6th Annual Summer Symposium-Less Familiar Elements
Behavior of Platinum Group Metals toward Ion Exchange Resins WILLI.431 M. MACNEVIN AND WARREN B. CRUMMETT' Ohio State University, Columbus, Ohio The complex chlorides of palladium, platinum, rhodium, andiridium behave as anions and are quantitatively adsorbed by the anion exchange resins Amberlite IR-4B, Dowex 1, and Dowex 2, but not at all by the cation resin Amberlite IR-100. The ammine complex of palladium behaves as a cation and is quantitatively adsorbed by Amberlite IR-100 but not by anion resins. Use of this idea has led to quantitative separation of palladium from iridium, from platinum, and from rhodium, singly and in mixtures. Complete separation of rhodium and platinum from iridium, but only 95% separation of rhodium and platinum, has been accomplished. These methods are adaptable to the established Gilchrist-Wichers methods for the separation and determination of the platinum group metals.
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EPARATION of the platinum group metals (platinum, palladium, iridium, rhodium, osmium, and ruthenium) is important in analysis, in industrial production of these metals, and in radiochemical separations. Outstanding among analytical methods of separation are those of Gilchrist and Wichers ( 7 ) of the National Bureau of Standards, and of Schoeller and Powell (19). These reliable methods are admittedly laborious, and anyone who has worked n-ith them Kill have wished for a simpler scheme. This is no criticism of these authors, but rather an emphasizing of the fact that the chemical separation of the platinum group metals is a complicated and difficult task (9). Following the unique success achieved by the use of ion exchange in separations of the rare earths (8, 12, 14, 22, BS), of sodium and potassiunl (5),of amino acids (W),and of sodium and potassium from alkaline earth nirtals (WO),it seemed worth nhile to investigate the behavior of these metals toward ion exchange resins. *4s far as could be determined, the only use of ion exchange resins with the platinum group reported in the literature was in the recovery of the metals from v, astes (16). The loaded resins were then incinerated and the metals recovered from the ash. The preparation, structure, and general behavior of ion exchange resins are described in several reference books ( I S , 15, 17, @). Both cation and anion exchangers were investigated in the work reported here. Preliminary work indicated that some resins were more effective than others. Some showed a strong affinity for the metals, as indicated by complete adsorption by a relatively small amount of resin. With others, adsorption was slight. Cation exchangers found useful are Amberlite IR-100 (Rohm & Haas) and Dowex 50 (The Dow Chemical Co.). These contain the sulfonic acid group, -SOsH. Anion exchangers used were Amberlite IR-4B, Dowex 1, and Dowex 2. The fact that both cation and anion exchangers are available makes i t possible to take advantage of the cationic and anionic complexes of the platinum metals. Each of the metals platinum, 1 Present
address, The Dow Chemical Co , Midland, hlich.
palladium, iridium, and rhodium forms ( 4 ) a complex anion of the genkal type ?*ieCls--. Palladium and platinum also form cationic ammines ( 5 ) of the type Pd(NHZ)&++.There is indication that rhodium and iridium form similar ammine compleses but to a much less extent. Thus i t was thought that the graded stabilities of chloride and ammine complexes (although not ex:tctly known) might form the basis of separations. Blthough the platinum group metals form many complex salts, it was the intention to work with the simpler forms first. The chlorides of the platinum metals appeared to be the simplest. Palladium, platinum, and iridium form complex chloride anions iihen placed in solution with excess chloride. Rhodium behaves peculiarly. Rhodium chloride made by ignition in chlorine is insoluble in water. Solutions of rhodium chloride that have been evaporated to dryness a t 100' C. dissolve to give aviolet-colored solution from which chloride ions cannot be precipitated Kith silver. Upon standing or