The extraction of trace amounts of gold from different aqueous mineral acid solutions by diphenyl-2-pyridylmethane dissolved in chloroform

Journal of Radioanalytical Chemistry, Vol. 42 (1978) 335-348

THE EXTRACTION OF TRACE AMOUNTS OF GOLD FROM DIFFERENT AQUEOUS MINERAL ACID

SOLUTIONS BY DIPHENYL-2-PYRIDYLMETHANE DISSOLVED IN CHLOROFORM

DETERMINATION OF GOLD BY NEUTRON ACTIVATION ANALYSIS

M. IQBAL, M. EJAZ*, SHAMIM A. CHAUDHRI, ZAMIRUDDIN

Nuclear Chemistry Division, PaMstan Institute of Nuclear Science and Technology, Nilore, Rawalpindi (Pakistan)

(Received April 9, 1977)

Diphenyl-2-pyridylmethane, a high molecular weight substituted pyridine has been

examined and found to be a useful solvent extraction reagent. Its behaviour is similar to amines in that it forms salts with mineral acids. The acid ionization constant (PKBH § determined spectrophotometrically has a value of 4.41 _+ 0.06 at 25 ~ A study of the

partition behaviour of trace amounts of gold between mineral acid solutions and 0.1M

diphenyl-2-pyridylrnethane dissolved in chloroform indicates that the metal can be quantitatively extracted from dilute mineral acid solutions and also from concentrated hydrochloric acid media in a single extraction. Attempts have been made to gain an understanding of factors affecting the extraction of gold. Common anions have little effect

on extraction in concentrations upto 1M. Separation factors of a number of metal ions relative to gold are reported for three mineral acid systems; and gold has been estimated in

some synthetic samples using neutron activation technique by prior extraction with 0.1M solution of diphenyl-2-pyridylmethane dissolved in chloroform.

Introduction

The high molecular weight substituted pyridines, 4- (5 -nonyl)pyridine (NPy) and 2-hexylpyridine (HPy) have been evaluated for use in the extraction and possible separation of metal ions. 1 In continuation of the study of the potential of this new class of solvent extraction reagents, the present paper describes the applicabil!ty of another high molecular weight substituted pyridine, diphenyl-2-pyridylmethane (DPPM) as an extractant. Preliminary investigations have shown that DPPM is a good extractant for metal ions which form complex metal acids in mineral acid so-

*Author to whom correspondence should be addres~d.

J. Radioanal. Chem. 42 (1978) 335 3

M. IQBAL et al.: THE EXTRACTION OF TRACE AMOUNTS

lutions. As a representative example of such metals, gold has now been studied by DPPM. Partition equilibria of gold between aqueous mineral acid solutions and NPy and I-IPy, respectively, have already been investigated. 1'2 Date on the extraction of gold with DPPM are interesting for comparison of the results with those obtained for long chain alkyl-substituted pyridines i.e. NPy and HPy. They also show potential for analytical extraction and/or preconcentration in conjunction with modern analytical techniques because the low concentrations of this metal ion are generally difficult to analyse directly due to the sensitivity limitations of the existing instru- mental methods.

Experimental

Reagents, tracers and equipment

DPPM was obtained from Aldrich Chemical Co. Inc. Milwaukee, Wis, USA. It has a melting point of 59-61 ~ Solutions of DPPM in chloroform were made before use. Mineral acid solutions were generally prepared from BDH volumetric solution ampoules or were of Merck Pro analysi grade. Their solutions were standard- ized against solutions of sodium hydroxide of appropriate concentrations. Standard- ization of the alkali solutions was done against N/5 potassium hydrogen phthalate solution prepared from the Primary standard grade sample of the salt after drying it at 110 ~ for 3 hrs. All other chemicals used were of analytical reagent grades.

