Spectrophotometric Determination of Rhodium with Tin( II) Chloride Simultaneous Determination of Rhodium and Platinum GILBERT H. AYRES, BARTHOLOMEW L. TUFFLYI, and JOHN S. FORRESTER The University o f Texas, Austin, Tex.
A method is given for the spectrophotometric determination of rhodium, based upon the pink to red color, absorbance maximum at 473 mp, formed by addition of tin(I1) chloride in hydrochloric acid solution. The color develops slowly at room temperature, but forms rapidly at the boiling point. The color is reproducible and stable. Wide variations in the amount of tin(I1) chloride or in the amount of hydrochloric acid are without effect on the absorbance. The optimum concentration range for measurement in 1.00-cm. cells is about 4 to 20 p.p.m. of rhodium. Ruthenium, osmium, palladium, gold, and chromium, which interfere with the determination of rhodium, are easily removed. A method is given for the simultaneous determination of rhodium and platinum from measurements of the absorbance at two wave lengths; the relative error of the simultaneous determination is approximately 1% for each element.
Tin(I1) chloride solution, prepared from the dihydrate, was 131 in tin(I1) chloride and 2.5M in hydrochloric acid. EXPERIMENTAL
Preparation of Standard Rhodium Solution. Rhodium metal powder, mixed with excess potassium chloride, was converted to the hexachlororhodate(II1) by high temperature treatment with chlorine, as described by Ayres and Young (3). Spectral Characteristics. Figure 1 shows spectral curves for rhodium(II1) solutions color-developed with tin(I1) chloride by the standardized procedure described below. I n addition to the absorption band a t 475 mp, there is strong absorption a t about 330 mp. Near 330 mp, absorbance is sensitive to small variations in the concentration of tin(I1) chloride, whereas this is not the case a t 476 mB; for this reason, absorbance measurements were made a t 476 mp in studying the influence of various factors on the color development. A region on either side of.475 mp was scanned in order to detect any possible shift in the position of maximum absorbance; no shift was observed in any case.
RHODIUM(III1
F
EW methods for the colorimetric determination of rhodium have appeared in the literature. Ayres and Young (3)
0 7 t
reported a spectrophotometric method based on the blue color (absorbance maximum a t 665 mp) formed by treating rhodium(111) solutions with excess hypochlorite; the system required about 1 hour for complete color development and also required rather close pH control. Ivanov (6) observed that rhodium(II1) salts in hydrochloric acid solution slowly developed a red color when treated with tin(I1) chloride. The reaction ha8 been used as a qualitative test for rhodium (IO, I I ) , and also for its estimation (4),but appears not to have been applied for the spectrophotometric determination of rhodium. The composition of the colored solute is not known (9). The present investigation was undertaken to study the color system produced by reaction of rhodium(II1) with tin(l1) chloride; to determine the optimum conditions for color development; to establish the range and reliability of the method; to determine the nature and extent of interferences and methods for their removal; and to test the method for the determination of rhodium, especially in samples containing other platinum metals and gold. APPARATUS
Absorbance measurements were made with a Beckman Model DU quartz spectrophotometer, using matched 1.00-cm. cells. The instrument was operated a t constant sensitivity. Calibrated weights and calibrated volumetric ware were used. REkGESTS
Spectrographically pure rhodium metal powder, obtained from A. D. Mackay, Inc., v-as used for preparation of the standard rhodium solution. Standard platinum solution was prepared from Grade 1 platinum thermocouple wire, using the procedure described by Ayres and Meyer ( 1 ) . Solutions of the other platinum elements were prepared from the metals or their compounds, obtained from A. D IIackay, Inc.; all these materials were checked spectrographically for purity with respect to foreign platinum elements. A11 other chemicals were analytical reagent grade. ~
1
Present address, Carbide and Carbon Chemicals Co., South Cliarleston,
w. Va,.
