of CoCl42- via the replacement of water

87

molecules bound to the Co(II) by chloride ion [24-27]. This can be explained by the

difference in atomic radius of oxygen (73pm) and ionic radius of Cl- (181pm). The

CoCl42- that has absorption band at 600-720 nm forms above 3.0 mol dm-3 CaCl2 and its

concentration increases with concentration of CaCl2 (Fig. 33).

400 500 600 700 8000

150

300

450

7654321

x 50 151413121110 9 8

x 1

ε

Wavelength / nm

Figure 33. Absorption spectra of cobalt(II) in aqueous solution without 2-propanol in

the presence of different concentrations of CaCl2 of (1) 0.0, (2) 0.1, (3) 0.5, (4) 1.0, (5)

1.5, (6) 2.0, (7) 2.5, (8) 3.0, (9) 3.5, (10) 4.0, (11) 4.5, (12) 5.0, (13) 5.5, (14) 6.0, (15)

6.5 mol dm-3. [Co2+]initial = 5.63x10-3 mol dm-3 for (1)-(11) and 1.12x10-3 mol dm-3 for

(12)-(15). [HCl]initial = 0.1 mol dm-3. ε = Absorbance / CCo. The absorption spectra of

(1)-(7) at 400-600 nm were shown by magnification of 50 times.

88

The absorption spectra of cobalt(II) chloro complexes in the aqueous and the organic

phases in the presence of various concentrations of CaCl2 are shown in Figs. 34 and 35.

The observed absorption spectra in the aqueous and the organic phases indicate clearly

the formation of tetrahedral cobalt(II) complexes (CoCl42-). For the aqueous phase, the

absorbance of CoCl42- increased with the concentrations of CaCl2 and then decreased

again at the concentrations of CaCl2 higher than 4.5 mol dm-3. For the organic phase,

the absorbance continued to increase with the concentration of CaCl2. This means that

CoCl42- formed in aqueous solution is transferred into organic phase by ion-pair

formation between CoCl42- and Ca2+.

A similar phenomenon has been observed in the extraction of Au(III) and Tl(III)

using sodium chloride, in which AuCl4- and TlCl4

- were extracted into 2-propanol with

Na+ as the counter ion [20,21].

However, the chemical species of Mn(II), Ni(II) and Co(II), are the mixtures of

many ionic species at high concentrations of Cl-: MnCl2, MnCl3-; NiCl3

- and NiCl42-;

CuCl+, CuCl2, CuCl3- and CuCl4

2- [28], which resulted in low extraction owing to their

strong hydration and a difficulty in charge neutralization by Ca2+. While CoCl42- is

tetrahedral and the coordination sites of Co(II) are fully occupied by chloride, so water

molecules do not bind to CoCl42- [29]. Thus, CoCl4

2- is extracted as the ion-pair

Ca2+[CoCl4]2- into the organic phase as compared to Mn(II), Ni(II) and Co(II), as shown

in Fig. 32b.

89

300 400 500 600 700 8000

40

80

120

ε

Wavelength / nm

(a)

12

43

300 400 500 600 700 8000

40

80

120

ε

Wavelength / nm

(b)

5678

Figure 34. Absorption spectra of cobalt(II) in the lower phase after the phase separation.

Arrows indicate the change in absorbance with increasing concentrations of CaCl2 in the

aqueous solutions: (1) 3.0, (2) 3.5, (3) 4.0, (4) 4.5, (5) 5.0, (6) 5.5, (7) 6.0, (8) 6.5 mol

dm-3. [Co2+]initial = 5.63x10-3 mol dm-3 for (a) and 1.12x10-3 mol dm-3 for (b).

90

300 400 500 600 700 8000

150

300

450

87654321

ε

Wavelength / nm

Figure 35. Absorption spectra of cobalt(II) in the organic phase. Arrows indicate the

change in absorbance with increasing the initial concentrations of CaCl2 in the aqueous

solution: (1) 3.0, (2) 3.5, (3) 4.0, (4) 4.5, (5) 5.0, (6) 5.5, (7) 6.0, (8) 6.5 mol dm-3.

