Spectrophotometric Determination of Ruthenium ROBERT
P.
LARSEN and LAURIDS
E.
ROSS
Chemical Engineering Division, Argonne National Laboratory, lemonf, 111.
b Conditions for the spectrophotometric determination of ruthenium as perruthenate have been established. Stable solutions of perruthenate are obtained only when the tetroxide i s distilled into dilute sodium hydroxide (1M ) containing hypochlorite. At higher hydroxide concentrations (6M) perruthenate i s reduced rapidly by water to ruthenate even in the presence of hypochlorite. From 200 to 400 y of ruthenium can be determined to an accuracy of +2% at the 95% confidence level. An improved procedure for the distillation of ruthenium tetroxide which i s particularly rapid and clean, uses a slurry of sodium bismuthate in dilute sulfuric acid as the oxidant.
S
methods for the determination of ruthenium in alkaline media have been reported by hfarshall and Richard ( 4 ) , Stoner (C), and Connick and Hurley (9). Rfarshall and Richard based their method on the color of the ruthenate ion in 1M sodium hydroxide after dissolving the sample by an alkaline nitrate fusion. Stoner’s method is based on the color of the perruthenate ion formed when distilled ruthenium tetroxide is caught in an alkaline hypochlorite medium. He reported that the perruthenate method n-as three times as sensitive as the ruthenate method. Connick and Hurley studied the reactions and spectra of the (VI),(VII), and (VIII) oxidation states of ruthenium and indicated that they used the spectra for quantitative work, but reported no details. Although the ruthenate and perruthenate methods are less sensitive than many methods using the intense colors formed in acid media between di-, tri-, and quadrivalent ruthenium and such reagents as thiourea (S), thiosemicarbazide (8), and 1,lO-o-phenanthroline, (1) they are, in general, easier to execute. The complex and contrary behavior of ruthenium in acid media necessitates meticulous control of time, temperature, and all ionic concentrations before and during development to obtain acceptable precision. Even then, undetected minor variations in reagent compositions may interfere in the complex equilibria of these solutions. In an oxidizing alkaline medium, the chemistry of ruthenium is limited to the uncomplexed sexivalent and septivalent PECTROPHOTOMETRIC
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ANALYTICAL CHEMISTRY
ions, and n-ide variations in the preparation of the sample and the concentrations of reagents can be tolerated with little or no effect on the result. The only satisfactory method for separating ruthenium from interferences is the distillation of the tetroxide, and the most effective catch solution is strong alkali. If some sensitivity can be sacrificed, it is advantageous to use this catch solution directly for the spectrophotometric determination. A method based on the color of the ruthenate ion in 9M sodium hydroxide had been used by the authors, but occasionally, for no apparent reason, the ruthenate was rapidly reduced to the insoluble dioxide before the absorbance could be measured. Stoner’s perruthenate method which incorporates hypochlorite as a holding oxidant was used in an attempt to eliminate the color instability. However, absorbance measurements a t 385 mp (perruthenate) and 485 mp (ruthenate) varied with the concentrations of hydroxide and hypochlorite in the catch solution, and the effect of the concentrations of these ions on the oxidation state of ruthenium had to be studied to establish a more satisfactory set of conditions for the determination. Because the molar absorbance indices of ruthenate and perruthenate a t their absorbance maxima are comparable, precision, color stability, and convenience were the criteria for selecting the best method. Distillation into a 1M sodium hydroxide-0.05M sodium hypochlorite solution gave solutions in which more than 99% of the ruthenium rras in the septivalent state, while distillation into 13M sodium hydroxide containing no hypochlorite resulted in a solution in which 100% of the ruthenium was in the sexivalent state. Between these two extremes unstable mixtures of the sexivalent and septivalent states are Table 1. Reduction of Perruthenate to Ruthenate as Function of Sodium Hydroxide Concentration Time, Ru(V1) in NaOH, yo Hours 1M 2M 3M 4M 6M 22 0.2 1 6 41 70 1 2 76 23 6 41 3 3 .. 55 72 97 24 30 95 100 100 95 48 100 .. .. 98 .. 72 50 .. ..