+ TlNilII
Rh, P
CHLORIDE
Pm
06-
w0.5
-
z 4
m
Eo4m a
03-
02-
01
-
01
350
I
400
I
I
I
450 500 550 WAVE LENGTH, m v
I
0
Figure 1. Spectral curves for rhodium(II1) solutions color-developed with tin(I1) chloride
A constant amount of rhodium, 10.0 p.p.m., was used in testing the effect of different variables on the color intensity. Rate of Color Development. At room temperature, about 12 hours were required for complete color development; a t temperatures near the boiling point, the full color intensity developed in 3 t o 5 minutes; additional heating for 30 minutes had no effect on the absorbance. Effect of Acid Concentration. The amount of hydrochloric acid (or perchloric acid) was varied from 1 t o 40 ml. of concentrated acid per 100 ml. of final solution. Absorbance readings were constant n-ithin the limit of accuracy of making the measurement. Effect of Tin(I1) Chloride Concentration. Using a constant amount of rhodium and of hydrochloric acid (10 ml. of concentrated acid per 100 ml. of final solution), the amount of tin(I1) chloride ieagent was varied from 5 to 50 ml. per 100 ml. of final volume. Absorbance readings at 476 mp were constant. Stability and Reproducibility. Solutions measured after 2 weeks had the same absorbance as when freshly prepared. Throughout the entire investigation, the measurements a t 4T5 mp for all solutions containing a constant amount of rhodium did not vary by more than 0.2% (absolute) transmittance. 1742
1743
V O L U M E 2 7 , NO. 11, N O V E M B E R 1 9 5 5 Table I. Ion Platinum(1V) Ruthenium(II1) Palladium(I1) Osmium(1V) Iridium(1V) Gold(II1) Chromium (VI) Iron(III)a Cobalt(I1) Copper(I1) Nickel (11) Iodide Bromide Sitrate= Sulfatea
Effect of Diverse Ions Tolerance, P,P. RI. 1
'
1
2 8 450 5 5 50
50 50 2500 75
x 104 200 x 104
1.5 1
These substances produced a decrease in absorbance, relative to rhodium alone: all others produced a n increase in absorbance.
Standardized Procedure. An aliquot of the working standard solution (or sample for analysis) to give the desired final concentration of rhodium was added to 10 ml. of concentrated hydrochloric acid. After addition of 10 ml. of 1M tin(I1) chloride, the solution was diluted to about 30 ml. with water, and boiled gently for about 10 minutes. The cooled solution was treated with an additional 10 ml. of tin(I1) chloride solution, to replace any tin(I1) that might have been air-oxidized, and finally diluted to 100 ml. The absorbance was measured a t 475 m*, using a reagent blank for comparison. In the concentration range studied, up to 24 p.p.m,, the system conforms to Beer's law. -4Ringboni plot (100 - yo?"us. log concentration) shows the optimum range, for measurement in 1.00-cm. cells, to be about 4 to 20 p.p.m. The range can be extended by any of the customary methods. Effect of Diverse Ions. Platinum is known to interfere by forming an amber to red solution with tin(I1) chloride ( I ) . Interference might be expected also from the other platinum elements, certain colored metal ions, and ions-e.g., gold-n-hich form colored reduction products n-ith tin(I1). For studying interference effects, the various elements were used in the oxidation states that would normally be present in the solution after the usual dissolution procedures. The tin( 11) reduced chroniinm(V1) to chromium(IlI), iron(II1) to iron(II), gold(II1) to blue colloidal gold, and palladium(I1) apparently to black colloidal metal. In order to establish the tolerance limit of the rhodium system for another element, solutions containing 10.0 p.p.ni. of rhodium and varying amounts of the foreign element (chloride solution for the metal ions, and alkali salts for the anions) were developed and measured in the usual way. The tolerance was taken as the largest amount of foreign substance that would give a per cent transmittance within 0.4 of that of the rhodium solution alone; for 10.0 p.p.m. of rhodium, this corresponded to an absorbance difference of about 0.005. The tolerances are given in Table I. Removal of Interfering Ions. Table I shows the principal interfering substances to be platinum, ruthenium, palladium, osmium, gold, and chromium; all of these except platinum are easily removed. Fuming down with perchloric acid, in the presence of excess chloride, volatilizes chromium as chromyl chloride, and osmium and ruthenium as their tetroxides; anions of volatile acids are also removed. Gold can be removed by extraction with amyl acetate from hydrochloric acid solution ( 7 ) . Palladium can be removed either by precipitation with dimethylglyoxime ( 6 ) , or by extraction of palladium-phenylthiourea with amyl acetate ( 2 ) . I n the latter process, rhodium is not extracted, but platinum is extracted to the extent of about 40%; a solution containing 10 p.p.m. of platinum would require several successive extractions with amyl acetate to lower the platinum concentration in the aqueous solution to its tolerance limit for the determination of rhodium with tin(I1) chloride. Platinum carried with the palladium into the organic solvent does not interfere with subsequent determination of palladium unless present in the extract in an amount about three times as much as the palladium (a).