[Co2+]initial = 5.63x10-3 mol dm-3 for (1)-(4) and 1.12x10-3 mol dm-3 for (5)-(8).

[HCl]initial = 0.1 mol dm-3. ε = Absorbance / CCo.

6. 3. 3. Effect of Water Concentration on the Extraction of Co(II)

The influence of water on the extraction of metal ions in the presence of NaCl and

CaCl2 is shown in Figs. 36a and b. It can be seen that the extraction of Co(II) increase

with increasing water concentration in the organic phase. Rapid increased distribution

of Co(II) at the concentration of CaCl2 higher than 4.5 mol dm-3 (9.17 mol dm-3 H2O)

was also observed in absorption spectra at 600-720 nm that correspond to the formation

of CoCl42- (see Fig. 34).

Water dissolved in the 2-propanol phase gives a large effect on the chemical

properties of the solvent. The high concentration of water in the organic phase

increases the polarity of 2-propanol [30] and can extracte ion-pairs which is followed by

dissociation in 2-propanol phase as observed in aqueous acetonitrile [31]. Furthermore,

91

water in the 2-propanol phase is to enhance the formation of solvent cluster of 2-

propanol that preferentially solvates to the ion-pairs [32]. These effects increased

extraction of metal ions with increasing concentration of water in the organic phase, as

shown in Fig. 6a. However, Fig. 6b shows that Co(II) is largely extracted at [H2O]Org >

9.17 mol dm-3 and the extraction of other metals was very small.

The different effects of NaCl and CaCl2 on the extraction of metal ions can be

explained by change in water in organic phases at different concentrations of NaCl or

CaCl2. The concentration of water in organic phase increased with concentration of

CaCl2 (Table 1), but decreased with that of NaCl [20,21]. That led to increased high

extraction of CoCl42- at high concentrations of CaCl2, but to decreased extraction of

AuCl4- or TlCl4

- at high concentration of NaCl [20,21]. Therefore, proper amount of

water in organic phase is an important causative factor for the extraction of charged

species with Ca2+ or Na+.

9 12 15 18 21 24

-1

-0.8

-0.6

-0.4

-0.2

[H 2O]Org / mol dm -3

logD

(a)

92

8 8.5 9 9.5 10-2

-1

0

1

2

[H2O] / mol dm -3

(b)lo

gD

Figure 36. Effects of water concentration in the 2-propanol separated from the mixture

of 2-propanol and aqueous solution containing NaCl (a) and CaCl2 (b), on the

extraction of Mn(II) (○), Co(II) (●), Ni(II) (■) and Cu(II) (▲) into 2-propanol. The

initial concentrations of metal ions are 5, 15 and 30 ppm.

6. 3. 4. Purification and Separation of Co(II) From Mn(II), Ni(II), Cu(II) and

Calcium Chloride in Organic Phase.

The organic phase still contains small amounts of Mn(II), Ni(II) and Cu(II) and

large amount of CaCl2. Therefore, cobalt(II) was separated from these metal ions by the

following method. The organic phase was transferred into a tube and followed by

addition of aqueous solution that containing CaCl2 (6.38 mol dm-3) and 0.1 HCl (0.1

mol dm-3) with volume ratio 6.00 : 3.86 of organic and aqueous solutions. By the

method, Mn(II), Ni(II) and Cu(II) remained in the organic phase transferred to the

aqueous phase. CaCl2 and water in organic phase were also removed as CaCl2. 2H2O

by the addition of excess anhydrous calcium chloride to the organic phase. Of course,

cobalt(II) concentration in the organic phase was not affected by the addition of excess

amount of CaCl2. By this procedure, the organic phase contains mainly Ca[CoCl4].

93

The calcium chloride precipitated was dried in an electric oven at 400oC and then it can

be reused for the extraction of cobalt from new samples using 2-propanol.

6. 3. 5. Mechanism of Extraction of Cobalt(II) by 2-Propanol

Based on the obtained results, the mechanism for the extraction of cobalt(II) is

suggested as Fig. 37, which shows an equilibrium scheme involving Ca2+, Cl- and

CoCl42. Cobalt(II) reacts with chloride ions to form CoCl4

2- at higher concentration of

CaCl2. Since the organic phase contains water, Ca2+ and Cl- , CoCl42- is extracted into

the organic phase with Ca2+ and partly ionize to CoCl42- and Ca2+ in the organic phase.