warming, the solution becomes orange and chloride can be precipitated A change from anion to cation character is indicated by these changes. The complex ammines of palladium, platinum, rhodium, and iridium behave as cations. Palladous ammine ion is the most itable; iridium ammine, if formed a t all, is least so. ThePe ammines are formed when ammonium hydroxide is added to a solution of the chlorides. A competition exists between the chloride and the ammonia for the platinum metal and this forme the basis of possible separations. The stabilities of both series of complexes are unknown and their relative behavior cannot be predicted. Solutions of platinum, palladium, iridium, and rhodium were prepared from chlorides obtained from the American Platinum Works. The solutions \yere evaporated with nitric acid, followed by repeated evaporations with hydrochloric acid. The residues were finally heated a t 100' C. to drive off excess hydrogen chloride. This method gave solutions of platinum, palladium, iridium, and rhodium of reproducible behavior. QU.4LITATIVE METHODS
The following qualitative methods were found useful for detecting platinum, palladium, iridium, and rhodium when each was present alone. Platinum. Evaporate the test portion t o dryness, add 5 ml. of concentrated sulfuric acid, and evaporate t o fumes. Add 5 ml. of a 40% solution of stannous chloride in 30% hydrochloric acid. An orange red color indicates platinum (21). Palladium. Place 1 drop of acidic test solution on filter paper impregnated with nickel dimethylglyoxime, dry the paper over a hot plate, and then bathe it in dilute hydrochloric acid. The paper turns white except where palladium is present. Fivehundredths microgram of palladium is detectable (6). Iridium. Evaporate the test solution t o dryness, add 4 to 5 ml. of concentrated sulfuric acid and 2 ml. of concentrated nitric acid, and evaporate t o fumes. A violet color indicates iridium. Although Sandell (18) describes this test as insensitive, Beamish and Hill ( 1 ) found it to be as sensitive as the highly recommended Pollard test, which uses perchloric acid and lithium sulfate and is sensitive to 0 . 5 ~ of iridium. Rhodium. Evaporate the test solution to dryness, add 5 ml. of concentrated sulfuric acid, and evaporate to fumes. Add 5
1628
4
1629
V O L U M E 2 5 , NO. 11, N O V E M B E R 1 9 5 3 ml: of a 40% solution of stannous chloride in 30% hydrochloric acid. 9 brown color turning red indicates rhodium (11). Solutions containing mixtures of the metals were evaporated to dryness and analyzed qualitatively by spectrographic means. QUANTITATIVE METHODS
Aniounts greater than I mg. in mixtures were estimated quantitatively by the Gilchrist-Wichers scheme. The solution to be analyzed was freed from organic matter and volatile salts by evaporation to dryness, ignition, treatment with hydrochloric and nitric acids, and evaporation to dryness. Residues, particularly those containing rhodium or iridium which did not dissolve in a mixture of hydrochloric and nitric acids, were heated in a sealed tube with aqua regia a t 300" C. The resulting solution was evaporated several times with hydrochloric acid and finally evaporated just to dryness. The resulting residue was dissolved in a small amount of water and transferred to a porcelain crucible for the determination of palladium, rhodium, iridium, or platinum. Palladium, rhodium, and iridium were separated hydrolytically. Platinum was ignited to the metal in air. After the hydrous oxides were dissolved, palladium was precipitated with dimethylglyoxime, filtered through a porous porcelain filter crucible, dried a t 110", and weighed. Rhodium and iridium were separated hydrolytically and each was determined by ignition. -4colorimetric method for estimating small amounts of iridium was developed, based on the formation of a violet color in the nitric-sulfuric acid test. Maximum extinction occurs a t 575 mp when a slit n-idth of 0.03 mm. is used. This solution obeys Beer's la,^ over the range 0.03 to 0.12 mg. of iridium per milliliter. RESIN EXCHANGE COLUMNS
