Anal. Chem.
(10) (11)
1981, 53, 1073-1076
Batchelor, B. G. “PracticalApproach to Pattern Classification”;Plenum Press: London, 1974: pp 46-49, 215-215, 222-225. Bogert, 8. P.; Healy, M. J. R.; Tukey, J. W. In “Proceedlngsof Time Serles Analysls”; Rosenbiutt, M., Ed.; Elsevier: Amsterdam: Chapter 15.
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Childers, D. 0.;Skinner, D, P.; Nemeriat, R. C. Roc. I€€€l977,65, 1428.
(13)
Horllck, G.; Hleftje, G. M. In “ContemporaryTopics in Analytical and Ciintcal Chemistry”;Ilercules, D. M., Hleftje, G. M., Snyder, L. R., Evenson, M. A., Eds.; Plenum Press: New York, 1978; Vol. 3, Chapter 4.
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Thierry, M de “L’Etudede la Chemie”; Masson: Paris,
1073 1906; pp
236-237.
RECEIVED for review October 28,1980. Accepted March 17, 1981. The provision of Studentships to M.T.J. and T.L. by the Science Research Council and support of IC1 Plastics, Welwyn Garden City, in a CASE Studentship (T.L.) are gratefully acknowledged.
Extraction of Rhodium and Iridium with Polyurethane Foam Sargon J. AI-Bazl and Arthur Chow* Department of Chemistty, Unlversity of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
the feaslblllty was StlJdled for preconcentratlng and separating rhodlum and lrldlum from acldlc thiocyanate solutions uslng polyether-type polyurethane foam. A detalled Investigatlon was made of the condltlons influenclng the extraction whlch Included thlocyanate concentration, acld concentration, tlme of heatlng, and tlme of extractlon. For both rhodlum and Irldluin, dlfferent complexes were formed when the acid was added before or afler heatlng. In the latter case the spectra lndlcated that the extracted rhodlum species was Rh(SCN),”. Dlstrlbutlon ratlos of more than lo4 were obtained wlth a capaclly of about 0.5 mol kg-’. By use of the Mst conditlons for separatlon, more than 90% of the lrldlum remained In the aqueous phase while more than 90% of the rhodlum was retalned by the polyurethane foam.
Polyurethane foam has been widely used for the extraction of inorganic and organic species from aqueous solutions ( I , 2). Bowen (3)pioneered work in this direction in 1970 when he used solid flexible polyurethane foam as an extractant for a number of substances. Considerable attention has been directed in recent yearri to the use of polyurethane foam after special treatment with a suitable reagent for separation and preconcentration processes. The use of thiocyaneites as complexing reagents has served as a basis for the extraction of a number of metals (4) including some platinum metalii (5-7) by several solvents. The extraction of cobalt as thiocyanate by polyurethane foam has been reported ( 4 9 ) with the distribution coefficient as high as 3 x IO6. The separation of rhodium and iridium is one of the most difficult problems in the analytical chemistry of the platinm group metals (IO). Several methods based on selective precipitation, chromatography, or solvent extraction have been suggested, which either are time-consuming or separate only very small masses (10-13). Ion-exchange methods have long been used (I4), but iridium is bonded so strongly by the anion-exchange resins that it is difficult to desorb it by any means. Baghai and Bowen (15)have reported the separation of milligram amounts of rhodium and iridium from hydrochloric acid solutions by a silicone-rubber foam treated with tri-noctylamine. The procedure required the continuous treatment of the sample in 6 M hydrochloric acid with chlorine gas to avoid reduction which restricts the general usefulness of this 0003-2700/81/0353-1073$01.25/0
method and precludes its use with polyurethane since the foam would rapidly decompose. The purpose of the present work was to establish the optimum conditions for the preconcentration and for the separation of rhodium and iridium as thiocyanate complexed from aqueous solutions using polyether type polyurethane foam.