CaCl2 plays the following roles in the present system: (1) phase separation from the

mixed aqueous solution of 2-propanol, (2) the formation of CoCl42- in both aqueous and

organic phases and (3) charge-neutralization of CoCl42- with Ca2+, resulting in the

extraction of cobalt(II) into the organic phase.

][CoClCa 2-2+

4

- 2+

4

- 2+4

-2+

[CoCl Ca Cl

ClCa+ CoCl4 ClCoAqueous phase

Organic phase

] 2-

] [2-

+

Figure 37. Reaction scheme for the extraction of cobalt(II) in the presence of calcium

chloride

6. 4. Conclusions

Cobalt(II) was extracted into the separated 2-propanol phase as CoCl42- to extent of

95.4 % at the concentration of CaCl2 of 6.5 mol dm-3, but Mn(II), Ni(II) and Cu(II) only

about 6.7%, 3.9% and 7.8%, respectively. Mn(II), Ni(II) and Cu(II) involved in the

organic phase were stripped to the aqueous phase by using an aqueous solution

containing of CaCl2. CaCl2 in the organic phase also was removed as precipitation of

CaCl2 . 2H2O.

94

Reference

[1] J. Rydberg, C. Musikas, Choppin, G. R. (Eds.)., Principles and Practices of

Solvent Extraction, Marcel Dekker, New York, 1992.

[2] J. E. Brady, General Chemistry Principles and Structure, Jonh Wiley & Sons,

New York, 1990.

[3] J. S. Preston, A. C. D. Preez, Solvent Extraction and Ion Exchange

13 (1995) 465.

[4] K. Yoshizuka, Y. Sakomoto, Y. Baba, K. Inoue, Hydrometallugy

23 (1990) 309.

[5] N. B. Devi, K. C. Nathsarma, V. Chakravortty, Hydrometallurgy 49 (1998) 47.

[6] K. Sarangi, B. R. Ready, R. P. Das, Hydrometallurgy 52 (1999) 253.

[7] P. Zhang, T. Yokoyama, T. M. Suzuki, K. Inoue, Hydrometallurgy

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[8] J. Chen, K. C. Teo, Anal. Chim. Acta 434 (2001) 325.

[9] S. Akita, L. P. Castillo, S. Nii, K. Takahashi, H. Takeuchi, J. Membrane

Science 162 (1999) 111.

[10] N. V. Thakur, Hydrometallurgy 48 (1998)125.

[11] R. Grimm, Z. Kolarrik, J. Inorg. Nucl. Chem. 36 (1974) 189.

[12] J. E. Barnes, J. H. Setchfield, G. O. R. Williams, J. Inorg. Nucl. Chem.

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[13] S. Ide, M. Takagi, Anal. Sci. 6 (1990) 599.

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Acta 441(2001)157.

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Talanta 44 (1997) 2019.

[16] M. Merdivan, A. Gungor, S. Savasci, R. S. Aygun, Talanta 53(2000)141.

[17] B. Gupta, A. Deep, P. Malik, S. N. Tandon, Solvent Extraction and Ion

Exchange 20 (2002) 81.

[18] T. Sekine, I. Ninomiya, M. Tebakari, J. Noro Bull. Chem. Soc. Jpn.

70(1997)1385.

[19] J. W. Ray, C. E. Bracker, J. Crystal Growth 76 (1986) 562.

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[20] N. H. Chung, M. Tabata, Talanta 58 (2002) 927.

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[22] G. H.Jeffery, J. Bassett, J. Mendham, R. C. Denney, Vogel’s Textbook of

Quantitative Chemical Analysis, Longman, London, 1989.

[23] B. Tremillon, Reaction in Solution, Wiley, New York, 1993.

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[25] L. Menabue, G. C. Pellacani, J. Coord. Chem. 7(1977) 1.