The columns used in this work were similar to those of Tompkins ( 2 5 ) ,but the sintered-glass disk used to support the resin was replaced by the male part of a ground-glass joint, which was sealed off and the seal perforated to permit passage of the eluate. A wad of glass wool was placed between the perforated glass base and the resin. The resin was prepared for use by allowing it to stand overnight in a 2% solution of the ion which would convert the resin to the desired form. This mixture was stirred until a slurry formed and then poured into the exchange column previously filled with distilled water. This procedure is imperative, to prevent air spaces in the resin bed which would lead to channeling and to prevent bursting of the column due to the swelling of the resin matrix. ADSORPTION
Tetrachloropalladous, hexachoroplatinic, heuachlororhodic, and hevachloroiridic anions are quantitatively adsorbed by the anion exchange resins Amberlite IR-4B, Doxex 1, and Dowex 2, but not a t all by cation resins Amberlite IR-100 and Dowex 50. Sdsorption bands lvere visible on Dowex 1 and Dowex 2. Colors were more intense than in the original solutions, indicating a concentration of the metals. Rhodium, present as R h C h - - , gave an orange-red band; platinum, as PtCI,--, an orange band; palladium, as PdCI,--, a dark red band; and iridium, as IrC&--, a very dark red band. Osmium, present as perosmic acid, H,OsOs, was quantitatively adsorbed by the anion resin Dowex 1 and gave a narrow black band near the top of the column. This was unexpected, as the ionization constant of the acid is 8 X lO-'3 and perosmate is colorless. Obviously, reduction t o free osmium occurred. Ruthenium, thought to be present as RuCI&--, 1%-asnot quantitatively adsorbed by the anion resin Dowex 1. Some of the solution, colored like the original, went through the column. Three adsorption bands appeared, dark brorvn, light brown, and green. This indicates the probable presence of three and possibly four valence states. This behavior could be eliminated by boiling the solution with a small amount of nitric acid, after which the ruthenium was quantitatively adsorbed in a dark brown band a t the top of the column. Because osmium and ruthenium can be quantitatively removed from the other platinum metals b y distillation (10)and because of
the evidence that compounds of both are strongly reduced h y the resin, it was decided to concentrate the present study upon the four remaining metals : platinum, palladium, iridium, and rhodium. ELUTION
.4 high concentration of chloride might be expected to displace the platinum metal anions from the resin. The displacement is, however, slight with 0.1 AI hydrochloric acid. More rapid elution of some of the metals was obtained with ammonium hydroxide. Complex ammines of the type lle(SH3)," + were formed. Selectivity in elution was attained, apparently as a result of the gradations in st,ability of the chloride anions and the ammine cations. Other bases thought, to give compleses of the ammine type w-ere also used as elutants. Of the bases tried, only aniline and piperidine shon-ed general elution behavior considered satisfactory. With ammonia, aniline, and piperidine, palladium is eluted most strongly; platinum is nest,, follon-ed h y rhodium and iridium. When ammonium chloride was added to the ammonium ti>-droxide, palladium and platinum were eluted more rapidly. Animonium chloride decreased the rate of elution of iridium, while t,he rate for rhodium was not appreciably affected by the presence of ammonium chloride. I t wa8 observed Tvith palladium that a mixture of ammonium chloride and ammonium hydroxide gave more rapid elution than either substance by itself. Apparently the PdC14-- ion is displaced from the resin by substitution of the chloride ion and by the formation of the ammine Pd(NH3)*++. SEPARATIOVS 4CHIEV ED
