Coulometric Determination of Rhodium and Iridium in a Concentrate G. W. VanLoon and J. A. Page Department of Chemistry, Queen’s University, Kingston, Ontario, Canada IN THE ANALYSIS of the platinum metals, many problems arise due to wide variations in oxidation states, to the many possible complex forms, and to the fact that the solution chemistry is not clearly understood. One such problem has been the separation and determination of rhodium and iridium. The separation procedures now used are of three types. There are two ion exchange methods ( I , 2). There are methods based on the fact that under certain conditions rhodium(II1) may be reduced to the metal using either chemical reducing agents (3-5) or a cathode at a fixed potential (6). These methods will reduce iridium only to the 3f oxidation state and in this way the rhodium is precipitated while the iridium remains in solution. Although they are capable of giving good results, these methods are time-consuming and may involve difficult separations of the precipitated metal from the precipitant. A solvent extraction procedure has also been developed (7) which allows for the separation and spectrophotometric determination of both the metals when present in microgram amounts. The present method is suitable for the consecutive determination of rhodium and iridium in a single solution. For milligram amounts of these metals, it is a rapid and accurate method. PRINCIPLE OF THE METHOD
A previous study (8) has shown that iridium(1V) can be determined quantitatively by a controlled potential coulometric reduction to the 3f oxidation state at +0.25 V (us. SCE) in a hydrochloric acid electrolyte. At this potential, in this medium, rhodium(II1) is neither oxidized nor reduced. Furthermore, at a potential of -0.20 V (us. SCE), rhodium (111) is quantitatively reduced to the metallic state while iridium(II1) is not reduced further (9). These observations suggest that rhodium and iridium could be determined in the same solution with no chemical separation by means of two separate electrolyses. This procedure should be applicable to concentrates containing milligram amounts of the two metals. In the analysis of such concentrates, rhodium and iridium generally remain in the final solution after separation of the base metals and the other platinum metals; a method which could be applied to this solution would be useful. EXPERIMENTAL
Apparatus. The coulometry was done using the types of cells and apparatus described by Zinser and Page (IO). For (1) E. W. Berg and W. L. Senn, ANAL.CHEM., 27, 1255 (1955). (2) S. S. Berman and W. A. McBryde, Carl.J . C/zem.,36,835 (1958). (3) R. Gilchrist, Nut. Bur. Stand. (U.S . ) J . Res., 9, 547 (1932). (4) A. D. Westland and F. E. Beamish, Microclzim. Acta, 10, 1474 (1956). ( 5 ) G. G. Tertipis and F. E. Beamish, ANAL.CHEM., 32, 486 (1960). (6) W. A. E. McBryde and N. A. Graham with W. L. Orr, Tulanta, 11, 797 (1964). (7) G. G. Tertipis and F. E. Beamish, ANAL.CHEM., 34, 623 (1962). (8) J. A. Page, Talunta, 9 , 365 (1962). (9) G. VanLoon and J. A. Page, ibid., 12, 227 (1965). (10) E. J. Zinser and J. A. Page, ANAL.CHEM., 42, 787 (1970).
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the determination of rhodium, the electrolyses were done in an H-cell with a mercury cathode. For iridium analyses, the cell was a 150-ml electrolysis beaker and the cathode a cylindrical platinum wire gauze. In both cells the anode compartment was separated from the working solution by a fritted glass disk backed by agar gel saturated with potassium chloride. The reference electrode was a saturated potassium chloride-calomel type. The current integrator was standardized before each determination. Procedure. In the analysis of the concentrate, two methods were used to obtain the sample solution. The first involved a fire assay extraction of the platinum metals into an iron-copper-nickel button (11). This method can be adapted to the determination of the precious metals in a variety of ores and concentrates. The procedure was that of J. C. VanLoon and Beamish (12) (as outlined in their Figure 2) except that 0.5 gram of concentrate and 26 grams of silica were substituted for their salted flotation concentrate in the assay charge. The method removes the base metals of the button by ion exchange, quantitatively separates Ru and Os by distillation as the tetroxides, and yields a sulfuric acid solution containing Au, Pt, Pd, Rh, and Ir. The procedure was found to be time-consuming. The second method was based on the dissolution of the sample according to Zachariasen and Beamish (13) (as outlined in their Figure 1) except that 4.5 grams of Na202were substituted for Na2C03in the sintering step. The method yields a hydrochloric acid solution of the precious metals and the Ru and Os were removed by distillation of the tetroxides using NaC103 and NaBrOa as oxidants according to Payne (14).
From this point, both sample solutions were treated similarly, using the procedure of Zachariasen and Beamish (13). The Rh, Ir, and any Pd and base metals were separated from Au and Pt by a double hydrolytic precipitation. The hydroxides were dissolved in hydrochloric acid and the base metals removed on a cation exchanger. The Pd was precipitated with dimethylglyoxime, and the excess precipitant was destroyed by two treatments with 2-3 ml of 30% H2O2on a steam bath. After the final treatment, the Rh and Ir solution was evaporated to near dryness and taken up in 100 ml of 0.2M hydrochloric acid solution. The solution was then chlorinated (8) to ensure Ir(1V) and the excess chlorine removed by gentle boiling for 20 to 30 min prior to the coulometry. The Rh and Ir were determined as follows: The solution, of volume about 60 ml and 0.2M in hydrochloric acid, is transferred to the cell, deaerated, and electrolyzed using a Pt cathode at $0.25 V. The reduction of Ir(1V) to Ir(II1) proceeds smoothly to a final low background current ( = l o PA). Typically the electrolyses consumed 1.5 to 3.0 coulombs. The quantity of Ir was calculated through Faraday’s law. Blank solutions electrolyzed in the same way consumed negligible amounts of electricity. To determine the Rh, the once-electrolyzed solution was quantitatively transferred to the second cell, deaerated, and (11) (12) (13) (14)
G. G. Tertipis and F. E. Beamish, ANAL.CHEM., 34,108 (1962). J. C. VanLoon and F. E. Beamish, ibid., 37, 113 (1965). H. Zachariasen and F. E. Beamish, Tulunta, 4,44 (1960). S. T. Payne, A m l y s f , 85, 698 (1960).