The radio-indicator for gold was 198§ (T = 64.8 h) which was obtained by neutron activation of spec-pure gold foil (> 99.9% purity) in the irradiation facil- ities of the swimming pool nuclear reactor PARR-1 of this Institute. It was dissolved in aqua regia, boiling the solution down to incipient dryness and then tak- ing up the residue in different mineral acid solutions by repeated evaporation with the corresponding acids. All other tracers used in this study were either obtained from the radiochemical Centre, Amersham, England or were prepared locally by (n, 7) reaction or by separation of the daughter nuclide from the parent without a carrier.

The equipment used for the radiochemical assay has been described in the pre- vious report.'

Procedure

Measurements o f the distribution coefficients. Distribution of gold (~< 10-TM) or the test element between aqueous and organic phase was followed radiometric- ally. The organic and aqueous solutions (usually 1 ml) were shaken in stoppered vials using a mechanical shaker. All systems were shaken for 5 min, although the

336 Z Radioanat. Chem. 42 (19.78)


time necessary to reach equilibrium was 2 rain. After separation of the layers, 500/al of each phase were taken for radiochemicat analysis. Distribution ratios were calculated for counts/100 sec of both phases. Experiments were designed to give at least 10 000 counts so that the standard deviation did not exceed 1%.

Extraction o f mineral acids. The procedure used for the extraction of mineral acids in this study was similar to the one described earlier. 3 The data reported are averages of duplicate runs except for the concentrated hydrochloric acid media where the data are averages of five to six readings.

pH measurements and spectral study. Measurements of the pH values for the determination of pKBH + by spectrophotometry were carried out on a Fischer Accumet model 320 pH meter, equipped with glass electrode and a calomel reference electrode (Fisher). The pH meter was calibrated using a Beckman buffer solution (pH 6.5 at 25 ~ and 0.05M potassium hydrogen phthalate (pH 4.00 at 25 ~ A Cary Model 15 recording spectrophotometer was used to obtain the spectra. Optical density measurements on the solutions at the analytical wavelength were, however, made manually using" Beckman DU-2 spectrophotometer for better con- trol and accuracy.

Results and discussion

Preliminary experiments were carried out to find the affmity of the compound for the mineral acids. The dependence on the acidity of the aqueous phase of the degree of extraction of HC104, HNO3, HC1 and H2 SO4 by 0.1M chloroform solution of DPPM is presented in Fig. 1. The order of acid extraction is:

HC104 > HNO3 > HC1 > H2 SO,

and mirrors the increasing importance of anion hydration in the aqueous phase, i.e. HSO4 > C1- > NO~ > C10;, which hinders extraction. The uptake of hydrochloric acid at 10M is higher than that of nitric acid which is very unusual and is not dearly understood. The reagent appears a considerably weaker base than aliphatic amines and exists completely in the salt form when the concentrations of the original aqueous acid solutions exceed IM. At high acid concentrations the acid to pyridine ratio in the organic phase exceeds even two. This addi- tional excess of acids can be attributed to simple distribution of molecular, perhaps hydrated acids without formation of any specific pyridine-acid complex richer in acids than DPPM (HX)2, and to the formation of a series of specific

J. Radioanal. Chem. 42 (1978) 337 3*


l

10-1

10-2

10-"

10 10 -2 10 -1 1 10

[Acid] initial 7M

Fig. 1. Concentration of HC104, HNO3, HCI, H2SO 4 in the organic phase (0.1M DPPM/chloroform) as a function of acid concentration in the aqueous phase. Curves: 1 - HC104, 2 - HNO 3, 3 - HC1, 4 - H 2 S O 4

pyridine acid adducts DPPM HX(HX)M similar to other amines, 4-6 The general

extract ion reaction of a mono-basic aqueous acid, HX, can be described by the equilibrium:

DPPMorg + HXaq ~ DPPM H +X~rg (1)

and the dissociation reaction o f the pro tona ted pyridine in aqueous solutions is

represented by the equilibrium:

DPPMH + ~ H + + DPPM

The ionization constant can be described as

KBW = [H +] [DPPM]

[DPPMH +]

(2)

(3)

338 .I. Radioanal. Chem. 42 (1978)