obtained. The use of dilute alkaline media and the superior color stability make the perruthenate method preferable to the ruthenate one. A new procedure for the distillation of ruthenium tetroxide, which is simple and rapid, and requires no volatilization of oxidant, has also been devised. Complete volatilization is effected from 6 N sulfuric acid-sodium bismuthate slurries in less than 5 minutes’ gentle boiling. EXPERIMENTAL
Effect of Hydroxide Concentration on Ruthenium Oxidation State. T o
test the effect of hydroxide concentration on the oxidation state of distilled ruthenium, 25 mg. of ruthenium was distilled as the tetroxide into 15 ml. of 1M sodium hydroxide, and diluted to 25 ml. with 1M hydroxide to form a 0.010M stock solution of perruthenate. Three-milliliter aliquots were then diluted to 25 d.in 1, 2, 3, 4, and 6M sodium hydroxide. These solutions were scanned a t frequent intervals with the Gary spectrophotometer. The results are given in Table I. The spectra of the 1M solution after 10 minutes (9970 perruthenate), the 6 N solution after 3 days (100% ruthenate), and the 111 solution after 3 days (a mixture) are given in Figure 1. Because the ratio of the absorbance a t 465 mp t o that a t 385 mp for ruthenate was somewhat higher (2.05) than the values reported by other in5
I
500
WAVE
400
30 0
LENGTH, m p
Figure 1. Spectra of ( 1 ) ruthenium(VI), (2) ruthenium(V1and VII), and (3) ruthenium(V1I) in 1M sodium hydroxide
vestigators (4, 6) spectra were also taken of other solutions judged to contain only ruthenate. Identical spectra were obtained when the tetroxide was distilled into 9M and 13M sodium hydroxide catch solutions, when the 9M hydroxide solution was boiled and diluted to lM, when a I M hydroxide solution of perruthenate mas boiled, and when spectra were taken over a period of several days while ruthenium dioxide mas precipitating from a 6M hydroxide solution. When unboiled 9M hydroxide catch solutions are diluted to 1111 the absorbance a t 385 m p is markedly increased. There is a similar increase in absorbance when dilute peroxide is added to a solution prepared by boiling the 9M hydroxide solution and diluting to 1M. Apparently the hydrogen peroxide formed in the reduction of the tetroxide and/or perruthenate by water will oxidize ruthenate to perruthenate when a strongly alkaline catch solution is diluted. The small amount of ruthenate formed d i e n milligram quantities of tetioxide were caught in 1M sodium hydroxide (Table I) can probably be ascribed t o reductants in the hydroxide solution. When the amount of tetroxide distilled was decreased by factors of 10 and 100, the concentration of ruthenate in the catch solution remained the same while the perruthenate disappeared. Such impurities would also explain the nearly pure ruthenate which Silverman and Levy (6) obtained when they caught tetroxide in 111l hydroxide. Catch solutions prepared from the commercial reagent grade concentrate which is marketed in polyethylene containers, or catch solutions stored in the laboratory for prolonged periods in polyethylene bottles often contain enough impurities to reduce a significant fraction of the 0.2 to 0.4 mg. of distilled ruthenium to the insoluble dioxide before completion of the analysis. Pellets of reagent grade sodium hydroxide are therefore used t o prepare the catch solution and glass is used for storage. Effect of Hypochlorite on Ruthenium Oxidation State. If sodium hypochlorite is added to a 1M hydroxide catch solution, no reduction of perruthenate t o ruthenate occurs for a t least a week. At higher hydrovide concentrations-Le., 6M-the ruthenium catalyzes decomposition of the hypochlorite. When a 6 M hydroxide solution, 0.001M in ruthenate, was made 0.05M in sodium hypochlorite, 46% of the ruthenate was oxidized to perruthenate over a 3-hour period. After 24 hours the solution again contained only ruthenate. When an identical solution was made 0.01BI in hypochlorite, 17% of the ruthenate was oxidized to perruthenate in 30 minutes
Figure 2. Apparatus for distillation of ruthenium tetroxide
with no evidence of perruthenate 24 hours later, There is no evidence that any perruthenate is oxidized to tetroxide in 1M hydroxide solution, which is 0.01 to 0.5M in hypochlorite. There is no decrease in absorbance a t 385 mp as there would be if oxidation occurred, and no loss of the volatile tetroxide after boiling these solutions. However, the addition of a large excess of 5% sodium hypochlorite solution and boiling results in complete volatilization of the ruthenium. The ratio of the absorbance a t 465 to the absorbance a t 385 mp for pure perruthenate is 0.125. This agrees within the limits of experimental errors with a value of 0.124 reported by Connick and Hurley (2). Distillation of Ruthenium Tetroxide. I n the distillation of ruthenium tetroxide, it is desirable to remove all reductants before the distillation is begun and to keep the distillate free of the oxidant and its decomposition or reduction products. Reductants may interfere in the distillation by reducing the tetroxide in the vapor phase or the catch solution; oxidants, such as perchloric acid, may interfere not only in the color development by introducing unknown quantities of acid into the receiver, but also in the trapping of the tetroxide by localized neutralization of the alkali. If ruthenium samples are first fumed with sulfuric acid to remove such volatile reductants and complexants as chloride, bromide, oxalate, nitrate, and nitrite, the ruthenium can be quantitatively volatilized in less than 5 minutes of gentle boiling from a 6N sulfuric acid-sodium bismuthate slurry. Osmium, which also forms a volatile tetroxide, is removed by nitric acid oxidation in the fuming operation. Using