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96

CHAPTER EIGHT DETERMINATION OF FORMATION CONSTANTS OF CHLORO

COMPLEXES OF COBALT(II) IN THE MIXTURE OF 2-PROPANOL

AND WATER BY SPECTROPHOTOMETRIC TITRATION

97

8. 1. Introduction

In our previous paper [1], we reported the quantitative extraction of cobalt(II) in the

presence of manganese(II), nickel(II) and copper(II) by salting-out method from 2-

propanol-water mixed solvents using CaCl2 as a salting out agent. We also pointed out

that the conversion of cationic Co2+ to anionic [CoCl4]2- in the salted-out aqueous phase

which contains large amount of CaCl2 is crucial for the extraction of cobalt(II).

The complexation of cobalt(II)-chloride system in aqueous solution have been

extensively studied [2-6]. The formation of halogeno complexes of cobalt(II) in various

solvents such as acetone [7], molten acetamide [8], nitromethane and nitrobenzene [9]

has been investigated. The color change of aqueous Co(II) solution from pink to blue

upon addition of Cl- is a well-known phenomenon and is ascribed to the conversion of

Co(H2O)62+ to CoCl4

2- [10-13]. However, no work has been studied on the formation of

CoCl42- in mixed aqueous solvents, especially in the mixture of 2-propanol and water.

The purpose of present study is to elucidate the extracted chemical species of Co(II) in

2-propanol-water mixed solvent containing CaCl2 and the extraction mechanism by the

determination of the formation constants of cobalt(II)-chloro complexes.

Spectrophotometric titration data was analyzed by non-linear regression analyses using

SPECFIT/32TM.

8. 2. Experimental

8. 2. 1. Apparatus

An auto spectrophotometric titration system consisting of an auto-titration unit

(ABP-118, Kyoto. Electronic, Japan) and a UV/VIS spectrophotometer (UV-2100,

Shimadzu, Japan) was used for the experiment. The system is completely controlled

and highly automatic by computer.

The simultaneous computation of the stability constants and the corresponding

characteristic spectra from multi-wavelength spectrophotometric measurements data

was carried out by using software SPECFIT/32TM. The essential details of the software

and the involved non-linear algorithms were given in the publications [14-17].

8. 2. 2. Reagent and Solutions

98

Hydrochloric acid, cobalt(II) oxide, calcium chloride, calcium nitrate and 2-propanol

(99.97%, Wako Pure Chemicals) were used without further purification.

The solutions of CaCl2 (titrant) were prepared in the same solvent compositions of 2-

propanol and water as those observed after the phase separation of 2-propanol-water by

salting-out. The cobalt(II) solutions (titrate) were prepared in the same way but

Ca(NO3)2 instead of CaCl2 was added to keep identical ionic strength of the titrate with

those of the titrants. The details of the solution compositions for the titrations are given

in Table 7. As the salting-out extraction of Co(II) involves a small amount of HCl in

the mixed solvents, HCl was also added to both the titrants and titrates in such a manner

that its concentration is the same as that of the salted-out mixtures.

Table 7. Compositions in mole fraction of titrant solutions for the titration. a

Aqueous phase Organic phase SolutionNo.b

2CaClx OH2x 2prx

SolutionNo.

2CaClx OH2x 2prx

S1 0.071 0.901 0.026 S1’ 0.045 0.431 0.522

S2 0.086 0.895 0.023 S2’ 0.046 0.441 0.514

S3 0.101 0.886 0.017 S3’ 0.048 0.444 0.508

S4 0.115 0.861 0.014 S4’ 0.051 0.447 0.500

S5 0.124 0.860 0.012 S5’ 0.054 0.451 0.494

a The titrates contain Ca(NO3)2 instead of CaCl2 . b Solutions Si and Si’ (i = 1 to 5) correspond to the aqueous and organic phases salted-

out from the mixtures of 1:1 (v/v) 2-propanol and water in the presence of different

initial concentrations of CaCl2 of 4.0, 4.5, 5.0, 5.5 and 6.0 mol dm-3, respectively.