Table 1 A typical set of the experimental data for determination of pK of DPPM

pH of the solution

3.9

4.1

4.3

4.5

4.7

4.9

5.1

d , c l n -1

0.698

0.670

0.650

0.610

0.582

0.553

0.539

d - d M, cm- '

0.219

0.191

0.171

0.131

0.103

0.074

0.060

d i - d , cm- '

0.057

0.083

0.105

0.145

0.173

0.202

0.216

d - d M lg - - pK

d i - d

+0.585 4.49

+0.352 4.45

+0.212 4.51

-0 .044 4.46

-0 .225 4.48

-0 .436 4.46

--0.556 4.54

Concentration of DPPM = 2 �9 l O - 4 M .

OD in 0.IM HC1 = d I = 0.755 cm -~ at 262 nm. OD in 1.OM KOH = d M = 0.479 crn -~.

Average PKa M = 4.48 -+ 0.06.

Ionic activity correction for ionic strength I of 2.7 �9 10-2M using formula 7

0.505 x/T = - 0 0 7

1+1.6

pK T (DPPMH) + = 4.41 -+ 0.06.

I f i t is w r i t t e n in the fami l ia r l oga r i t hmic form, we get:

[DPPMH +] pKBH+ = p H + lg (4)

[DPPM]

pKBH + values were d e t e r m i n e d b y s p e c t r o p h o t o m e t r y f r o m the s t u d y o f the spec-

t ra a t var ious accu ra t e ly k n o w n pHs, the r a t io [DPPMH § / [ D P P M I in the above

e q u a t i o n can be d e t e r m i n e d b y m e a s u r i n g the e x t i n c t i o n coe f f i c i en t o f the an ion ,

o f t he mo lecu l e and o f the m i x t u r e o f the a n i o n and the mo lecu l e a t the analy t -

ical wavelength . Eq. (4) can be w r i t t e n as: 7

d - d M p K a = p H + lg - - (5)

di - d

whe re d[ - t he op t ica l dens i t y ( O D ) o f the ionic f o r m o f the py r id ine a t the ana-

ly t ical wave leng th ,

dM -- the OD o f t he m o l e c u l a r fo rm.

J. Radioanal. Chem. 42 (1978) 339


09 ~ ~

,d i

I 07!-

i I J

06 i I

osH

j 2

0 4

0 .3

L I I L _t 0.1 230 250 270 290 310 330 350

Wavelenght 7 nm

Fig. 2. Absorption spectra of 2.10-4M DPPM in; 1 - 0.1M HC1, 2 - 1M KOH, 3 - 0.1M CH3COOH at pH 4.6

The experimental data for a typical set are given in Table 1 and the absorption spectra are depicted in Fig. 2.

The dependence of the extraction coefficient of gold on the acidity of the aque-

ous phase over a wide range of mineral acid concentration by 0.IM DPPM/chloroform is shown in Fig. 3. The concentration of mineral acids has a significant in-

fluence on the equilibrium of the reaction of gold with DPPM. In the hydrochloric

acid system, the extractability of gold goes through a minimum in the HC1 con-

centration range of 2-6M; extraction coefficients of 580 and 620 were found for

extraction from 0.25M and 10M HCI solutions, respectively. In dilute acid solu-

tions (0.25M) gold species o f the type AuCI4 may be extracted either through ion-association with amine cations or as complex metal acid, HAuC14, salted out

by the supporting acid. In concentrated acid solutions gold complexes of the type

H2AuCls and HaAuC16 [acid combined species HAuCI4 �9 HC1 and HAuC14 �9 (HC1)2 ]

are perhaps salted out by the supporting acid. From the data obtained for the ex-

340 J. Radioanal. Chem. 42 {19781

M. IQBAL et al.: THE EXTRACrlON OF TRACE AMOUNTS

r'

