Table I. Determination of Rhodium and Iridium in a Concentratea Wt of Fraction of Rhodium Iridium concentrate, solution found, found, g determined 7i Fire assay method 5.44 2.75 0.5866 114 5.47 2.80 0.5408 114 5.54 2.88 1/10 Wet method 5.52 2.86 0.5144 114 5.52 2.78 114 0.4975 112 5.58 2.70 5.62 2.85 0.2362 112 5.56 2.83 0.5085 114 112 5.48 2.78 5.52 2.66 0.5115 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
603
i
F e ( I I ) - + Fe(m)
!
7 5 0 0 #eq
0
s
CURVE 8
U
I
1
5
0 00
1
02
06 0I8 0 4 POTENTIAL, V o l t vs S C E
L 10
Figure 1. Coulograms for oxidation and reduction of plutonium and iron in the presence of BPSA plutonium has been the quantitative interference of iron in the determination (1). Iron interferes because the formal potentials of the Fe(I1)-Fe(II1) and Pu(II1)-Pu(1V) couples are nearly equal (< 50 mV difference) in sulfuric acid. Even small quantities of iron relative to plutonium can cause an appreciable error in the analysis since the equivalent weight ratio of Pu:Fe is about 5:l. In the past there has been no easy way of eliminating or minimizing iron interference, and either the iron had to be separated from the plutonium prior to analysis or a correction had to be applied for the iron content to obtain an accurate analysis. We have investigated the problem of iron interference in this procedure, and have modified the coulometric method to make it insensitive to relatively large quantities of iron. The modification involves addition of the disodium salt of bathophenanthroline sulfonate (BPSA) to the supporting electrolyte. Complexation of BPSA with iron shifts the formal potential of the iron couple and permits oxidation and reduction of Pu(II1) and Pu(1V) without simultaneously affecting the oxidation state of iron. BPSA was selected for this work over other phenanthroline derivatives because the reagent is available commercially and its solubility is high in dilute sulfuric acid. Previous applications of this reagent to the spectrophotometric determination of iron and as a redox indicator in volumetric titrimetry have been published (2).
EXPERIMENTAL Reagents. Aqueous solutions of BPSA were prepared from the commercial product (G. F. Smith Chemical Co., Columbus, Ohio) without additional purification. Standard plutonium solutions were prepared from high-purity metal (99.97J argon was passed through the solution before and during the elec(2) David Blair and Harvey Diehl, Talanta, 7, 163 (1961). (3) H. C. Jones, W. D. Shults, and J. M. Dale, ANAL.CHEM., 37,680
(1965). (4) T. R. Mueller, Coulometric Titrator (ORNL Model 4-4010): Prototype and Instruments for Analytical Services, A n d . Chem. Diu. Ann. Progr. Rep., January 1970, p 2. 604
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
trolysis to remove oxygen. A platinum gauze electrode (2 cm X 6 cm-45 mesh) was used for the working electrode. If erratic results or slow titrations were observed, the electrode was placed in aqua regia for 1-2 minutes, rinsed thoroughly with water, and soaked overnight in 1SMperchloric acid. Plutonium Analysis. A sample aliquot containing 1-10 mg of plutonium and up to 1 mg of iron is added to the coulometric cell. Sufficient 0.1F BPSA is added so that the final concentration will be 0.03F. Sulfuric acid is added so that the final concentration will be 0.03F. BPSA is added so that the final sulfuric acid concentration is between 0.05 and 0.4F and the total solution volume is 10 ml. The cell is placed under the electrodes, and argon is passed through the solution for 5 minutes with stirring. Plutonium and iron are reduced at a potential of $0.30 V tis. SCE. The titrator is set to terminate the reduction when the current reaches 50 PA. After this electrolysis, the solution is allowed to stand for 30 minutes to permit complete complexation of Fe(I1) with BPSA. Plutonium(II1) is then oxidized at +0.66 V us. SCE with integration of the electrolysis current. Again the titration is terminated when the current reaches 50 PA. The integrator output is obtained and the plutonium content of the sample aliquot is calculated using Faraday's law with n = 1.
RESULTS AND DISCUSSION The essential features of the modified coulometric procedure are illustrated in Figure 1. Curve A in this figure is a coulogram obtained by reduction of a sulfuric acid solution which initially contained Pu(IV), Fe(III), and BPSA. Both iron and plutonium are reduced in the potential span of $0.65 to $0.30 V DS. SCE. The initial reduction of iron and plutonium, therefore, takes place at essentially the same potential whether BPSA is present or absent (5). The failure of BPSA to shift the potential of either or both couples suggests that neither plutonium nor iron is complexed strongly by the reagent in the initial reduction. When both ions are reduced, however, and allowed to stand in the presence of BPSA, the BPSA complex for a least one of the ions slowly forms which is evident from the intense red color that develops. Curve B in Figure 1 is a coulogram obtained by oxidation of the reduced solution after allowing 30 minutes for reaction of BPSA. Two discrete oxidation steps are observed. The couple involved in the first step has a formal potential of +0.50 V us. SCE, and the integrator readout at f0.66 volt agrees within 1 with the value expected for oxidation of Pu(II1) to Pu(1V). The first step is therefore due to oxidation of Pu(III), and the potential for the couple is essentially the same as that observed in sulfuric acid alone. Failure of BPSA to change the potential of the plutonium couple in both the oxidation and reduction suggests that neither oxidation state of plutonium is complexed with BPSA. Oxidation of the BPSA complex of Fe(I1) is responsible for the second step on Curve B. The potential of the complexed iron couple is near +0.86 V tis. SCE (oxidation of excess BPSA in the solution begins at about $0.90 DS. SCE, hence the entire coulogram for the iron couple could not be obtained). Complexation shifts the potential of the iron couple such that the potential of the plutonium and iron couples are separated by 0.36 volt. This difference, although not large, is sufficient to allow oxidation of Pu(II1) without simultaneously oxidizing appreciable amounts of Fe(I1). A coulometric titration of plutonium at +0.66 V US. SCE after a prereduction in the presence of BPSA forms the basis for the determination of plutonium in the presence of iron. (5) W. D. Shults, U.S.A t . Energy Comm. Rep., ORNL-2921(1960).
Table I. Effect of BPSA Concentration (5.000 mg of Pu; 0.3F H S 0 4 , 1 mg of Fe; 30-minute reduction time) Pu found, Error, BPSA, F mg 7.227 +44.6 0.005 5.879 $17.5 0.010 0.015 5.061 +l .o 0.025 4.987 -0.3 0.030 5.012 +0.2
z
Table 11. Effects of Sulfuric Acid and Iron Concentration on Plutonium Results (4.000 mg of Pu added; 0.03F BPSA) Fe HzS04 PU found, Error, conc., added, mg F mg 4.001
