6th Annual Summer Symposium-Less Familiar Elements
Reactions Involving Colored Complexes of the Platinum Metals GILBERT H. AYRES, The C'nicersity of Texas, Austin, Tex.
Spectrophotometric measurements have been used in studying the composition of several colored products formed from solutions of the platinum metals. For the determination of reaction stoichiometry, the method of mole ratios is more w7idely applicable than the method of continuous variation; the latter method is not generally suitable for determining ratios higher than 4 to 1. Formation of the wellknown tetrachloro complex of palladium(I1) has been confirmed spectrophotometrically by the method of continuous variation. The yellow product formed b y reaction of thiocyanate with osmium solution showed a 4 to 1 reaction ratio by the continuous variation method, and both 4 to 1 and 6 to 1 by the method of mole ratios. Hypochlorite reacted with rhodium(II1) in a 1 to 1 mole ratio; the product was deep blue in solutions of
T
pH near 7, yellow- a t high pH, and purple a t low pH; the purple color could not be developed directly in acidic solution. Electromigration tests and ion exchange resins showed the blue material to be anionic and the purple substance to be cationic. The orange to red solutions resulting from the reaetion of tin(I1) chloride with platinum(I1) contained complexes involving tin to platinum ratios of 1 to 4, 1 to 2, 1 to 1, 3 to 2, 2 to 1, 3 to 1, and 5 to 1. In the 5 to 1reaction, oxidation of only one tin(I1) to tin(1V) occurred, apparently by reduction of platinum(I1) to platinum(0). On the basis of precipitation tests with strong alkali, with phenylarsonic acid, and with silver nitrate, as well as on spectrophotometric eviwas dedence, a cationic complex, [PtSn&14]++++, duced; this complex contains platinum in zero oxidation state.
HE chemical properties of the platinum elements are so
strikingly similar that their separations from one another are among the more difficult of analytical procedures. Although many color reactions of the platinum metal solutions with a variety of reagents-both inorganic and organic-have been reported, the reactions applied to analytical procedures have been largely confined to microscopic examination and to spot-test methods. Many of these reactions are suited to spectrophotometric determination of the platinum elements, n i t h the advantages that accrue in this method of measurement. The high selectivity of many organic reagents made it appear not impossible that a scheme might be devised whereby each metal in solution could be estimated by the use of a Ppecific (or highly selective) reagent for that element, thus eliminating a t least some of the tedious analytical separations involved in gravimet1ic methods. Furthermore, a study of the systems by methods that nould give some insight into the color-forming reaction might thronadditional light on the chemistry of the platinum metals. Thc above factors served as the inipetu? for initiation of a reseaich program a t The University of Texas on spectrophotometiw methods for the determination of the platinum metals During the past four years. this laboratory has invehtigated the following color reactions: ruthenium u ith thiouiea ( b ) , with dithio-oxamide ( 6 ) ) and with thiocyanate ( 2 3 ) ; rhodium n ith tin(I1) chloride (20) and u i t h sodium hvpochloiite ( 7 , 2 6 ) ; palladium Kith hydrobromic acid (3);osmium with thiourea ( 4 ) and with thiocyanate ( 2 3 ) ; iridium with mixed per( u 2
a 0
m
a
8 m U
in sulfuric acid solution oxidized rhodium(II1) to give a purple solution containing rhodium(V). A comparison of the products obtained by bismuthate reaction and by hypochlorite reaction was made by determining the spectral curves for the two purple solutions (Figure 5). Although the curves of log absorbancy against wave length have the same general shape, they are not superimposable by shift along the ordinate axis, and the intensity for a given amount of rhodium is much greater in the case of the hypochlorite reaction than for the bismuthate reaction. I n electromigration tests, the blue reaction product migrated toward the positive electrode. The product was not adsorbed by Dowex-50 cation exchange resin, but was adsorbed by IRA-400 anion exchange resin. Assuming rhodium(V) as the oxidation product, the blue reaction product might be Rhos- or RhCIe-. The purple reaction product migrated toward the negative electrode; it was adsorbed on Dowex-50, but not on IRA-400 resin. Cations of the type R h o + + +and/or Rho*" may be present in the purple solutions. Rhodium in oxidation state +4 or +6 is a good oxidizing agent ( 2 6 ) ; from these oxidation states, rhodium has been quantitatively determined iodometrically (11), It would seem plausible that if the reaction product with hypochlorite is rhodium(V), it should be an active oxidizing agent, and might be determined titrimetrically if separation from excess hypochlorite could be accomplished. The purple solution, prepared in the usual way, was passed onto a column of Dowex-50 resin; after rinsing the column to remove hypochlorite, the rhodium on the column was eluted with hydrochloric acid-sodium chloride solution; the elutriate contained 99% of the original rhodium. Excess potassium iodide was added, and the liberated iodine was titrated with sodium thiosulfate solution.