-2 -I I

Ig [Acid] ~ M

Fig. 3. Extraction of gold from mineral acid solutions by 0.1M DPPM. Curves: 1 - HC1. 2 - HNO3, 3 - H2SO *

traction from nitric and sulphuric acid systems, it is seen that the distribution coef-

ficient values reach a broad maximum at 0.1-1M acid concentration. The decrease

in the extraction of gold above 7M nitric and sulphuric acid systems can be attri-

buted to excess acid extraction into the organic phase or to the formation of in-

extractable complexes in the aqueous phase. The present data are similar to those reported for NPy and HPy systems. The D values are comparable to NPy and are almost one order of magnitude greater than those of the HPy systems although

the pKm~. value of this compound is lower than those of alkyl-substituted pyridines. This is perhaps due to the intermolecular interactions of the extracted gold-

DPPM complex with chloroform diluent being much more stronger than those of

alkyl substituted pyridines with benzene which was used as diluent in those systems. As compared to NPy, this compound extracts gold much more efficiently

in concentrated hydrochloric acid media with an abrupt increase beyond 5M HCI;

and is almost similar to the simple HC1 extraction. This is not dearly understood and could be attributed to structural difference.

The data on the extraction of gold from 0.25M nitric hydrochloric and sulphuric acid solutions b y 0.1M' DPPM/chloroform were obtained at various concen-

trations of the pyridine. The lg-lg plots of the extraction coefficient vs, lg [DPPM]


M. IQBAL et al.: THE EXTRACTION OF "fRACE AMOUNTS

- ~tg[DPP.], i

3 E3

- ?

Fig. 4. Influence of the concentration of DPPM/chloroform on the extraction coefficient of gold. Curves: 1 - 0.25M HC1, 2 - 0,25M HNO 3, 3 - 0.25M H~SO4

a

2 -2 -1 Ig [Anions] ~ M

Fig. 5. Effect of salting-out agents on the distribution coefficient of gold by 0.1M DPPM/chloroform from constant acidity solutions. Curves: 1 - chloride (0.25M HCI), 2 - nitrate

(0.25M HNO3), 3 - sulphate (0.25M H2SO,)

for the three acid media are plotted in Fig. 4. The slope of the plots at relatively higher concentration (0.05-0.1M) of the extractant is close to one, while at lower pyridine concentrations the slope is close to two. Gold complexes o f the type

AuCI~, Au(NO3)4 and Au(SO4)~ are perhaps extracted through the association o f

anionic complexes with the amine cations or most probably salted out as complex

metal acids. The higher slopes at low reagent concentrat ion can be explained as

342 J. Radioanal. Chem. 42 (1978)

M. IQBAL et al.: THE EXTRACI'ION OF TRACE AMOUNTS

Ig [Anions] ~ N

Fig. 6. Effect of various anions on the extraction of gold from 0.25M HC1 by 0.1M DPPM/ chloroform. Curves: 1 - acetate, 2 - ascorbate, 3 - citrate, 4 - oxalate, 5 - thiocyanate

due to the formation of mixcJ quadrupoles formed by the association of (DPPMH)+X -

with (DPPM)§ or as having two DPPMH § cations hydrogen honded to one chloride or nitrate anion.

The variation of the partition coefficient, DAu, with sodium chloride, nitrate

and sulphate ions added to the corresponding constant acidity solutions is shown

in Fig. 5. There is a decrease in the D value with the increase in concentration

of these salts in the original aqueous solutions but the percentage extraction

remains nearly constant and the extraction may be regarded as independent of the amount of the salts present.

The effects of the addition of acetate, ascorbate, citrate, oxalate and thiocya-

nate ions were investigated from constant acidity solutions of hydrochloric, nitric

and sulphuric acids. The results are presented in Figs 6, 7 and 8, respectively. In

the case of hydrochloric acid system the addition of acetate ions to 0.25M HC1

leads to a steady decrease in the partition coefficient of gold, with its increase in

the aqueous phase. The addition of ascorbate, oxalate and thiocyanate ions leads

to a decrease in the D value but the extraction still remains quantitative. In the

case of ascorbate and citrate ions, loss of activity was observed beyond 0.2M and 0.05M concentrations of these ions in the aqueous phase; respectively.