8. 2. 3. Spectrophotometric Titrations Procedure

Titration was carried out in a water jacketed-titration vessel at a constant temperature

of 25.0±0.5 °C. The titrant solution (CaCl2 solution) was added to a cobalt(II) solution

through an auto-burette controlled by a computer. After each addition, the solution was

circulated to a flow cell by a peristaltic pump, where the absorption spectrum was

recorded.

99


8. 3. 1. Characteristics of the Cobalt(II) Chloro Complexes in Absorption Spectra.

Typical results of the spectrophotometric titration are shown in Fig. 38, where the

mole fraction of 2-propanol is 0.494. Before the titration the cobalt(II) is present as an

octahedral complexe of Co(H2O)62+ that has absorption maximum at 510 nm. Upon

addition of calcium chloride, the octahedral of Co(H2O)62+ gradually converts to the

anionic tetrahedral complex of [CoCl4]2-. The spectra observed at 600-720 nm indicate

clearly the formation of tetrahedral cobalt(II) complexes (CoCl42-) [10-13 ].

Similar result were observed at all the solution composition listed in Table 8,

indicating the formation of the tetrahedrally coordinated [CoCl4]2- in all these solutions

under the present experimental conditions.

400 500 600 700 8000

0.2

0.4

0.6

0.8

1

Wavelength / nm

Abs

orba

nce

Figure 38. Changes in absorption spectrum of cobalt(II) (6.976x10-3 mol dm-3) upon

titration of CaCl2 (1.165 mol dm-3) in 2-propanol-water mixtures of x2pr = 0.494 at 25oC.

The ionic strength was kept at I = 3.5. Concentration of HCl is 0.032 mol dm-3.

100

8. 3. 2. Determination of Stability Constant of CoCl42-

Figure 38 clearly shows that the conversion of [Co(H2O)6]2+ to [CoCl4]2- is

predominant and the formation of other complexes of CoClx(H2O)y is negligible. Thus,

the equilibrium is simplified as:

Co(H2O)62+ ( pink) + 4 Cl- ⇔ CoCl4

2- (blue) + 6H2O (26)

-24

4-262

62

24

4-262

62

24

-24

CoCl6

24-262

24

][Cl]O)[Co(H

O][H][CoCl

][Cl]O)[Co(H

O][H][CoClCoCl

O][H]][ClO)[Co(H

][CoCl βK

aa

aaK

γ+

−

=⋅γγ

γγ=

=

+

−

+

−

(27)

62

CoClCoCl ]OH[

-24

-24 γ=

K

Kβ (28)

where and ia iγ are the activity and activity coefficient of species i, respectively.

As the ionic strength has been kept in constant, the γK is constant during the titration

and the stability constant can be determined by eq.28. The value and the

corresponding electronic spectra of [Co(H

−24CoCl

β −24CoCl

β

2O)6]2+ and [CoCl4]2- were simultaneously

determined from the titration spectra using a software SPECFIT/32TM. The electronic

spectra of [Co(H2O)6]2+ and [CoCl4]2- are depicted in Fig. 39, and the β values were

summarized in Table 8. The formation constants of the tetrachlorocobalt(II) in 2-

propanol phase are higher by 8 to 10 orders than those in aqueous phase. Strong

solvation of water to cobalt(II) makes it difficult to form [CoCl4]2-. It is also attributed

to the relatively low dielectric constant of the 2-propanol (εT = 18.3) than in water

(εT=78.0) [18-21], as has been observed for copper(II) chloro complexes in 2-propanol

[22,23]. Moreover, in 2-propanol phase, the values decrease with decreasing x−24CoCl

β 2-

propanol and increasing xH2O (see Tables 7 and 8). Hence the contents of 2-propanol and

water in the solutions are the most important factor that affects the formation of

[CoCl4]2-, which is apparent from the definition of (eqs.1 and 2). On the other

hand, in aqueous phase where the 2-propanol mole fractions are small, the

−24CoCl

β

−24CoCl

β

101

values are apparently affected by the ionic strength of the solutions. As the ionic

strength increases from 10.6 to 17.6, the corresponding value increases from 10−24CoCl

β -

4.26 to 10-3.46, almost 10 times larger. This can be explained by the fact that the activity

of water molecules decreases with increasing ionic strength, resulting in a smaller γK

and consequently a larger formation constant, . −24CoCl

β

400 500 600 700 8000

100

200

300

400

Wavelength / nm

(2)

ε / (

M c

m-1

)

(1)

Figure 39. Calculated electronic spectra of cobalt(II) complexes in aqueous and 2-

propanol phase: (1), [Co(H2O)6]2+ and (2), [CoCl4]2-.