0 OXIDATION OF RHODIUM SOLUTION
\
I. BISMUTHATE OXIDATION, Rh .002 M 2 HYPOCHLORITE OXIDATION, Rh ,001 M.
1.0
.9
.8
C
I I I I CONTINUOUS VARIATIONS METHOD PLATINUMW -TIN(IU SYSTEM
I
I
320mp
-
.7 -
Figure 5 . Comparison of Spectral Curves of Rhodium(II1) Solutions Oxidized by Bismuthate and Hypochlorite Mole Ratio Method. YELLOWSOLUTIONS. Attempts t o apply the mole ratio method t o the yellow solutions (high pH) were unsuccessful because of poorly reproducible absorbancies of mixtures in which the hypochlorite to rhodium ratio was less than 1 to 1. Under these conditions a colloidal system or a yellow precipitate was formed. BLUESOLUTIONS.Figure 4 shows the plots of absorbancy a t several wave lengths against the mole ratio of hypochlorite to rhodium [as rhodium(II1) chloride] ; a 1 to 1 mole ratio is clearly indicated. Strangely, no b e a k in the curve was obtained from measurements a t 610 mH, which is the wave length of maximum absorption of the blue solutions formed under the experimental conditions used. Entirely similar results were obtained by the use of sodium hexachlororhodate(111) as the rhodium source. PURPLE SOLUTIONS.The solutions were prepared in the usual way, finally adjusting to pH 1.5. I n the plots a t several wave lengths, sharp breaks in the curves occurred a t a 1 to 1 mole ratio of reactants. Continuous Variation Method. YELLOWSOLUTIOKS.The method of continuous variation was not applicable to the yellow solutions, for reason# given under the mole ratio method. BLUESOLUTIONS.In the typical continuous variation curves a t different wave lengths, the maxima a t 0.5 mole fraction indicated a 1 to 1 mole ratio of reactants in the color reaction. PURPLE SOLUTIONS.A 1 to 1 mole ratio in the reaction was also shown by the continuous variation curves, which maximized a t 0.5 mole fraction. Rhodium(II1)-Hypochlorite Reaction. The 1 t o 1 mole ratio indicated by all of the above methods, and the known reduction of hypochlorite to chloride by gain of two electrons per mole of hypochlorite, suggests that rhodium(II1) is oxidized to rhodium(V) by hypochlorite under all the conditions tested. Although rhodium in oxidation state +5 appears not to have been well characterized, it has been reported (19) that sodium bismuthate
0.3
0.4
0.5
0.6
0.7'
0.8
0.9
1.0
X
Figure 6. Reaction of Platinum Solutions with Tin(I1) Chloride
x = mole fraction of tin(I1) hlinimum at 0.5 corresponds to reduction of platinum(1V) t o platinurn(I1) When the results were calculated on the basis of electron change per mole of rhodium, it was found that the electron change varied from 1 t o about 2.3 per mole of rhodium, depending upon the amount of alkali in the rhodium-hypochlorite mixture before acidification t o develop the purple color. Attempts a t potentiometric titrations with arsenite and with iron(I1)asreductantswere not successful; the titration curves did not show significant breaks. Direct potentiometric titration of rhodium(II1) solution in phosphate buffer with hypochlorite solution of the same molar concentration showed a poorly defined inflection a t about a 1 t o 1 mole ratio of the reactants.