Palladium from Iridium. Palladium as PdCIb-- and iridium as IrC16-- are both quantitatively adsorbed by Dowex 1 and Dowe\ 2 . Elution of the Dowex 1 with ammonium hydroxideammonium chloride did not give a sharp separation of the two metals. With Dowex 2, palladium is quantitatively eluted in a spectrographically pure form vhen the eluent is not more than 0.025 M in both constituents. Iridium could not be quantitatively removed from these resins by either strong acid or strong ammonium hydroxide. The separation of palladium from iridium is better n i t h Amberlite 1R-100, a cation resin. This has the advantage that both metals can be recovered. The mixture of IrC16-- and PdCI4-ions is treated with ammonium hydroxide, which converts the palladium to the cation ammine P d ( S H & + + but has no appreciable effect on the IrCl6-- ion. %-hen this mixture is passed through Amberlite IR-100, palladium is quantitatively retained while iridium is eluted, also quantitatively. Palladium can then be removed from the resin by elution -with 1 M hydrochloric acid. Palladium from Platinum. Like the previous pair, palladium and iridium, these two are not completely separable on Dowex 1. With Amberlite IR-100, complete separation is achieved. . 4 mixture of PtC16-- and PdCI*-- was treated with ammonium hydroxide and introduced into an Amberlite IR-100 cation resin column. Platinum was eluted quantitatively with an eluent containing 0.025 M ammonium hydroxide and 0.025 M ammonium chloride, while palladium remained quantitatively adsorbed. Again the palladium could be removed from the resin with 1.0 JI hydrochloric acid. Apparently the platinum remains in the anion form in the presence of ammonium chloride. Palladium from Iridium and Platinum. When the above experiments with Amberlite IR-100 were repeated with a mixture of palladium, iridium, and platinum as Pd(SHa)*++, IrC16--, PtCl,--, and Pt(NHy),++'+, palladium nas retained by the column while iridium and platinum \+ere eluted quantitatively, showing that the presence of the third metal had no adverse effect. Rhodium from Iridium. .4n attempt was made t o separate rhodium from iridium by taking advantage of the observation that Rhen individually adsorbed on Dowex 1 or Dowex 2,
A N A L Y T I C A L CHEMISTRY
1630 rhodium is quantitatively eluted by ammonium chloride while iridium is not removed at all by this reagent. However, when the two met,als are present as complex chlorides, rhodium is not quantitatively eluted from the resin by this eluent. The maximum elution of rhodium amounted t o about 9570 of that present. However, the rhodium eluted was spectrographically free from iridium and this method has been useful in separations of rhodium from iridium for radiochemical purposes. Separation of Palladium, Platinum, Rhodium, and Iridium. Attempts were made to separate palladium, platinum, rhodium, and iridium by adsorbing the mixture of the complex chlorides on the Dowes 2 resin (anion) followed by elution with an animonium hydroxide-ammonium chloride solution. Palladium was first quantitatively eluted and was free from rhodium, platinum, and iridium. Sext came rhodium, followed by a mixture of rhodium and platinum. Iridium was quantitatively retained by the resin. Better results were obtained with a combination of resin treatments. When a mixture of t,he four chlorides is treated wit,h ammonia and pamed through Amberlite IR-100, palladium is quantit,atively retained and the rhodium, platinum and iridium are quantitatively eluted. If the eluate is then made acid lvith h p drochloric acid and passed through Dowex 2, the remaining three are quantitatively adsorbed. Elution with a mixture of ammonium chloride and ammonium hydroxide (0.025 JI in each) gradually removes the rhodium and platinum and in this order. Iridium is quantitatively retained by the resin. So far the separation of rhodium and platinum has not been better than 95%that is, platinum will appear in the eluate when 95% of the rhodium has been collected. However, all the platinum can be removed from the resin by prolonged elution. The results t’hus far indicate probable eventual sucress in making this a quantitative separation. ADAPTATION T O ANALYTICAL SCHEME