In nitric acid media, the addition of acetate ions decreases the D value similarly

to the HC1 system. In the case of thiocyanate, the extraction remains more or less

the same. Oxalate ions have no effect upto 0.1M concentration beyond which loss

of activity was observed. In the case of ascorbate and citrate ions loss of activity

was noted beyond 0.05M concentration of these ions in the aqueous phase.

a

4

lg [Anions]~bl J Y

Fig. 7. Effect of various anions on the extraction of gold from 0.25M HNO a by 0.1M DPPM/ chloroform. Curves: 1 - acetate, 2 - ascorbate, 3 - citrate, 4 - oxalate, 5 - thio- eyanate

In the sulphuric acid system, the effect of the addition of acetate ions is similar

to those observed in the case of HC1 and HNO3 systems. Loss of activity was ob-

served in the presence of ascorbate ions as in the case of other two acids. Thio-

cyanate ions have little effect on extraction. Unlike the other two mineral acid

systems, no loss of activity was observed in the case of citrate ions. The loss of

activity that occurs in these systems with the addition of ascorbate and oxalate

ions is probably due to the formation and mutual immiscibility of different gold

species. In the oxalate system, AuCI~ has been reported a to undergo reduction

through the formation of AuC13(C204H)- and AUC13(C204) -2 which undergo ring

closure followed by a rapid electron transfer process during which AuCI~, Au(O)

and CO2 are produced. The study of the effect of these anions on the extraction

of gold from the three systems indicates that none of these anions can be used

344 J. Radioanal. Chem. 42 (1978)

M. IQBAL et al,: THE EXTRACTION OF TRACE AMOUNTS

for the back extraction of gold from the organic phase. However, diluted ammonia (8% solution) was found suitable for the back-extraction of the metal.

The selectivity of the extraction separation of gold with 0.1M DPPM/chloroform was investigated from 0.25M hydrochloric, nitric and sulphuric acid solutions and

\

"~o x3 2

- 2 -1

Ig [Anions~ ~M

Fig. 8.Effect of various anions on the extraction of gold from 0.25M H2SO 4 by 0.1M DPPM[ chloroform. Curves: 1 - acetate, 2 - ascorbate, 3 - citrate, 4 - oxalate, 5 - thiocyanate

also from 1M HCI. The behaviour of a number of metal ions including, copper,

zinc and iron was studied. The data are presented in Table 2. It is seen that the separation factor of gold with respect to iron(Ill) is fairly good even in 1M hydrochloric acid media. In all the commonly used solvating reagents and liquid anion exchangers, iron(III) is coextracted at this acidity. Although, 2-hexylpyridine also extracts gold very selectively at this acidity, 1 but this reagent is superior to 2-hexylpyridine in that this compound does not have the pungent odor like that Of 2-hexyl-

pyridine. In addition, the organic layer in this system is the lower phase and repeated extractions if desired can be carried out in the same separatory funnel. Utilizing this data separation of gold from a mixture of different metal ions (in


M. I Q B A L et al . : T H E E X T R A C T I O N O F T R A C E A M O U N T S

T a b l e 2

S e p a r a t i o n f a c t o r s o f d i f f e r e n t m e t a l s i ons re la t ive

t o g o l d in 0 .1M D P P M / c h l o r o f o r m - m i n e r a l a c id e x t r a c t i o n s y s t e m s

Meta l

i on

UO 2+ C r 6+

Cr 6+

Th 4+

F e 3+

e r a+

Ce 3+

W3+

L a 3*

Ba2+

Sr 2+

M n 2+

Co 2+

C u z+

Z n 2+

Cs +

C o n c e n t r a t i o n ,

m / l

1 0 -3

10 -~

5 " 102

0 . 2 5 M HCI

C . F .

1 0 - 7

l O s

10-8

C . F .

C . F .