1626
ANALYTICAL CHEMISTRY
INTERACTION OF PLATINUM(I1) A N D TIN(I1) CHLORIDES
The reaction of pIatinum(1V) with tin( 11) chloride in hydrochloric acid solution is the basis for a spectrophotometric method for platinum ( 1 ) . The red solutions are stable in the absence of oxygen, they show strict conformity to Beer’s law over wide concentration ranges, readily pass through semipermeable membranes, and are well extracted by a variety of organic solvmts. The properties of the system are not compatible with earlier explanations that the colored material was a protected metal colloid, or was chloroplatinous acid. The color differed from that of chloroplatinous acid, both in intensity and in the shape of the spectral curve, and identical colors were produced by the reaction of platinum( IV) and platinum( 11) chlorides with tin(I1) chloride. Studies designed to elucidate the reaction are described below ( 1 7 ) .
Continuous Variation Method. The method was first applied using hexachloroplatinate( IV) and tin( 11), each a t 0.0005 M concentration in 1.2 ilf hydrochloric acid. Absorbancies were measured a t many wave lengths between 320 and 450 mp. I n Figure 6, the minima at 0.5 mole fraction, or a 1 to 1 mole ratio, correspond to the reduction of platinum( IV) t o platinum(I1) as the first step in the reaction. At higher mole fractions of tin(II), maxima in the curves correspond to platinum-tin ratios of 5 to 1 or 6 to 1, and marked changes in slope of the curves for wave lengths 400 mp and above indicate a 2 t o I reaction ratio (mole fraction about 0.75).
NI
to 10 to 5 to 1 were constructed in order to select the most appropriate wave lengths for use in making subsequent measurements. Upon adding tin(I1) chloride in definite increments to the platinum(I1) solution, the original pinkish-yellow color changed to bright yellow, to orange, and finally to deep red at a mole ratio of 5 to 1 or higher. In all cases where the ratio was less than 5 to 1, the solutions showed further color development on standing. This effect was most pronounced for solutions of tin to platinum ratio of 1.5 to 1 in which a brown color developed in 20 to 30 minutes, and in one or two days the solutions were so opaque that the absorbancy could not be measured in 1-cm. cells a t wave lengths shorter than about 700 mp. Addition of excess tin(I1) chloride to these brown solutions gave the clear, deep red color characteristic of the 5 to 1 solutions. K i t h solutions of mole ratio less than 1 to 1, heating for several hours caused most of the colored material to be precipitated, apparently as metal; solutions of mole ratio greater than 2 t o 1 gave no precipitate under the same conditions.
80
MOLE RATIO PLOT FOR HIGHER RATIO COMPLEXES PLATINUMfII)-TIN(II) SYSTEM
70
P
0
2
1
2
3
MOLE RATIO OF TIN(JI)
Figure 8.
0 0.25 0.5
I .o
1.5
MOLE RATIO OF TIN(II)-
Figure 7.
2.0
2.5
PLATINUM(n)
Platinum(I1)-Tin(I1) Reaction
In the continuous variation method, unambiguous deduction of reaction ratio cannot be made when the ratio is more than 4 to 1 (mole fraction 0.80); a 2% error in preparation of solutions or in estimation of the position of maximum is sufficient to produce a unit change in the mole ratio. For this reason, subsequent studies were made by the mole ratio method. Furthermore, because the first step in the reaction was shown to be the reduction of platinum(1V) to platinum(II), platinum(I1) in the form of potassium tetrachloroplatinate(I1) was used as the source of platinum in subsequent experiments. Mole Ratio Method. All solutions measured were 0.010 M in platinum, provided by the use of weighed amounts of potassium tetrachloroplatinate(I1) [KsPtCla]; tin(I1) chloride solution was added to give the desired concentration, and the final solution was 3.6 M in hydrochloric acid. All mixing, transfers, and measurements were made in nonoxidizing atmosphere. Spectral curves for solutions in which the tin to platinum ratio was varied from 1
4
5
6
- PLATlNUM(n)
7
Platinum(I1)-Tin(I1) Reaction