One application of these results is the separation of palladium from platinum, rhodium, and iridium or from rhodium and iridium a t the startling point, of these separations. I n the GilchristWichers scheme, palladium, rhodium, and iridium are precipitated as hydrated oxides, leaving platinum in solution. After the oxides are redissolved in acid, palladium is separated with dimethylglyoxime. The excess organic matter must t’hen he destroyed before rhodium and iridium can be separated. The separation of palladium from rhodium and iridium by ion exchange as described seems a valuable simplification a t this point. At the preFent time, allowing for limitations in t,heion exchange separation, these four elements are separated in the following way. First palladium, rhodium, and iridium are separated from platinum as the hydrated oxides. S e x t the oxides are dissolved in acid and made slightly alkaline with ammonia. The mixture is passed through Amberlite IR-100 resin, which retains palladium, andrhodium mdiridium appear in the filtrate. Palladium is eluted from the resin with 1 .lii hydrochloric acid. Because rhodium and iridium are not yet rompletely separable by ion exchange, a method developed several years ago in this laboratory b y Tuthill was used for t,he electrolytic separation of rhodium from iridium. Rhodium is dpposited electrolytically and weighed. Iridium is
then precipitated hydrolytically or reduced to the free metal. This procedure takes about 4 hours. SUMMARY
The chloride complexes of palladium, platinum, rhodium, and iridium behave as anions and are quantitatively adsorbed by anion exchange resins but are not adsorbed by cation resins. The ammine complex of palladium behaves as a cation and is quantitatively adsorbed by cation exchange resins but not b y anion resins. Palladium has been separated from iridium, from platinum, and from rhodium singly and in a mixture by ion exchange. Rhodium and platinum have also been separated from iridium by ion exchange, but only 95% separation of rhodium and platinumhas so far been achieved. An adaptation of the GilchristWichers scheme for the separation of palladium from a mixture of palladium, rhodium, and iridium is suggested. The rhodium and iridium ma>-then he separated electrol~tically. LITER4TURE CITED
Beamish, F.E., and Hill, AI. A . , .\N\L. CHEM.,22, 590 (1950). Cleaver, C. S., and Cassidy, H. Q., J . Am. Chem. Soc., 72, 1147 (1950). Ibid., 70, 1986 (1948). Cohn, W.E., and Kohn, H. W., Ephraim, F., “Inorganic Chemistry,” 5th ed., p. 240, Xew York. Interscience Publishers. 1948. Ibid., p. 308. Feigl, F., Chemistry and Industry, 57, 1161 (1938). Gilchrist, R.. and Wichers, E., J . Am. Chenz. Soc., 57, 2565 (1935). Harris, D. H., and Tompkins, E. R., Ibid., 69,2792 (1947). Hillebrand, W.F., and Lundell, G. E. F., “Applied Inorganic Analysis,” 2nd ed., rev. by Lundell, G. E. F.. Bright, H. A , , and Hoffman, J. I., p. 339, S ~ York, N John Wiley R: Sons, 1953. Ibid., p. 354. Ivanov, V. N., J . R ~ L SPhys. S . Chem. Soc., 50, I, 460 (1918). Ketelle, B. H., and Boyd, G. E., J . Am. Chem. Soc., 69, 2800 (1947). Kunin, R., and Llyers, R. J., “Ion-Exchange Resins,” S e w York, John Wiley 8: Sons, 1950. Marinsky. J. A., Glendenin, L. E., and Coryell, C. D., J . A m . Chem. Soc., 69,2781 (1947). Nachod, F. C . , “Ion-Exchange,” Kew Tork, -4cademic Press, 1949. Patent 2,371,119 (March 6, 1945). on-Exchangers in Analytical Chemistry,” New Ilork, John Wiley 8: Sons, 1953. Sandell, E. R., “Colorinietric Determination of Traces of Rletals,” p. 259, Sew Tork, Intersrience Publishers, 1944. Schoeller, W ,R., and Powell,,;1. R., “The Analysis of Minerals and Ores of the Rarer Elements,” 2nd ed., p. 240, London, Charles Griffin and Co., Ltd.. 1940. Schubert, J., “Ion-Exchange.” ed. by F. C . Sachod, p. 208, Sew York, Academic Press, 1949. Snell, F. D., and Snell, C . T.. “Colorimetric JIethods of .4nalysis,” Vol. I, p. 413, Sew Tork, D. Van Nostrand Co., 1936. Spedding, F. H., Voigt, Ai,F., Gladrow. E. lI.,and Sleight, K. R., J . Am. Chem. Soc., 69,2777 (1947). Spedding, F. H., Voigt, A . F.. Gladrolv, E. AI., Sleight, S . R., Powell, J. E., Wright, J. 31.. Butler, 1 Ibid., 69,2786 (1947). Strain, H. H., “Chromatographic Adsorption Analysis,” Sew York, Interscience Publishers, 1941. Tompkins, E. R., Khym, ,J, Y., and Cohn, IT. E., J . A m . Chenz. Soc., 69,2769 (1947). R E C E I V Lfor D rerim
Aiigiist
END OF 6TH ANNUAL SUMMER SYMPOSIUM
G, 1933. A c r r p t e d September 17, 1953.