Analysis of Mixtures. Sample 1 contained 10.0 mg. each of rhodium, palladium, gold, and iron. The solution, acidic with hydrochloric acid, was extracted with amyl acetate to remove the gold. The aqueous layer was treated with phenylthiourea solution and again extracted with amyl acetate; after separation, the organic layer was used for the spectrophotometric determination of palladium with bromide ( 2 ) ; the aqueous layer was treated with nitric acid then with perchloric acid to remove organic matter, and the rhodium was determined with tin(I1) chloride. The folloTving results, on triplicate samples, are typical: palladium found, 10.0, 10.1, and 9.8 mg.; rhodium found, 10.1, 10.1, and 10.3 mg. Sample 2 contained 10.0 mg. each of rhodium, palladium, cobalt, nickel, and copper in solution. Palladium was removed by precipitation, from hydrochloric acid solution, with dimethylglyoxime; the palladium dimethylglyoximate was decomposed with nitric and hydrochloric acids, and the palladium was determined spectrophotometrically \vith bromide. The filtrate from the palladium separation Tvas evaporated with nitric acid then fumed with perchloric acid, and the rhodium was determined with tin(I1) chloride. In typical triplicate samples, the palladium found was 10.1, 9.9, and 9.9 mg.; rhodium found !vas 10.2, 10 1, and 9.9 mg.
I
2
RHODIUM, 8 p . p m. PLATINUM, 6p.p.m.
3
Figure 2.
Spectral curves for rhodium, platinum, and mixture
When platinum is present along Tvith palladium, and the latter is separated from rhodium by treatment n ith phenylthiourea and extraction with amyl acetate, platinum is also extracted to a moderate extent. Use of this method for the removal of platinum would require several successive extractions, and subsequent separation of palladium and platinum if these elements are to be determined. Removal of platinum is unnecessary, and both rhodium and platinum can be determined by making measurements of the absorbance at two different wave lengths, appropriately chosen. SIMULTAVEOUS DETERMINATION OF RHODIUM AND PLATINUM
Solutions of platinum(IV, 11) react with tin(I1) chloride in hydrochloric acid to form orange to red solutions having maximum absorbance a t 403 mp ( I , 9 ) . The colored product formed by reaction of rhodium(II1) with tin(I1) has an absorbance minimum a t 403 mp and a maximum at 475 mp. The spectral
1744
ANALYTICAL CHEMISTRY
curves for rhodium, platinum, and a mixture of the two are shown in Figure 2. Simultaneous determination of two components from the absorbances a t two different wave lengths is based upon the additivity of absorbances of the components. Application of the spectrophotometric law to the mixture of rhodium and platinum, color-developed with tin(I1) and measured a t 403 and a t 475 mp in cells of the same optical path, gives A475
=
+
~ I C H I ~ a>Cpt
=
a?mtI + a o t
S i m u l t a n e o u s Determination of Rhodiuni and P l a t i n u m
Taken, P.P.hI. Rh Pt 2.0 4.0 8.0
10.0 10.0 2.0
4.0
(1)
nnd Ad03
Table 11.
2.0 20.0
2.0 4.0 8.0 10.0 16.0 16.0 16.0 20.0 2.0
_____ Difference, %
Found, P.P.11. Rli Pt 2.0 4.0 7.7 9.9 16.0 2.0 4.0 2.1 1R 9
Rh 0 0
2.0 4.0 8.0
4 1 0
9.Y Itj 2
0 0
18.1 1ti.l 19.7 1.9
4V.