EXPEBIMENTAL SECTION Apparatus and Reagents. Rhodium was determined with a Model 306 Perkin-Elmer atomic absorption spectrometer. A Baird Atomic Model 708 Iso/matic system with a 2-in. NaI well-type y-detector was used for the measurement of iridium. A multiple automatic unit was used to squeeze foam in up to 10 sample cells simultaneously. Sodium hexachlororhodate(II1)(Na3RhC&.12H20)and sodium hexachloroiridate(1V)(Na21rC16.6H20)were supplied by Johnson-Matthey Chemicals Ltd., Toronto. Iridium-192in the form of ammonium hexachloroiridate(1V)was obtained from Amersham-Searle Ltd., Don Mills, Ontario. All other chemicals used were of &&tical grade. Polyether type polyurethane foam sheets were obtained locally and cut into cubes of approximately 1.0 g, soaked in 1 hydrochloric acid for 24 h with occasional squeezing, washed with distilled water until acid free, Soxhlet extracted with acetone for several hours, and finally air-dried. Stock solutions (500 pg mL-’) of rhodium and iridium were prepared from their salts in 0.1 M hydrochloric acid, and a 3 M stock solution of potassium thiocyanate was prepared in water. Procedure. A known amount of the stock solution of rhodium or iridium W&S put into a 100-mLvolumetric flask with the desired amount of thiocyanate and adjusted to 60 mL with distilled water. For iridium solutions sufficient tracer was added to yield a count rate of at least 160 counts/s for 10 mL of sample contained in a test tube of 15 mm i.d. The solution was heated at 90 OC for the desired period of time, cooled to room temperature, and f i d y diluted to 100 mL before the procedure. To fix the acidity of the solution we added the acid either before heating or after heating. In both caes, the addition of the acid was done after the dilution of the thiocyanate solution to minimize the decomposition (16) which occurs by direct contact of concentrated thiocyanate solution with concentrated acid. The sample solution was placed in a 250-mL glass cell containing about 0.05 g of foam which was squeezed with a glass plunger to bring fresh solution into contact with the foam. The plunger was operated by a multiple automatic apparatus (8)with an eccelltric cam which gave a 5-cm stroke at a rate of 24/min. The extractions were carried out at a temperature of 25.0 & 0.05
“C. The percentage of extraction of the metal was calculated by measuring the concentration before and after extraction. A distribution coefficient (D) for the extraction process was calculated from the ratio of the concentration of the metal in the 0 1981 American Chemical Society
1074
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
D
m
0
3
L
2O
0
0.1
0 25
A 0.2
0 50
0.3
0 75
o e
ISCN-1
Flgve 1. The effect of thkcyanate on the extraction of h d l u m : initial rhodium concentration 16.67 l g mL-'; volume of solution 145 mL; weight of foam 0.1 g; (0)0.8 M HCI added before h e a mthe solution; ( 0 )2 M HCI added after heating the solution.
foam to the concentration of the metal left in solution with units of L kg-', although a true equilibrium may not have been reached.
