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
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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
4. 1. Introduction 45
4. 2. Experimental 46
4. 3. Results and Discussion 49
4. 4. Conclusions 56
References 57
(To be continued)
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v
Chapter 5. Selective Extraction of Thallium(III) in the Presence of 59
Gallium(III), Indium(III), Bismuth(III) and Antimony(III)
5. 1. Introduction 60
5. 2. Experimental 61
5. 3. Results and Discussion 62
5. 4. Conclusions 70
References 71
Chapter 6. Phase Separation Occurs by the Addition of CaCl2 72
to a Mixture of 2-propanol and Water
6. 1. Introduction 73
6. 2. Experimental 73
6. 3. Results and Discussion 74
6. 4. Conclusions 78
References 80
Chapter 7. Selective Extraction of Cobalt(II) in the Presence of 81 Manganese(II), Nickel(II) and Copper(II)
6. 1. Introduction 82
6. 2. Experimental 83
6. 3. Results and Discussion 84
6. 4. Conclusions 94
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
7. 1. Introduction 98
7. 2. Experimental 98
7. 3. Results and Discussion 100
References 105
Chapter 9. General Conclusions 106
List of Publications 109
Presentation Conferences 110
List of Figures
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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.
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19
18
34
36
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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.
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67
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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.
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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
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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
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62
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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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[26] M. I. Martín, F. J. Alguacil, Hydrometallurgy 48 (1998) 309.
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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
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[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.
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Bergstrasse, 1955.
[6] R. E. Treybal, Liquid Extraction, McGraw-Hill, New York, 1963.
[7] Y. Marcus, J. Solution Chem. 21 (1992) 1217.
[8] Y. Marcus, J. Solution Chem. 20 (1991) 929.
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[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.
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Mallard, J. Phys. Chem. Ref. Data 17, Suppl. 1 (1988).
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New York, 4th ed. 1986.
[16] Y. Marcus, J. Solution Chem. 13 (1984) 599.
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[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.
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29
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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
[NaCl] initial / mol dm -3
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
[NaCl] initial / mol dm -3
logD
(Cl )-
2 2.5 3 3.5 4 4.5
-0.8
-0.6
-0.4
-0.2
[NaCl] initial / mol dm -3
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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[14] G. Kiss, Z. Varga-Puchony, J. Hlavay, J. Chromatogr. A, 725 (1996) 261.
Press, New York, 1978.
[16] M. Tabata, M. Kumamoto, J. Nishimoto, Analytical Science 10 (1994) 383.
[17] P. Debye, J. Chem. Phys. 31 (1959) 680.
[18] R. Aveyard and R. Hedelden, J. Chem. Soc. Faraday Trans. 1, 71(1975) 312.
[19] R. A. Pierotti, Chem. Rev. 76 (1976) 717.
[20] K. Yoshida, T. Yamaguchi, Z. Naturforsch. 56a (2001) 529.
[21] M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Sci. 10 (1994) 383.
[22] Y. Nagao, Anal. Chim. Acta 120 (1980) 279.
[23] T. Fujinaga, Y. Nagao, Bull. Chem. Soc. Jpn. 53 (1980) 416.
[24] M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Chem. 68 (1996) 758.
[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
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).
4. 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.
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. Results and Discussion
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
[NaCl] initial / mol dm -3
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.
[14] M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Sci. 10 (1994) 383.
[15] M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Chem. 68 (1996) 758.
[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. Results and Discussion
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
[NaCl] initial / mol dm -3
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
[NaCl] initial / mol dm -3
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
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 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
2-Propanol (Wako Pure Chemicals) was purified by drying over 4 Å molecular
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. Results and Discussion
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
[CaCl 2]initial / mol dm -3
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
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[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.
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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
[NaCl] initial / mol dm -3
Ext
ract
ion
perc
enta
ge(a)
3 4 5 6 70
20
40
60
80
100
[CaCl 2]initial / mol dm -3
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
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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. Results and Discussion
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.
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Chemical Society, Burlingson House, London, 1964.
[3] L. I. Katzin, E. Gebert, J. Am. Chem. Soc. 72 (1950) 5464.
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83 (1961) 4690.
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[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.
[16] H. Gampp, M. Maeder, C. J. Meyer, A. D. Zuberbühler, Talanta 32 (1985) 1133.
[17] H. Gampp, M. Maeder, C. J. Meyer, A. D. Zuberbühler, Talanta 33 (1986) 943.
[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.
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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