8. 3. 3. Mechanism of Extraction of Co(II) in the Mixture of 2-Propanol and Water

Table 8 showed the distribution fraction of [CoCl4]2- in the aqueous and 2-propanol

phases for a series of salting-out phase separations. Apparently, [CoCl4]2- forms

quantitatively in the 2-propanol phases, and the fraction of [CoCl4]2- increases with

increasing CaCl2 concentration in the aqueous phase. Such a tendency is in good

agreement with the cobalt(II) extraction results: the extraction efficiency increases with

increasing CaCl2 concentrations [1].

102

Table 8. Formation constants for the cobalt(II) chloro complexes in the mixture of 2-

propanol with water.

Aqueous phase Organic phase

[CaCl2]

mol dm-3−2

4CoCllog β a

(%)CoCl42- [CaCl2]

mol dm-3−2

4CoCllog β (%)CoCl4

2-

3.555 (I = 10.6) -4.26 ± 0.03 22.5 0.941(I = 2.8) 5.70 ± 0.06 100.0

4.276 (I = 12.8) -4.03 ± 0.07 46.2 0.943 (I = 2.8) 5.44 ± 0.03 100.0

4.916 (I = 14.7) -3.83 ± 0.04 67.3 1.013 (I = 3.0) 5.36 ± 0.06 100.0

5.444 (I = 16.3) -3.69 ± 0.03 79.3 1.090 (I = 3.3) 5.10 ± 0.04 100.0

5.892 (I = 17.6) -3.46 ± 0.01 90.1 1.165 (I = 3.5) 4.84 ± 0.05 100.0

a: see the definition of in eq.28 −24CoCl

β

The extraction of [CoCl4]2- from aqueous phase to 2-propanol phase can be written as

follows:

[Co(H O) ]262+

4Cl-+

[CoCl ]42-

2-4[CoCl ]

log β6H O2+ Ca

Ca2+

2+

KD2-Propanol phase

Aqueous phase

Thus the extraction constant KD is calculated as

)1]Cl[

1()1]Cl[

1(]CoCl[

]CoCl[

]CoCl[]O)Co(H[]CoCl[

4CoCl

4CoCl

aq24

org24

aq24aq

262

org24

aqCo(II),

orgCo(II),

-24

-24

+=

+=

+==

−−−

−

−+

−

β

K

β

CC

D

D (29)

where D is the extraction ratio and KD = [CoCl4]2-Org / [CoCl4]2-

Aq. Figure 40 depicted

the KD together with the extraction percentage as a function of initial concentration of

CaCl2. Surprisingly, although the extraction percentage of Co(II) increases with

103

increasing initial concentration of CaCl2, the KD decreases monotonously with the CaCl2

concentration. Looking at Table 6 one notices that as the initial concentration of CaCl2

increases the mole fraction of water increases in the salted-out organic phases while the

mole fraction of 2-propanol decreases in the aqueous phases. It is thus clear that the

polarity of the salted-out organic phases increases with increasing CaCl2 initial

concentration. The decrease in KD obtained at higher initial concentration of CaCl2 is

then ascribed to the increased water content in the salted-out organic phase. The

decreases in KD at higher initial concentration of CaCl2 are, however, compensated by

the large increases in the distribution fraction of [CoCl4]2- (as a result of increased

and [Cl−24CoCl

β -]) in the aqueous phase. Thus, the overall extraction efficiency increased

with increasing the initial concentration of CaCl2, as we already reported.

igure 40. The extraction percentage (%) and the extraction constant (KD) of Co(II) as a

3.5 4 4.5 5 5.5 6 6.50

20

40

60

80

100

1.2

1.5

1.8

2.1

2.4

[CaCl 2]initial/mol dm -3

%

%

KD

logK

D

F

function of the initial concentration of CaCl2 for the salting-out extraction using 2-

propanol and water mixed solvents.