RESULTS AND DISCUSSION The Extraction of Rhodium. The extraction behavior into polyurethane foam from aqueous solutions of 15 pg mL-' rhodium(II1) in 2 M hydrochloric acid and 0.1-0.7M thiocyanate was studied. The extraction was carried out for 24 h, immediately after the preparation of the solutions to minimize the decomposition of thiocyanic acid (16) and trimerization (17) which forms 5-amino-1,2,4-dithiazole-3thione. The results showed about 5% extraction of rhodium which was independent of the thiocyanate concentration. Furthermore, the absorption spectra of the 0.1M thiocyanate solution gave an absorbance band at 288 nm as was obtained for Rh(SCN)63- by Schmidtke (18); this indicates that a rhodium thiocyanate complex was formed which is not extractable, perhaps due to a significant influence of thiocyanic acid and 5-amino-1,2,4-dithiazole-3-thione. In order to minimize the effect of these interferences, we used low concentrations of thiocyanate and it was found that heating was necessary to increase the rate of formation of rhodium thiocyanate complex. The time used for heating the solution at 90 OC was varied from 0.5 to 8.0h, while the concentrations of thiocyanate and hydrochloric acid were kept constant at 0.004and 2 M,respectively, with the acid added after heating and the extraction process was carried out for 12 h. About 80% (log D = 3.77)of rhodium was extracted from the solution heated for 2 h with the extraction increasing slowly to 95% (log D = 4.46)after 8 h of heating. The absorption spectra show that rhodium thiocyanate Rh(SCN)63- was the extractable species. When the solutions had the acid added before heating with the thiocyanate and hydrochloric acid concentrations fixed a t 0.1 and 0.3 M,respectively, a different complex was obtained with an absorption band at 306 nm and the extraction increased sharply with time of heating reaching about 80% (log D = 3.79)after 1.5 h and then increasing slowly to about 92% (log D = 4.24) after 7.0 h. The absorbance band at 288 nm indicated that Rh(SCN)6" was initially formed and after 0.5 h of heating, the absorbance of this band decreased, and a more extractable complex with a band at 306 nm was formed which increased with increasing time of heating. For 0.004 M thiocyanate solutions which had been heated for 4 h before the hydrochloric acid was adjusted to 2 M, the extraction of rhodium increased sharply to 0.5 h and then slowly up to 3 h after which it remained constant to 27 h of extraction. The extraction of rhodium from solutions 0.1 and 0.3M in thiocyanate and hydrochloric acid, respectively, with the acid added before heating, increased up to 2.25h and then remained constant up to 22.5 h, the longest time studied.
d
0'2
O'L
0
1
2
0'6
0'8
1'0
3
I
5
0
e
[HCII
Flgure 2. The effect of hydrochkric acid on the exiraction of rhodium:
initial rhodium concentration 16.67 pg mL-'; volume of solution 145 mL; wewt of foam 0.1 g; (0)0.15 M thiocyanate, acid added before heating; (0)0.004 M thiocyanate, acid added after heating.
The effect of varying the thiocyanate concentration of the rhodium solution with the hydrochloric acid added before or after heating is shown in Figure 1. With both methods, at high concentrations of thiocyanate, the extraction of rhodium decreased which may be attributed to the increased influence of thiocyanic acid and Bamino-l,2,4-dithiazole-3-tone. With thiocyanate concentrations less than 0.05 M and acid added before heating, it was found that a brown rhodium precipitate formed, which increased with decreasing thiocyanate concentration. A similar precipitate was obtained when a solution with a thiocyanate concentration less than 0.002M was adjusted to 2 M hydrochloric acid after heating. The effect of hydrochloric acid concentration on the extraction efficiency of polyurethane foam is shown in Figure 2. A maximum extraction of 96% was obtained at about 4 M for solutions which were heated before adjusting the acid concentration. For rhodium thiocyanate solutions with the acid added before heating, the extraction increased to 91% at 0.3 M hydrochloric acid. The spectra of these solutions indicated the formation of Rh(SCN)63-at hydrochloric acid concentrations less than 0.01 M by the presence of a band at 288 nm which shifted to 306 nm for higher acid concentrations and reached a maximum at 0.3 M. With both methods, the absorption spectra showed that 5-amino-1,2,4-dithiaole-3thione was not formed. The extraction of two solutions of rhodium chloride, one of which was prepared from a stock solution which was 1 day old and the second from a solution which was 7 months old were compared. The thiocyanate and the hydrochloric acid concentrations were fixed at 0.003 and 2 M, respectively, with the acid added after heating the solutions for 4 h. After 3 h, 92% (log D = 4.34)of the rhodium was extracted from the fresh solution, while only 78% (log D = 3.49)was extracted from the older solution. The absorbance at 288 nm indicated that more rhodium thiocyanate was formed in and extracted from the fresh solution. The spectra showed that the bands at 490 and 390 nm of the fresh rhodium chloride solution were shifted to 466 and 366 nm for the older solution. This aging effect has been attributed to the hydrolysis of RhCb3- to (RhC1,(H20),& complexes (19). Thus the differences in the amount of rhodium thiocyanate formed in the two solutions can be attributed to the rate of reaction of rhodium chloride with thiocyanate which decreases with increasing number of water molecules in rhodium chloride complex. Rhodium solutions containing 3.3-33.3 pg mL-' in 0.005M thiocyanate and 2 M hydrochloric acid with the acid added after heating were extracted with 0.05 g of foam and indicated a capacity of 54.5 g kg-' or 0.53 mol kg-' foam. When the hydrochloric acid was added before heating the solution for 4 h, with 0.1 M thiocyanate and 0.3 M hydrochloric acid, the capacity was 0.50 mol kg-I (51.5g kg-') of foam.