1 0 - 8

1 0 - 9

1 0 - 8

1 0 - 8

l O - S

1 0 - 8

1 0 - 8

10 6

3 " 10 3

7 .5 " 10 ~

S e p a r a t i o n f a c t o r s > D A u / D M

10 7

1 0 6

10 6

1 0 - 7

10 7

10 7

10 6

10 6

10 6

10 6

lO s

lO s

1 0 6

I M H C 1

�9 1 0 2

8 .3

10 6

l O s

10 5

10 6

10 6

10 6

10 6

10,~

l O s

lO s

lO s

10 2

lO s

0 . 2 5 M H N O 3

10 6

1.3 �9 1 0 "

1 . 5 ' 1 0 3

10 6

10 6

10 6

1 0 6

10 6

10 6

10 s

10 s

10 6

1 0 6

10 6

1 0 7

10 6

0.25M H2SO 4

10"

2 . 5 . 108

106

106

107

107

10 7

1 0 6

106

106

105

106

the form of chloride) was carried out for subsequent determination by neutron activation analysis by the following procedure.

The mixture was dissolved in aqua regia, boiling the solution down to incipient dryness and then taken up in 0.25M HC1. The aqueous phase was equilibrated only once with 0.1M DPPM/chloroform for 5 min and scrubbed twice with the barren aqueous phase. Because of the high distribution coefficient for gold, a single equili- bration suffices, and the subsequent scrubbing stages barely affect the recovery of gold whilst eliminating other metal ions from the organic phase. The radiochemical purity of the extract was checked by "),-spectra and decay studies, and the amount of gold in the sample was calculated by neutron activation analysis using the com- parative method. The results of various samples of different matrix composition are presented in Table 3.

3 4 6 J. Radioanal. Chem. 42 {1978)


Cu ~

Zn 2.

Fe 3+

Foreign ions

Cu 2. + Zn 2+ + Fe 3+

Cu 2+ + Zn ~§ + Co 2+ + Fe a§

Cu 2+ + Mg 2§ + Mn 2§ +

Ni 2§ + Na +

Table 3 Se 9aration of gold from different metal ions

Zn 2§ + Ca 2+ + Mg 2+ +

Fe s+ + A13. + Ce s+

I Added, mg

I0

50

100 50

i

10 100

50 100

100 100

10 100

50 100

100- 100

10 (each) 1 O0

50 (each) 100

100 (each) 100

10 (each) 100

50 (each) 100

100 (each) 100

10 (each) 100

50 (each) 100

100 (each) 100

10 (each) 100

50 (each) 100

100 (each) 100

Taken

50

50

Av.

Av.

Av,

Av.

av.

7

Av.

2 ~ V ,

Au, ~g

Found

48.2

50.1

50.3

49.53

99,2

103.3

98.7

100.4

102.0

98.1

100.0

100.03

99.3

98,2

99.7

99.0

96.0

97.3

93,8

95,7

96.8

98,3

99,1

98,06

99.8

96.4

95.8

97,33

Standard deviation,

g g

1.156

2.523

1.949

0.781

1.769

1.1658

2.156

a r. Radioanal . Chem. 4 2 (1978) 347


References

1. M. IQBAL, M. EJAZ, S. A. CHAUDHRI, R. AHMED, Separ. Sci., 11 (1976) 255, and references cited therein.

2. M. IQBAL, M. EJAZ, Radioehim. Acta, 22 (1975) 37. 3. S. A. CHAUDHRI, R. AHMED, M. EJAZ, J. Radioanal. Chem., 35 (1976) 235. 4. D. J. CARSWELL, J. J. LAWRENCE, J. Inorg. Nucl. Chem., 11 (1959) 69. 5. A. S. KERTES, I. T. KALZNER, J. lnorg. Nucl. Chem., 24 (1962) 1417. 6. W. KNOCH, J. Inorg. Nucl. Chem., 27 (1965) 2075. 7. A. ALBERT, E. P. SERJEANT, Ionization Constants of Acids and Bases, Methuen, London,

1962, p. 69. 8. B. S. MARITZ, R. VAN ELDIK, Inorg. Claim. Acta, 17 (1976) 21.

348 J. Radiocmal. Chem. 42 (1978]

Documents

The extraction of trace amounts of gold from different aqueous mineral acid solutions by diphenyl-2-pyridylmethane dissolved in chloroform