Typical results of the spectrophotometric measurements are illustrated in Figures 7 and 8. At short wave lengths, especially below 300 mp for the higher ratio complexes, molar extinction coefficients were of the order of 30,000 to 40,000, and the color was so intense that optical paths of 0.10 cm. (in some cases 0.005 cm.) were required to bring the absorbancy into an appropriate range for measurement. The curves of these (and other similar) figures show clearly the changes of slope corresponding to tin t o platinum ratios of 1 to 4, 1 to 2, 1 to 1, 3 t o 2, 2 t o 1, 3 t o 1, and 5 to 1. Precipitation Methods. HYDROLYTIC PRECIPITATION. Known amounts of tin(I1) chloride and potassium tetrachloroplatinate(11) in mole ratio of about 6 to 1 were treated with excess of carbonate-free sodium hydroxide and diluted to known volume. After settling the dark brown precipitate by centrifugation, an aliquot of the supernatant liquid was acidified, and the tin(I1) was determined by titration with standard iodide-iodate solution; a titration control was made using the same amount of tin(I1) chloride and alkali. The unprecipitated platinum was determined spectrophotometrically. From the amounts of tin and platinum taken, and the amounts of each found in the solution after precipitation, the tin to platinum ratio in the reaction was calculated to be 4.93 to 1, or approximately 5 to 1. Under proper PRECIPITBTION WITH PHESYL.4RSOSIC A C I D . conditions of acidity, phenylarsonic acid selectively precipitates tetrapositive ions. Tests showed that neither tin(I1) nor platinum(I1) was precipitated by this reagent from solutions 0.5 to 1.0 J! in hydrochloric acid, and that tin(IV-) was quantitatively precipitated. Mixtures of platinum(I1) and excess tin(I1) were treated with excess phenylarsonic acid; a voluminous precipitate formed, con! taining all of the platinum. The precipitate was soluble in 4 & hydrochloric acid with return of the original red color. By following the same general method as used in the hydrolytic precipitation, the decrease in tin(I1) concentration resulting from precipi-
V O L U M E 25, NO. 11, N O V E M B E R 1 9 5 3 tation was determined. The results, calculated as before, indicated a tin t o platinurn ratio of 5 to 1 in the color reaction. A mixture of 0.60 millimole of tin(I1) and 0.10 millimole of platinum(I1) in solution was treated with phenylarsonic acid in 0.10-millimole increments. The first increment of phenylarsenic acid produced a dark brown precipitate, and 46% of the original platinum still remained in solution. The second 0.10-millimole quantity of phenylarsonic acid produced a similar amount of dark brown precipitate, and left only 1% of the original platinum in solution. The next two increments of phenylarsonic acid produced similar amounts of white precipitate. Further amounts of the reagent caused no visible change. Since two moles of phenylarsonic acid are required for the precipitation of one mole of a tetrapositive ion, the above results indicate that the platinum is in the form of a tetrapositive complex, and that one mole of tin(1V) is formed in the color reaction. The above experiment u-as repeated, starting with 0.10 millimole of platinum(I1) and 0.30, 0.10, and 0.05 millimole of tin(I1) in a series of three experiments. On the basis of the 5 to 1 mole ratio previously indicated for the reaction, these quantities of reactants should leave, respectively, 0.04,0.08, and 0.09 millimole, or 40,80, and 90% of the original platinum unreacted. -4fter adding phenylarsonic acid to the solutions in amounts in excess of requirements t o precipitate all of the colored reaction product, the platinum remaining in the solution was found t o be 38, 72, and 91 %, respectively. Considering the adsorptive properties of the phenylarsonate precipitate, these findings are in good agreement with the assumption of a 5 to 1 mole ratio in the color reaction. DETERMINATIOX O F CHLORIDE I N THE COMPLEX. LfiXtUreS O f platinum(I1) and excess tin(I1) were prepared, and the chloride content of the solution determined before and again after precipitation with phenylarsonic acid, t o determine the decrease in chloride resulting from the main reaction. The mole ratio of chloride to platinum in the reaction was found t o be very close t o 4 t o 1. A mixture of 0.10 millimole of platinum(II), 0.50 millimole of tin(II), and 0.50 millimole (an excess) of phenylarsonic acid, in sufficient hydrochloric acid to prevent precipitation, was diluted to a final acidity of 0.4 M , to permit precipitation. The precipitate was filtered, washed, and dried to constant weight. Based upon the platinum content of the precipitate, the calculated formula weight of the precipitate was 1727. On the basis of the precipitation tests outlined above, the precipitate should consist of (PtSnaCl4)(C6H5As03)2 Sn(CsHaAs08)2, for which the summation of molecular weights is 1730.
