Anal. Chem. 1983, 55, 1094-1098
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(13) Kabachnlk, M. I.; Lastovskll, R. P.; Medved, T. Y.; Medyntsev, V. V.; Kolpakova, I . D.; Dyatiova, N. M.; Dokl. Chem. (Engl. Trans/.) 1987, 777, 1060-1063. (14) Gravenstetter, R. J.; Cliley, W. A. J. Phys. Chem. 1971, 75, 676-682. (15) Wada, H.; Fernando, Q . Anal. Chem. 1971, 4 3 , 751-755. (16) Wada, H.; Fernando, Q . Anal. Chem. 1972, 4 4 , 1640-1643. (17) Burton, D. J.; Pletrzyk, D. J.; Ishihara, T.; Fonong, T.; Flynn, R. M. J . FlUOfhe Chem. 1982, 20, 617-626. (18) Gordon, G. “Executive Computer Programs as Applied to Handling Chemical Data” (Modified by K. Sando); University of Iowa: Iowa City, IA, 1972. (19) Blackburn, G. M.; England, D. A.; Kolkman, F. J . Chem. Soc., Chem. Comm~n.1961, 930-932. (20) Grabenstetter, R. J.; Quimby, 0. T.; Fiautt, T. J. J. Phys. Chem. 1967, 7 7 , 4194-4202.
(21) Barnett, B. L.; Strlckland, L. C. Acta Crystallogr.,Sect. 6 1979, 835, 1212-1 214. (22) Maruta, M., unpublished work, University of Iowa, 1982.
RECEIVED for review November 22, 1982. Accepted March 3, 1983. This work was presented at The 184th American Chemical Society Meeting, Kansas City, MO, Sept 1982. This investigation was supported by Grant AM 28077 awarded by the National Institute of Arthritis, Diabetes, Digestive, and Kidney Diseases. D.J.B. would like to thank NSF and AFOSR for support of organofluorine research at the University of Iowa.
Extraction of Platinum and Its Separation from Palladium by Polyurethane Foam Sargon J. AI-Bazl and Arthur Chow* Department of Chemistty, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
The Influence of thiocyanate, hydrochloric acld, and pH of the solutlon on the extraction of platinum( I I ) and platinum( I V ) by polyether-type polyurethane foam was demonstrated. Dlstrlbution coefflclents of more than I O 4 were obtalned. The effect of the chlorlde salts of dlfferent cations on the extractlon of platinum( I I ) Increased In the order of LI’ < Na+ < K+ 5 NH4+ lndlcatlng that platinum( I I )-thlocyanate complex [most probably Pt(SCN):-] Is extracted through the “catlonchelation” mechanlsm. The separatlon of platinum( I V ) and palladium(11) was also studied and the resuns Indicated that 20-fold excess of platlnum has no effect on the extractlon of palladlum.
Several solvent extraction methods have been used for the preconcentration of platinum from aqueous solution including the use of methyl isobutyl ketone (I),tri-n-butyl phosphate (2), tri-n-octylamine hydrochloride in toluene (3), 4-(5nony1)pyridine in benzene ( 4 ) , and di-n-octyl sulfite in cyclohexane ( 5 ) . Two different mechanisms have been suggested, the “ion-association” mechanism (6-9) in which platinum is extracted as an ion pair of the type 2C+-PtX82or C+.HPtX6- (C is a bulky cation or a protonated organic solvent, X is an anion) and the extraction of platinum as a neutral species of the type PtX,L, (X is a halide or pseudohalide, L is a neutral ligand) by several organic solvents (10-1 2). For the separation of platinum and palladium, Forsythe et al. (13)used solvent extraction with methyl isobutyl ketone and Gregoire and Chow (14) employed silicone-rubber foam treated with dimethylglyoxime. The anion exchanger Amberlite IRA-400 (15)and the cation exchanger Amberlite IR400 (16) have both been used to separate Pd(I1) and Pt(1V). The purpose of the present work was to find the optimum conditions for the preconcentration of platinum [as Pt(I1) and Pt(IV)] from thiocyanate solutions by polyether-type polyurethane foam. The study also involved the separation of platinum and palladium and the mechanism by which platinum(I1)-thiocyanate complex is distributed between the foam and the aqueous phase.
EXPERIMENTAL SECTION Apparatus and Reagents. A Model 306 Perkin-Elmer atomic-absorption spectrometer was used for platinum and palladium determinations, a Fisher Accumet Model 520 for pH measurements, and a Varian 634 UV-visible spectrophotometer for absorbance measurements. Platinum and palladium solutions were made from K2PtC1, and Na2PtC&.6H20(Johnson-MattheyChemicals, Ltd.) and PdClz (Matheson, Coleman and Bell) in 0.1 M hydrochloric acid. All other chemicals used were of analytical grade. Polyether-type polyurethane foam (no. 1338 M) was obtained from G. N. Jackson Ltd. (Winnipeg, Manitoba) and washed by using the procedure previously reported ( 1 7 ) . A 5.0 M stock solution of potassium thiocyanate was prepared in doubly distilled and deionized water. Extraction Procedure. Aliquots of the stock solutions of Pt(I1) or Pt(1V) chloride and potassium thiocyanate were diluted to 100 mL with distilled water. Any addition of acid was made after diluting the thiocyanate solution in order to minimize the decomposition of thiocyanate caused by direct contact with concentrated acid (18). The extraction of 3 X lod M platinum by 99 1 mg foam and the calculations of percentage extracted (%E) and distribution coefficient (D)were done as described previously (17).
