Spectrophotometric Determination of Rhodium and Platinum in Plutonium MAYNARD E. SMITH 10s Alamos Scientific laboratory, University of California, 10s Alamos, N. M. ,The spectrophotometric method for the simultaneous determination of rhodium and platinum has been adapted for use with plutonium samples. N o separation from plutonium is necessary but a correction must b e applied for the absorbance of plutonium(lll) a t the wave lengths used to determine the rhodium and platinum. Alloy samples are readily dissolved in sealed glass tubes a t elevated temperatures. The standard deviation for the simultaneous determination of rhodium and platinum in plutonium solutions in the range of about 5 to 14 p.p.m. is approximately 2 relative %.
2.0
I
I
400
5 00
1.5
I .o
B
of certain chemical-metallurgical investigations carried out a t this laboratory, a need arose for the determination of small quantities of rhodium and platinum in plutonium. The spectrophotometric determination of rhodium and platinum reported by Ayres, Tuffly, and Forrester (1) is based on the color formed on adding tin(I1) chloride to solutions of these elements. Under the experimental conditions of this 11-ork plutonium is reduced to plutonium(II1) which has an absorption peak a t 665 mp. The absorbance of platinum and rhodium is negligibfe a t this wave length. The absorbance of the plutonium at the wave lengths used t o determine the rhodium and platinum (470 and 399 mp, respectively) was directly related to the absorbance of the plutonium a t 665 mp. The w v e lengths used for determining the rhodium and platinum are essentially the same as those used by Ayres, Tuffly, and Forrester ( I ) , but the procedure was scaled down for convenience in working with radioactive solutions. Approximately 1 hour was allowed for development of the platinum color when plutonium ions n-ere present in the solution. After this time, the color was extremely stable as described by the previous investigators. Inasmuch as the radiation from the plutonium used in this ivork was predominantly alpha particles, shielding was not necessary. However, because of the health hazard involved in handling plutonium, the work Tvas done in special, ventilated enclosures; rubber gloves and suitable protective clothing were worn. ECAUSE
912
ANALYTICAL CHEMISTRY
0.5
0 300
600
700
800
WAVE LENGTH, m p Figure 1. Absorption curves for tin(ll) chloride solution containing plutonium, rhodium, and platinum 1 . Pu (10 mg. per ml.)
2. Pu (10 mg. per ml.) and Rh (10 y per ml.) 3. Pu (10 mg. per ml.) and Pt (17 y per ml.)
APPARATUS A N D REAGENTS
Spectrophotometer, Beckman Model DU with 1-cm. Corex cells (for absorbance measurements), and Gary Recording, Model 11 (for absorption curves). Steel shell, for protection of sealed glass tube, construction and use described by Gordon, Schlecht, and Wichers (2). Furnace, suitable for heating 15-inch steel shell to 300' C. Glass tubing of borosilicate glass for dissolving samples a t elevated temperatures: outside diameter, 12 mm.; wall thickness, 2 mm.; length when sealed, 10 to 12 inches.