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
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Table I. Effect of Hydrochloric Acid on Iridium Extractiona w11, M %E log D
2
IIfI'0
0 02
0 OL
0 06
[SCN-I
Figure 3. The
effect of thiocyanate on the separation of modium and
iridium (acid added after heating the solution): (0)extraction of rhodium, initial rhodium concentration 16.67 pg mL-', volume of solution 145 mL, weight of foam 0.1 g, hydrochloric acid concentration 2 M; (0)extractbn of Hdium, initial iridiun concentration 15 pg d-' volume , of solution 100 mL, weight of foam 0.05 g, hydrochloric acid con-
0 0.01 0.05 0.1 0.5 1.0 2.0 3.0 4.0 5.0
6.23 7.61 8.05 7.72 6.00 6.08 6.67 6.75 7.01 5.78
2.12 2.22 2.28 2.24 2.12 2.15 2.16 2.16 2.18 2.15
Acid added after heating the solution: initial iridium concentration 15 wg mL-'; volume of solution 100 mL; weight of foam 0.05 g; thiocyanate concentration 0.004 M.
centration 2 M.
The Extraction of Iridium. A maximum of 1% extraction was obtained with aqueous solutions containing 15 pg mL-' of iridium into polyurethane foam for thiocyanate concentrations from 0.0075 to 0.9 M with the hydrochloric acid fixed at 0.8 M, using a 3-h extraction. Even when the solutions were left in contact with the foam for 40 h, the percentage extraction only increased to 4% which is due to the slow rate of formation of iridium thiocyanate complex at room temperature. The spectra showed that the absorbance bands at 490 and 430 nm which correspond to IrCb2- completely disappeared when thiocyanate was added to the solution, indicating the reduction to IrCh" (20,21) in the presence of this weak reducing agent. When the acid was added after heating the solution for 4 h and the extraction was carried out for 4 h, the extraction increased to a maximum at 0.03 M thiocyanate where about 40% of the iridium was extracted. The optimum conditions for the extraction of iridium by polyurethane foam when the hydrochloric acid was added after 4 h of heating were the solutions 0.03 M in thiocyanate and 5 M in hydrochloric acid, followed by a 4 h extraction, but only 54% extraction was obtained. When the hydrochloric acid was added before heating the 0.3 M thiocyanate and 1.0 M hydrochloric acid solutions for 0-7 h, the iridium extracted increased sharply up to 79% for 3.5 h and then slowly to 87% (log D = 4.14) for 7 h heating using an 11 h extraction. By use of 0.1 M thiocyanate and 0.3 M hydrochloric acid solutions heated for 4 h, the extraction increased sharply until 0.5 h and then slowly to 22.5 h. A minimum time required for the extraction was fixed at 3 h. The effect of thiocyanate concentration on the extraction of 15 pg mL-' of iridium from aqueous solution, when the acid was added before heating, showed that the extraction increased sharply until 0.005 M thiocyanate where 73% of the iridium was extracted, beyond which it increased slowly to a maximum of 81% a t 0.04 M thiocyanate. At high thiocyanate concentrations the extraction decreases due to the presence of the other extractable species. The effect of hydrochloric acid