Pt 0 0 0 1 1.2 0.6 0.ti
5
1.5
0.5
5 1.0
1.1
(2)
where a1 and a2 are the absorptivities of rhodium and platinum, respectively, a t 475 mp, and a3 arid a4 are the absorptivities of rhodium and platinum at 403 inp. Solving Equations 1 and 2 gives
allti
Proof of Additivity of Absorbances. Before applying the method, additivity of absorbances of the two components was proved by measuring the absorbance of solutions of rhodium, phtinum, and mixtures of rhodium and platinum, color-developed with tin( 11) by the standardized procedure given previously. blany mixtures were measured, from 2 to 16 p.p.m. of each elenirnt, present in equal amounts and in widely different amounts. T h e absorbances were additive over the entire region from 375 t o GOO mp, within the limits of error of the absorbance measurements (about 0.001 to 0.005 ‘absorbance, depending on the magnitude of the absorbance measured). The absorptivities, al, az, a3, and a4, were determined from the inensured absorbances, a t 403 and a t 475 mp) of color-developed solutions of each element; each absorptivity used in Equations :iarid 4 was the average from several concentrations of each clement a t each wave length, and was equal to the slope of the plot of absorbance against concentration. Analysis of Mixtures. The method was applied to synthetic mixtures of rhodium and platinum, varying widely in amounts of these elements and in their ratio. Typical results are shown in Table 11. I n the analysis of 20 samples (including the 10 samples given in Table 11),the average relative error in the estimation of rhodium was 1.1%)and in the estimation of platinum, 1.0%. The selection of the wave lengths to be used in the analysis of :t mixture is based mainly on two considerations: the spectral poqition of the peak of the absorption band of the principal absorber and/or the position a t which only one of the components +bows appreciable absorption (8). The wave lengths 403 and
475 mp were chosen on the basis of the first consideratioii. At 475 mp, however, the platinum contributes an appreciable fraction of the total absorbance, especially in mixtures containing a high ratio of platinum to rhodium. At wave lengths in the region 560 to 600 mp the platinum system shows very little absorption, but the absorption due to rhodium is only about onr third of its value a t 475 mp, Several mixtures m-ere analyzed from the absorbances measured a t 403 and 560 mp; the accurac\ of the determinations a t these Tyave lengths was the same as that obtained from measurements a t 403 and 475 mp. Either pair oi wive lengths is equally satisfactory, provided that the absorptivities for use in Equations 3 and 4 are determined accurately. ACKNOW LEDGhIENT
Part of this work was supported by The University of Texae and the United States Atomic Energy Commission, under thth terms of Contract No. AT-(40-11-1037, and part by The I-niversity of Texas Research Institute, Project N o . 440. LITERATURE CITED
(1) (2) (3) (4) (5)
(6) (7) (8)
(9)
(10) (11)
S.. Jr., ASAL. CHEM..23,299 (1951). Ayres, G. H., and hleyer, Ayres, G. H., and Tuffly, B. L., Ibid.,24, 949 (1952). Ayres, G. H., and Young, H. F., Ibid., 22, 1403 (1950). Bouvet, Pierre, Ann. pharm. franc., 5 , 293 (1947). Hillebrand, W. F., and Lundell, G. E. F., “Applied 1norg:aiiii. Analysis,” p. 281, Wiley, New York, 1929. Ivanov, V. N., J. Rum. Phys.-Chem. Soc., 50, 460 (1913). Lehner, V., and Kao, C. H., J . Phus. Chem., 30, 126 (1926). Mellon, hI. G., ed., “Analytical Absorption Spectroscopy,” 11. 493, Wiley, New York, 1950. lleyer, A. S., Jr., and Ayres, G. H., J . Ani. Chern. Soc., 77, 2671 (1955). . m. Thompson, S. O., Beamish. F. E.. and Scott, AI., I N U E CHEM.,ANAL.ED..9, 420 (1937). Wolbling, H., Ber., 67, 7T3 (1934).
RECEIVED for review February 25, 1955. Accepted
.iugust 1, lY55. Condensed, in part, from a dissertation by Bartliolomeff L. Tuffly submitted tu the Graduate School of The University of Texas in partial fulfillment of t h r requirements for the degree of doctor of philosophy. 1952. Presented in part a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroacopy, Pittsburgh, Pa., 1953.