Table I. Determination of Rhodium and Iridium in a Concentratea Wt of concentrate, g
0.5866 0.5408
0.5144 0.4975 0.2362 0.5085 0.5115
Fraction of Rhodium Iridium solution found, found, determined 7i Fire assay method 5.44 2.75 114 5.47 2.80 114 5.54 2.88 1/10 Wet method 5.52 2.86 114 5.52 2.78 114 112 5.58 2.70 5.62 2.85 112 5.56 2.83 114 112 5.48 2.78 5.52 2.66 1/4 5.55 2.67 114 Fire assay Wet __ Rhodium, Iridium, Rhodium, Iridium,
z
x
z
z
z
2.79 Average values 5.49 2.81 5.54 0.08 Standard deviation 0.05 0.07 0.04 a The concentrate, obtained from Falconbridge Nickel Mines Ltd., Metallurgical Laboratories, Richvale, Ontario, Canada, was a byproduct of an electrolytic refining process. The previously recorded values (12) for this concentrate are Iridium 2.78 % (Zachariasen-Bearnish) and 2.80% (Falconbridge). Rhodium 5.56 (Z-B) and 5.65 % (F).
electrolyzed using a Hg cathode at -0.20 V. The reduction of Rh(II1) to the metal proceeded smoothly but more slowly than in the case of the Ir. The final background current was =20 PA; typically the electrolyses consumed 9 to 40 coulombs of electricity and required 75 min. A correction for this background was made before the Faraday law calculation (9). The results are listed in Table I.
analysis of pure solutions of these metals. These procedures have been combined and adapted to the successive determination of rhodium and iridium in a precious metal concentrate. The somewhat larger deviations in these results are probably due to operations involved in solution preparation. Nevertheless the values obtained are comparable with those obtained by other workers using gravimetric techniques (13). The results for rhodium are lower than those reported by Falconbridge (23) in a spectrographic or spectrophotometric determination. The coulometric method includes a separation of rhodium and iridium; no other separation need be applied. The chief advantage of the coulometric method is its speed; analysis for both rhodium and iridium in the same solution can be made in about three hours. It is possible that the technique of controlled potential coulometry can be further applied to platinum metal analysis. The distillation of ruthenium provides an excellent method of separation from almost all other interferences. If a receiver solution consisting of 1 : 1 hydrochloric acid and 3 hydrogen peroxide is used to collect the distillate, it appears that the ruthenium is quantitatively present in the 4+ oxidation state. The excess H202may be removed by heating on a steam bath. Preliminary coulometric investigations have indicated that ruthenium may be quantitatively reduced from ruthenium(1V) to ruthenium(II1) by electrolysis at 0 volt using a mercury cathode. Analogous results have been reported elsewhere for similar solutions (15). This reduction might be applied to the quantitative estimation of the amount of ruthenium in such receiver solutions.
RECEIVED for review September 9,1970. Accepted November 30, 1970. This study was supported by operating grants and a fellowship from the National Research Council of Canada. Thanks are also due to the Falconbridge Nickel Mines Ltd. for supplying the concentrate and for financial support.
CONCLUSIONS
Previous studies had shown that the coulometry of rhodium (9) and iridium (8) yielded precise and accurate results in the
(15) G. Weldrick, G. Phillips, and G. W. C. Milner, analyst 94,
840 (1969).
Controlled-Potential Coulometric Determination of Plutonium in the Presence of Iron J. R. Stokely, Jr., and W. D. Shults Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
PLUTONIUM IS OFTEN DETERMINED by controlled-potential coulometry when accurate results are required. Various procedures for analyzing plutonium solutions by controlledpotential coulometry have been devised (1). One widely used procedure is based upon primary coulometric electrolysis : coulometric oxidation or reduction of plutonium between the trivalent and tetravalent oxidation states. This electrolysis can be carried out in nitric, hydrochloric, perchloric, or sulfuric acid media, with proper selection of electrolysis poten(1) W. D. Shults, Talanta, 10,833 (1963).
tials. In many instances, however, sulfuric acid is the preferred medium for the electrolysis. For example, when the titration follows depolymerization or an anion-exchange separation, the solution is fumed with sulfuric acid to remove fluoride and(or) organic impurities, is diluted, and then electrolyzed. Also samples that contain appreciable amounts of Pu(V1) are most conveniently electrolyzed in sulfuric acid because Pu(V1) can be electrolytically reduced to Pu(II1) in this medium; Pu(V1) is only partially reduced in perchloric, nitric, or hydrochloric acid solutions. A problem associated with the use of sulfuric acid as the supporting medium in the primary coulometric titration of ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
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