concentration from 0 to 1.0 M added before heating for 4 h on the extraction of iridium was studied with the thiocyanate concentration fixed at 0.03 M. The results indicated a sharp increase in the extraction up to 0.4 M where 63% of the iridium was extracted and then a slow increase to 83% at 1 M hydrochloric acid. The capacity of the polyurethane foam for the extraction of iridium was determined by using 0.05 g of foam with solutions 0.03 M in thiocyanate and 1.0 M in hydrochloric acid which was added before heating the solution for 4 h. After a 12-h extraction the capacity of the foam for iridium thio-
;
OO
1
2
3
4
time lhrl
effect of time of heating at 90 O C on Separation of rhodium and iridium (acid was added after heating the solution): (0) extraction of modiw, initial rhodium concentration 15 pg rk-', volume of solution 95 mL, weight of foam 0.05 g, hydrochloric acid concentration 2 M; ( 0 )extraction of iridium, initiii iridium concentration 15 pg mL-', volume of solution 100 mL, weight of foam 0.05 g, hydroFigure 4. The
chloric acid concentration 2 M.
cyanate was 0.60 mol kg-' (115 g kg-') of foam. Separation of Rhodium and Iridium. From Figure 3, it is clear that decreasing the thiocyanate concentration below 0.02 M decreases the extraction of iridium and increases it for rhodium. For determination of the minimum thiocyanate concentration which gives the maximum separation of rhodium and iridium, the extraction of 15 pg mL-' rhodium was investigated over a range of 5 X to 9 X M of thiocyanate by 0.05 g of polyurethane foam. The hydrochloric acid concentrationwas fixed at 2 M after heating the solutions for 4 h, and the samples were extracted for 3 h. The results indicated that 2 X M thiocyanate was the minimum concentration that could be used without the precipitation of rhodium after the addition of hydrochloric acid and under these conditions 87% (log D = 4.12) of the rhodium together with about 8% (log D = 2.24) of iridium was extracted. The effect of hydrochloric acid concentration on the extraction of iridium is shown in Table I, which indicates that the distribution coefficient is nearly independent of acid concentration. Therefore 2 M hydrochloric acid, which is the optimum for the extraction of rhodium, was chosen for the separation of rhodium and iridium. The extraction of iridium under the worst conditions of thiocyanate and acid concentrations, Le., 2 X M thiocyanate and 2 M hydrochloric acid, which was added after heating the solutions for 4 h showed little change in percentage of iridium extracted from 0.25 to 25.5 h so a time of 3 h was
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Table 11. Separation of Rhodium from Iridium by Polyurethane Foama Rh(II1) Ir( IV) [Ir(IV)I, pg mL-’ % extraction log D % extraction log D 0 15 50 100 200
89.36 88.12 87.35 86.20 85.92
4.20 4.17 4.13 4.01 4.06
9.44 9.12 9.42 8.99
2.32 2.30 2.32 2.21
a Acid added after heating the solution: initial rhodium concentration 1 5 p g mL-‘; thiocyanate concentration 0.002 M; hydrochloric acid concentration 2 M; volume of solution 95 mL-’; weight of foam 0.05 g.