104

Reference

[1] N. H. Chung, M. Tabata, Hydrometallurgy 73 (2004) 81-89.

[2] L.G. Sillén, A.E. Martell, Stability Constants of Metal-ion Complexes, The


[3] L. I. Katzin, E. Gebert, J. Am. Chem. Soc. 72 (1950) 5464.

[4] L. I. Katzin, J. Am. Chem. Soc. 76 (1954) 3089.

[5] F. A. Cotton, D. M. L. Goodgame, M. Goodgame, J. Am. Chem. Soc.

83 (1961) 4690.

[6] A. H. Zeltmann, N. A. Tiatwiyoff, L. O. Morgan, J. Phys. Chem. 72 (1968) 121.

[7] A. Dwight, J. Fine, Am. Chem. Soc. 84 (1962) 1139.

[8] J. Savovic, R. Nikolic, D. H. Kerridge, Fluid Phase Equilibria 118 (1996) 143.

[9] M. A. Khan, G. M. Bouet, Transition Met. Chem. 22 (1997) 604.

[10] L. G. Spear, G. S. Larry, J. Chem. Educ. 61 (1984) 252.

[11] L. J. A. Martins, J. B. D. Costa, J. Chem. Educ. 63 (1986) 989.

[12] S. F. A. Kettle, Physical Inorganic Chemistry, Oxford University Press, New

York, 1998.

[13] F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New

York, 1976.

[14] H. Gampp, M. Maeder, J. C. Meyer, A. D. Zuberbühler, Talanta 32 (1985) 95.

[15] H. Gampp, M. Maeder, C. J. Meyer, A. D. Zuberbühler, Talanta 32 (1985) 251.



[18] M. A. Khan, J. Meullemeestre, M. J. Schwing, F. Vierling, Inorg. Chem.

28 (1989) 3306.

[19] M. A. Khan, S. K. Ali, G. M. Bouet, Inorganic Biochemistry 90 (2002) 67.

[20] K. Sawada, T. Onoda, T. Suzuki, Inorg. Nucl. Chem. 43 (1981) 3263.

[21] S. Lechat, M. A. khan, G. Bouet, Inor. Chim. Acta. 211 (1993) 33.

[22] E. Bentouhami, M. A. Khan, J. Meullemeestre, F. Vierling, Polyhedron

11(1992) 2179.

[23] S. Dali, F. Benghanem, M. A. Khan, Polyhedron 10 (1991) 2529.

105

CHAPTER NINE GENERAL CONCLUTIONS

106

We conclude that this research has achieved advanced applications in solvent

extraction field. A new separation technology has been developed for separation metal

by different charge species. The phase separation occurs by the addition of NaCl or

CaCl2 to a mixture of 2-propanol and water could be quantitatively extracted tetrachloro

metals into 2-propanol phase without using any extracting reagent. It is simple method

and not toxic compared to traditional extraction technique using conventional solvent

with various extracting reagents. Thus, the conclusions from this thesis are as follows:

1. The result indicates that phase separation occurs at 0.1-0.4 in mole fraction of 2-

propanol and concentrations range of NaCl from 2.0-4.0 mol dm-3. Compared to the

phase diagram of acetonitrile, the phase separation occurs in a small concentration range

of NaCl and 2-propanol. This suggests strong interaction of 2-propanol with water

compared to acetonitrile. The water content of 2-propanol is higher (0.45 in mole

fraction) than that of acetonitrile (0.19 in mole fraction). The high concentrations of

water in the organic phase increase the polarity of the 2-propanol phase. The extracted

ion-pair complexes can dissociate into ions in the mixture due to the effect of water

involved in the 2-propanol phase with increasing water content. Thus, 2-propanol is

suitable for the extraction of ionic species by salting-out.

The interesting points are that under these experimental conditions gold is the only

metal that can be extracted and selectively separated from several precious metals in an

aqueous NaCl solution below 4 mol dm-3 without any reagents extracting. Au(III) is

quantitatively extracted into the organic phase by salting-out phase separation.