RESULTS A N D DISCUSSION The Extraction of Platinum(I1). To establish the optimum conditions for the extraction of Pt(I1) from thiocyanate solutions with polyurethane foam, the effect of various parameters was investigated. The extraction of platinum from solutions 0.5 M in thiocyanate and 0.4 M in hydrochloric acid was studied from 5 min to 30 h. The extraction increased sharply up to 15 min and then slowly until 45 min, beyond which it was almost time independent. A minimum time of 45 min was regarded convenient for further studies. To determine the optimum range of thiocyanate for the extraction of Pt(I1) from 0.8 M hydrochloric acid solutions, the effect of thiocyanate concentrations varying from zero to 0.9 M was studied. At zero thiocyanate, less than 1% of platinum was extracted and the UV spectrum showed a characteristic absorbance at 230 nm, which has been assigned to PtC1:- by several workers (19-21). For solutions containing thiocyanate, the UV spectra showed a broad absorbance below 330 nm partially overlapped with the intense absorbance of
0003-2700/83/0355-1094$01.50/00 1983 American Chemical Society
ANALYTICAL. CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
1095
4 5,
zcLL-30
-25
-I
-20
Log
5
-IO
-05
00
[SCN-]
Flgure 1. Effect of thiocyanate on the extiraction of piatinum(I1)thiocyanate complex.
thiocyanate ion that likely corresponds to Pt(SCN)4z-complex which absorbs at 243 nm (22). The resullts (Figure 1)indicate that 97 f 1% (log D = 4.53 f 0.16) platinum is extracted between 4 x M thiocyanate. From 5 X and 6 x M to 0.15 M thiocyanate, the extraction decreases mainly due to thiocyanic acid interference, but at higher concentrations, the decrease is most likely due to the combined interference of thiocyanic acid and 5-amino-1,2,4-dithiazole-3-thione (23) as indicated by absorlbance bands at 284 and 342 nm. The extraction of Pt(I1) from 6 X lo-' M thiocyanate solutions as a function of hydrochloric acid indicated a slow increase from 87% (log D = 3.79)at 0.l M to 96% (log D = 4.40) at 0.36 M and slower to 98% (log D = 4.67)a t 1.1 M. The effect of pII on Pt(I1)extraction showed a sharp decrease from 87% (log D = 3.79) at pH 1.0 to 19% (log D = 2.35)at pH 4.0and a slow decirease to 10% (log D = 2.05) at pH 9.9. The intensity of the greenish yellow platinum(I1)-thiocyanate complex of solutions in both studies was not affected by acidity of the solution, confirming the stability of this complex (24). Thus the acidity of the solution affects mainly the distribution of platinum(I1)-thiocyanate complex between foam and aqueous phase rather than the formation of the extractable species. T o establish the mechanism of platiiium(I1)-thiocyanate extraction by polyether foam, the influence of chloride salts of the cations LP, Naf, K+, and NH4+over the range zero to 2 M was investigated. 'The pH of these solutions was adjusted to 6.1 f 0.2 by the addition of lithium hydroxide or hydrochloric acid except for those containing ammonium chloride which were fixed at pH 5.1 f 0.1 to prevent the formation of Pt(NH3)42+complex (16). The results indicated that the extraction increased linearly with the increase of lithium chloride concentration. For solutions containing sodium chloride the extraction increased slowly, while in the presence of potassium or ammcmium chloride it increased sharply to 0.1 M and then slowly with further increase in salt concentration. These solutions before extraction had the same intensity of the gireenish yellow color which indicates that the added salt mainly affects the distribution of platinum(I1)thiocyanate complex rather than the formation. These results plotted vs. cation radius (Figure 2) indicate that the extraction of platinum(I1)-thiocyanate complex in the presence of potassium or ammonium ion is much higher than that obtained for solutions containing sodium or lithium ion. The results obtained are similar to those reported for Pd(SCN)?- (17)and in agreement with the "cation-chelation" mechanism (25,26). Since the "cation-chelation" mechanism is playing the major role in the extraction of platinum(I1)-thiocyanate complex by polyether foam, the sorption a t low acidities should be improved in the presence of potassium chloride (17). An average of 94 i 3% of Pt(I1) was extracted from 6 X M thiocyanate solutions in the presence of 2 M potassium
K
Na
06
08
I O
12
Cation Radius
NH4
14
(i)
Figure 2. Effect of the size of various monovalent cations on the distribution of platinum(I1) thiocyanate between foam and aqueous phase: Salt concentration = (0)5 X lo-' M, (A) 0.1 M, (0)0.5 M.
Table I. Effect of Ammonium Hydroxide on the Extraction of Platinum( 11) from Solutions Containing Potassium Chloride w11, M
a
t ",OH
I,
M
0.5 0.5 0.5 0.5
1 x 10-2 2x 4 x
0.5
0.1
6x
% Pt extracted
log D
32 13
2.66 2.14
N.D.a N.D.a N.D.a
N.D. means less than 0.5% extraction (loa D