chosen for the separation of rhodium and iridium. Figure 4 shows the effect of time of heating the solutions of rhodium and iridium on their extraction by 0.05 g of foam. These results indicate that when the solutions were heated for 1.25 h, about 56% (log D = 3.39) of the rhodium was extracted with only 0.06% (log D = 0.07) of the iridium. Attempts to improve the extraction of rhodium by increasing the weight of the foam were unsuccessful, which suggests that low extraction was due to less formation of Rh(SCN)& so that this method may not be fully convenient for the separation unless the equilibriumcan be shifted and the rate of formation increased. The effect of the presence of up to 200 pg mL-l iridium on the extraction of rhodium showed that a 12-fold excess of iridium had little effect on the extraction of rhodium, as given in Table 11. The formation of the extractable species of both rhodium and iridium was slow at room temperature which necessitated heating the solutions to 90 “C before extraction. The results obtained for the separate extraction of rhodium and iridium indicate that (a) a low concentration of hydrochloric acid and thiocyanate minimizes the extraction interference caused by thiocyanic acid and 5-amino-1,2,4-dithiazole-&thione,(b) the extraction of both rhodium and iridium depends on the amount of the complex formed rather than
on the distribution of the complex between the foam and the aqueous phase, (c) because of the high capacity and large distribution coefficients the polyether polyurethane foam is a highly efficient method for the preconcentration of rhodium and iridium from aqueous solution. Rhodium and iridium could be separated with reasonable success compared with other techniques and considering the difficulty of this separation, but the method is not fully quantitative.
LITERATURE CITED Braun, T.; Farag, A. B. Tabnta 1975, 22, 699-705. Braun, T.; Farag, A. B. Anal. Chim. Acta 1978, 99, 1-36. Bowen, H. J. M. J. Chem. Soc. A. 1970, 1082-1085. Sultanova, Z. Kh.; Chuchallu, L. K.; Iofa, B. 2.; Zolotov, Yu. A. J. Anal. Chem. USSR (Engl. Trans/.) 1973, 28, 389-389. (5) Rigamonti, R.; Spaccamela Marchetti, E. AM. Accad. Scl. Torlno, Cl. Sci. Fls. Mat. Nat. 1959, 94, 25-35; Chem. Abstf. 1983, 5 9 , 14644e. (6) Di Casa, M.; Stella, R. Radiochem. Radioanal. Lett. 1972, 70, 331-337. (7) Berg, E. W.; Lau, E. Y. Anal. Chim. Acta 1982, 27, 248-252. (8) Hamon, R. F. Ph.D. Thesls Chemlstry Department Unlversky of Manit+ ba, 1980. (9) Hamon, R. F.; Chow, A,, Abstracts, 60th CIC Conference Montreal, 1977. (IO) Beamish, F. E. “Analytical Chemlstry of the Noble Metals”, 1st ed.; Pergamon Press: Oxford, 1966; Chapter 2. (1 1) Beamish, F. E.; Van Loon, J. C. “Recent Advances in the Analytical Chemlstry of the Noble Metals”, 1st ed.;Pergamon Press: Oxford, 1972; Chapter 1. (12) Beamish, F. E. Tabnta 1987, 74, 991-1009. (13) Ashy, M. A,; Headridge, J. B. Analyst (London) 1974, 99, 285-295. (14) Kuroda, R.; Yoshlkunl, N.; Tateno, K. Fresenius’ Z. Anal. Chem. 1978, 290, 46-47; Chem. Abstr. 1978, 89, 35899q. (15) Baghai, A.; Bowen, H. J. M. Analyst (London) 1978, 707, 661-665. (16) Crowell, T. I.; Hankins, M. G. J . Phys. Chem. 1989, 73, 1380-1383. (17) Hall, W. H.; Wilson, I. R. Aust. J . Chem. 1989, 22, 513-518. (16) Schmidtke, H. 2. Phys. Chem. (Wiesbaden) 1984, 40, 96-108; Chem. Abstr. 1984, 61, 185g. (19) Wolsey, W. C.; Reynolds, C. A.; Kleinberg, J. Inorg. Chem. 1983, 2 , 463-468. (20) Fine, D. A. Inorg. Chem. 1989, 8 , 1014-1016. (21) Stanbury, D. M.; Wilmarth, W. K.; Khalaf, S.; Po, H. N.; Byrd, J. E. Inorg. Chem. 1980, 79, 2715-2722. (1) (2) (3) (4)
RECEIVEDfor review January 5, 1981. Accepted March 23, 1981. This work was supported by the Natural Sciences and Engineering Research Council of Canada.