ruthenium tagged with ruthenium-lo6 (365 d), gamma assays of the still pots after distillation routinely showed them to contain less than 0.5% of the activity. At higher sulfuric acid concentrations, the bismuthate is reduced by water before the ruthenium is completely distilled. In 8 N acid partial volatilization can be obtained; in 12N acid the bismuthate is reduced so rapidly that little or no volatilization of the ruthenium occurs. In perchloric acid the behavior is similar. Bismuthate has been used with perchloric acid as an oxidant for ruthenium by DeFord (3) and Stoner (6) and has been unsuccessfully tried in sulfuric acid by Banks and O'Laughlin (1). The effective oxidant in both DeFord's and Stoner's work was fuming perchloric acid, because water will reduce bismuthate rapidly a t the concentration of perchloric acid which they recommend. Banks and O'Laughlin did not report the details but were unable to get complete volatilization after 1 hour's boiling. DeFord (3) has pointed out the importance of the sulfuric acid fuming for removing reductants from the solution. The fuming is probably even more important for destroying the strong chloride and nitroso complexes which are very hard to oxidize. Once the sample has been fumed, several oxidants besides bismuthate appear to work equally well, particularly periodate and ceric. Bromate and permanganate appear definitely inferior. The apparatus for these distillations is shown in Figure 2. Quantitative trapping of the tetroxide is best attained by using a condenser in the down leg, and keeping the hydroxide receiver a t 0" C. Material balances on six distillations using tagged ruthenium were between 99.7 and 100%. The reaction between 1N hydroxide and ruthenium tetroxide is slow, particularly a t 0" C., and the quantitative recovery depends almost entirely on condensation. If the receiver is not kept cold, losses can easily occur. Because ruthenium tetroxide is readily reduced and deposited on the walls of the still as dioxide by all manner of reductants, and the dioxide catalyzes further reduction, particular care must be taken to ensure the cleanliness of the apparatus. Washing the still with hot sodium hypochlorite is the most effective way to remove both deposited ruthenium dioxide and the greases which accumulate on dry glassware from prolonged contact with laboratory air. REAGENTS
Only reagent grade materials were used. The sodium hydroxide solutions should be prepared from reagent grade VOL. 31, NO. 2, FEBRUARY 1959
177
pellets, not commercially available concentrate, and should not be stored in polyethylene. The ruthenium chloride solutions were standardized by ignition of the chloride to dryness, reduction in hydrogen, and weighing as the metal. PROCEDURE
Fume aliquots containing from 100 to 300 y of ruthenium with 3 ml. of sulfuric acid to remove chloride and nitrate. If nitrate is present, add a fourfold excess of 12M hydrochloric acid to prevent the premature volatilization of the tetroxide. Transfer the sample with water to a still of the type shown in Figure 2 and add a slurry of 0.5 gram of sodium bismuthate in 6N sulfuric acid with the aid of several 6N acid washes. Add water if necessary to keep the acid normality a t 6 or less. Adjust the air supply to about 2 bubbles per second, and distill the ruthenium by heating the mixture to boiling for about 5 minutes. Catch the condensate in 15 ml. of iced 1N sodium hydroxide to which 1 ml. of 5% sodium hypochlorite has been added. Warm the catch solution to room temperature and dilute to volume with 1N sodium hydroxide. Measure the absorbance in 1-em. cells at 385 mp against a reagent blank. If 5-cm. cells are used, 40 y of ruthenium can be determined in the initial aliquot. Calculate the concentrations by comparing the absorbance measurements for the samples with the values obtained for a pair of standards on the same day. There are small day to day variations in the standard values as a result of unavoidable reduction of the tetroxide to dioxide by mater betn-een the still pot and the receiver. The suspended dioxide can be removed by prolonged centrifugation but it is unnecessary. For any set of standards determined on the same day by the
same analyst, there is no observable deviation from Beer’s law.