2. We have utilized the phase separation processes for the quantitatively and selectively

extracted of thallium(III) in the presence of other trivalent metals ions such as gallium,

indium, bismuth and antimony into 2-propanol phase without extracting reagents. The

extraction efficiencies of gallium(III), indium(III), bismuth(III) and antimony(III) were

much lower than that of thallium(III). Thus a maximal selective separation of

thallium(III) from these metals could be attained using the mixture of 2-propanol with

water.

107

3. The mixed solvent of 2-propanol and water by addition of CaCl2 indicate that phase

separation occurs at 0.1－0.5 mole fractions of 2-propanol and 0.03 – 0.12 mole

fractions of CaCl2. Compared to the phase separation of the aqueous mixture with 2-

propanol by addition of NaCl (2.0- 4.0 mol dm-3) the phase separation occurs at higher

concentration range of CaCl2 (2.5 - 6.5 mol dm-3). Cobalt(II) was extracted into the

separated 2-propanol phase as CoCl42- to extent of 95.4 % at the concentration of CaCl2

of 6.5 mol dm-3, but Mn(II), Ni(II) and Cu(II) only about 6.7%, 3.9% and 7.8%,

respectively. Mn(II), Ni(II) and Cu(II) involved in the organic phase were stripped to

the aqueous phase by using an aqueous solution containing of CaCl2. CaCl2 in the

organic phase was also removed by precipitation as CaCl2 . 2H2O.

108

List of Publications

1. Nguyen Huu Chung and Masaaki Tabata, Salting-out phase separation of the

mixture of 2-propanol and water for selective extraction of cobalt(II) in the

presence of manganese(II), nickel(II), and copper(II), Hydrometallurgy 73

( 2004) 81-89.

2. Nguyen Huu Chung, Jun Nishimoto, Osamu Kato and Masaaki Tabata, Selective

extraction of thallium(III) in the presence of gallium(III), indium(III),

bismuth(III) and antimony(III) by salting-out of an aqueous mixture of 2-

propanol, Analytica Chimica Acta 477 ( 2003) 243-249.

3. Nguyen Huu Chung and Masaaki Tabata , Selective extraction of gold(III) in the

presence of Pd(II) and Pt(IV) by salting-out of the mixture of 2-propanol and

water, Talanta 58(2002) 927-933.

4. Ying Guang Wu, Masaaki Tabata, Toshiyuki Takamuku, Atsushi Yamaguchi,

Tomomi Kawaguchi and Nguyen Huu Chung , An extended Johnson–Furter

equation to salting-out phase separation of aqueous solution of water-miscible

organic solvents, Fluid Phase Equilibria 192 (2001) 1-12.

109

Presentation Conferences

1. Nguyen Huu Chung and Masaaki Tabata, Simple and selective extraction of

metal chloro complexes from homogeneous mixed solvents by addition of NaCl,

The 7th Asian Conference on Analytical Science, Hong Kong, July 28-31, 2004,

page 431.

2. Nguyen Huu Chung and Masaaki Tabata, Extraction of metal ions using the

mixture of 2-propanol with water, 10th Asian Chemical Congress 10 ACC, 8th

Eurasia Conference on Chemical Sciences EuAsC2S-8, Hanoi Viet Nam,

October 21-24, 2003, page 21.

3. Nguyen Huu Chung and Masaaki Tabata, Homogeneous liquid-liquid extraction

of metal ions using the mixtures of 2-propanol with water, Nihon Bunseki

Kagakukai Dai 50 nenkai, Kumamoto University, Japan, November 23-25,

2001, page 420.

4. Nguyen Huu Chung and Masaaki Tabata, Extraction of transition metals using

2-propanol-water mixtures by salting-out technique, Nihon Bunseki Kagakukai

Dai 49 nenkai, Okayama University, Japan, September 26-28, 2000 page 303.

110

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STUDIES ON THE HOMOGENEOUS LIQUID-LIQUID EXTRACTION … · A schematic representation of solvent extraction. Liquid-liquid distribution plots. The distribution ratio D for two different