+
DISCUSSION
The spectrophotometric methods for determining the mole ratio of reactants and the results of precipitation reactions from solutions containing an excess of tin(I1) confirm the 5 to 1 ratio of tin to platinum for the principal reaction product and indicate that reaction products of higher mole ratios are not formed. The precipitation reactions require the assumption of a tetrapositive ion (PtSn4C14) or possibly its halogenated compound in solution; this complex contains platinum in zero oxidation state, formed b y reduction by one tin(I1) in the reaction. Similar zero oxidation states of metals have been shown to exist in tetracyanonickelate(0) ( I O ) and in tetracyanopalladate(0) ( 9 ) , and in the lower halide complexes of tantalum and niobium which have been ++++
1627 reported to contain metallic bonds, and which are colored, stable in aqueous solutions, and very soluble in organic solvents (21). I n this study, little information was obtained about the lower ratio complexes; a brief study of distribution equilibrium between water and capryl alcohol indicated that a reaction product of 3 to 1 ratio of tin to platinum was the predominant species in the organic phase. I t has not yet been possible to isolate any crystalline products for making x-ray studies:. ACKNOWLEDGMENT
The experimental work by B. L. Tuffly, It. F. Wilson, H. F. Young, and A. S. hleyer, Jr., is acknowledged, as is also the financial support of the United States Atomic Energy Commission, under the terms of Contract No. AT (40-1)-1037 with The University of Texas. LITERATURE CITED
Ayres, G. H., and Meyer, A. S., Jr., ANAL.CHEM.,23,299 (1951). Ayres, G. H., and Quick, Quentin, Ibid., 22, 1403 (1950). Ayres, G. H., and Tuffly, B. L., Ibid., 24, 949 (1952). Ayres, G. H., and Wells, W. N., Ibid., 22, 317 (1950). Ayres, G. H., and Young, H. F., Ibid., 22, 1277 (1950). Ibid., p. 1281. Ibid., 24, 165 (1952).
Bent, H. E., and French, C. L., J . A7n. Chem. Soc.. 63, 568 (1941)
.
Burbage, J. J., and Fernelius, W. C., Ibid., 65, 1484 (1943). Eastes, J. W., and Burgess, W. &I., Ibid., 64, 2715 (1942). Grube, G., and Gu, B., 2. Elelttrochem., 43, 397 (1937). Harvey, A . E., and Manning, D. L., J . A m . Chem. SOC.,72,4488 (1950). (14) (15) (16) (17)
Job, P., A n n , Chim. (Paris), (10)9,113 (1928). Kraus. F.. and Umbach. H.. Z . anoru. Chem.. 180. 42 (1929). Latimer, W.M., “The Oxidation States of the Elements and Their Potentials in Aqueous Solutions,” p. 204, New York, Prentice-Hall, 1938. Mellon, RI. G., ed., “Analytical Absorption Spectroscopy,” pp. 307-10, New York, John Wiley & Sons, 1950. Meyer, A. S., Jr., Ph.D. dissertation, University of Texas, May
1953. (18) Meyer, J., Kawocryk, AI., and Hoehne, K., Z . anorg. C‘hcm., 232, 410 (1937). (19) Syrokomskii, V. S., and Proshenkova, N. N., Zhur. Anal. Khim., 2,247 (1947). (20) Tuffly, B. L., Ph.D. dissertation, Universityof Texas, May 1952. (21) Taught, P. A , , Sturdivant, J. H., and Pauling, Linus, J . A m . Chem. Soc., 72, 5477 (1950). (22) Vosburgh, W.C., and Cooper, G. R., Ibid., 63, 437 (1941). (23) Wilson, R. F., Ph.D. dissertation, University of Texas, hlay 1953. (24) Yoe, J. H., and Harvey, A. E., J . Am. Chem. SOC.,70, 648 (1948). (25) Yoe, J. H., and Jones, -4. L., ISD. EXG.CHEM.,ANAL.ED., 16, 111 (1944). (26) Young, H. F., Ph.D. dlssertation, University of Texas, May 1953. RECEIVED July 6, 1953. Accepted July 30, 1953.