Dissolution of Uranium-Ruthenium Alloys. Uranium alloys containing u p to 5% ruthenium have been dissolved in a 4 t o 1 hydrochloric t o nitric aqua regia. The dissolution is carried out in a n Erlenmeyer flask by covering the sample with water and adding the acid mixture as rapidly as the vigor of the reaction will safely permit. When hydrogen evolution has ceased, a fourfold excess of acid is added, and the sample boiled to solubilize the ruthenium residues. If the dissolution is not pushed, or if the aqua regia is added sparingly, the ruthenium will not dissolve completely. When this occurs, it is necessary to separate the finely divided ruthenium by a tedious centrifugation, dissolve it in a sodium hypochlorite-sodium hydroxide mixture, acidify with hydrochloric acid, and combine this with the rest of the solution. DISCUSSION
The precision obtained in the analysis of uranium alloys for ruthenium using this method has been very good. In 12 duplicate determinations selected a t random from the files, the precision a t the 95% confidence level was =t1.5% and the maximum deviation was 2.0%. Although no exacting confirmation of the accuracy of the method has been attempted, the completeness of the distillation as checked by the use of tagged ruthenium, the simple nature of the colored ruthenium species, and the precision also indicate that the accuracy is good. Negative bias may occur from carelessness in the distillation steps, but this is readily detected by obvious de-
creases in the precision. One independent accuracy check mas made against a ruthenium metal powder sample with a reported purity of 99 %. A weighed amount was dissolved in sodium hypochlorite, acidified with hydrochloric acid, and diluted to volume. By analysis the metal was 98.5% ruthenium. The molar absorbance index obtained for perruthenate at 385 mp is 2150 10, for ruthenate at 465 mp, 1730 It 10. The values reported by Connick and Hurley ( 2 ) at the same absorbance maxima appear about 47, high. Their value for ruthenium tetroxide a t 315 mp is also 4% higher than the values reported by TT’ehner and Hindman (7) and Silverman and Levy (6).
+
*
ACKNOWLEDGMENT
The authors thank Luke DeGraff and Donald AI. RIacDonnell for technical aid in preparing this paper. LITERATURE CITED
(1) Banks, C. V., O’Laughlin, J. W,, AKAL.CHEM. 29. 1412 (19571.
Kansas 1948.
(4) Marshall, E. D., Richard, R. R., ANAL. CHEM.22. 795 (19,501. (5) Silverman, M.’ D., Leiy, H. A,, Oak Ridge Natl. Lab., “Studies of
Upper Valence States of Ruthenium in Aqueous Solutions,” ORNL 746 (August 1950). ( 6 ) Stoner. G. A.. ASAL. C H E h f . 27. ‘ 1186 (1955). (7) Wehner, Philip, Hindmnn. J. C., J . Am. Chem. SOC.72, 3911 (1950). (8) Yaffe, R. P., Voigt, A. F., Ibid., 74, 5043 (1952). RECEIVEDfor reviem June 30, 1958. Accepted September 26, 1958.
High-Sensitivity, Recording, Scanning Flame Spectrophotometer M.
T. KELLEY,
D. J. FISHER, and H. C. JONES
Analytical Chemistry Division, Oak Ridge National laboratory, Union Carbide Nuclear Co., Oak Ridge, Tenn.
b The design and performance of a high-sensitivity, recording, wave length-scanning, flame spectrophotometer are described. Considerations are discussed that govern choices of recorder, monochromator, multiplier phototube, and filter components. Special advantages of this instrument include gains in precision and in dependability resulting from the use of a chart recorder, the reduction of hazards arising from the analysis of 178 *
ANALYTICAL CHEMISTRY
radioactive samples, exceptional performance in the red spectral region, great stability and sensitivity, unusual operational versatility, minimizing of anion and salt interferences by the high sample dilution possible, and reduction of spectral and flame background interferences by use of a good monochromator. Generally, the relative standard deviation for analyses is less than 1%, and linear calibration curves are obtained.
T
of the high-sensitivity, recording, scanning flame spectrophotometer described has evolved from that of single-beam instruments designed and built at the Oak Ridge National Laboratory in February 1953 (6) and from the design of a doublebeam flame spectrophotometer built in June 1952 (4). Two instruments of the latest design, described herein, have been built and are in use: ORNL Models Q-1457A and Q-1887. ComHE DESIGN