RHODIUM STANDARD SOLUTION. Add a known weight of pure rhodium foil (approximately 100 mg.) to a mixture of 4 ml. of 12M hydrochloric acid and 5 drops of 16M nitric acid in a thick-
walled sample tube of borosilicate glass. In no case should the volume of the liquid in the tube be greater than one half of the volume of the tube. Seal the tube, place in a protective steel shell, and heat a t 200' C. for approximately 4 hours. Cool the tube in a freezing mixture, open, and transfer the solution to a 100-nil. volumetric flask with several rinses of distilled water. Dilute to volume with distilled water. PLATIXUM STAXDARD SOLUTIOK. Dis-
solve a known weight of pure platinum foil (approximately 100 mg.) in aqua regia. Evaporate this solution to dryness on the steam bath and repeat three times, dissolving the residue each time in 10 ml. of 6M hydrochloric acid. Dissolve the final residue with distilled water and dilute to 100 ml. TIN(II) CHLORIDESOLUTIOK,1M. Dissolve 22.6 grams of tin(I1) chloride dihydrate in 21 ml. of 12.11 hydrochloric acid and dilute to 100 ml. with distilled water. This solution should be freshly prepared every week. DISSOLUTION OF SAMPLE
Plutonium alloys containing platinum and rhodium are conveniently dissolved by heating with a mixture of hydrochloric and perchloric acids in a sealed tube. A detailed study of this technique was reported by \Tichers, Schlecht, and Gordon (3). Transfer a weighed quantity of the alloy (100 mg. or less) to a thick-walled tube (borosilicate glass) sealed a t one end and carefully annealed. Add slowly a total of 4 ml. of 12M hydrochloric acid and, after the reaction is complete, 2 drops of 72% perchloric acid. In no case should the volume of the liquid in the tube be greater than one half of the volume of the tube. Seal off the tube a t a length of 10 to 12 inches using a gas-oxygen torch, and anneal the seal in a gas flame. Place the tube in the steel shell and add 40 grams of solid carbon dioxide. Immediately screw on the cap and tighten to prevent leakage. Test for leakage by immersing the shell under water and retighten the cap if necessary. Place the shell in a furnace maintained a t approximately 300" C. for about 3 hours. Remove the shell, cool to room temperature, and remove the tube. If the sample is completely dissolved, cool the tube in solid carbon dioxide and open. Transfer the contents to a 50-ml. beaker with several rinses of distilled water. Evaporate the solution on a steam bath to a volume slightly less than 5 ml. RECOMMENDED PROCEDURE
Transfer an aliquot of the sample not exceeding 5 ml. to a 10-ml. volumetric flask. The total amount of rhodium plus platinum in the aliquot taken should not be more than 200 y. Add 1 ml. of 12M hydrochloric acid and 1 ml. of 1M tin(I1) chloride solution. Immerse the flask in a steam bath and heat for 1 hour. Remove, add 1 ml. of 1M tin(I1) chloride, and cool to room temperature before diluting to the mark with distilled water. L4110w the solution to stand for 1 hour, then measure the absorbance a t 399, 470, and 665 mp,, using a reagent blank for comparison. T o determine the plutonium absorbance a t 470 and 399 mp, use the following equations: APU470
0.216
A666
AP" 399 = 0.157
A665
=
- 0.004 - 0.004
(1) (2)
Table I.
Simultaneous Determination of Rhodium and Platinum in Plutonium Solutions
Taken, P.P.M. Rh Pt 5.96 5.96 8.35 8.35 8.35 8 35 11.Y2 11.92 14.30 14.30 14.30 14.30
9.15 9.15 6.11 6.11 9.15 9.15 6.11 6.11 4.58 4.58 4.58 4.58
Found, %
Found, P.P.M. Rh Pt 6.20 6.20 8.32 8.39 8.74 8.60 11.92 11.92 14.24 14.24 14.38 14.38
where APu470 and are the absorbances of the plutonium a t 470 and 399 mp, respectively. Determine the net absorbance of the rhodium and platinum a t these two wave lengths by subtracting the calculated absorbance of the plutonium a t these wave lengths from the measured absorbance of the solution. The rhodium and platinum concentration of the solution may then be obtained from these net absorbances according to the method of Ayres, Tuffly, and Forrester ( I ) . RESULTS AND DISCUSSION
Correction for Plutonium Absorbance. Rhodium-plutonium and platinum-plutonium solutions were treated according to the iecommended procedure. Figure 1 shon-s that rhodium and platinum have no effect on the plutonium(II1) absorption peak a t 665 nip. To determine the dependence of the plutonium absorbance, measurements xere made on treated solutions of varying plutonium concentration. The absorbance of the plutonium a t 470 and 399 mp is a linear function of the absorbance a t 665 mp. By applying the statistical method of least squares to the data, Equations 1 and 2 were derived. The numerical values in the equations are the result of 24 absorbance determinations at each wave length made on solutions varying from 2.5 to 15 mg. of plutonium per ml. The values 0.216 and 0.157 are the slopes of the curves formed when the absorbances of the plutonium solutions a t 665 mp are plotted against their corresponding absorbances a t 470 and 399 mM, respectively. The value 0.004 is the intercept of these curves with the origin. Interferences. As expected, ruthenium, palladium, osmium, gold, and chromium (the principal substances reported t o interfere with the rhodium and platinum determination) interfered n i t h the determination in plutonium solutions. Of these substances, only gold appreciably changed
9.24 9.10 6.06 5.99 9.10 9.38 6.28 6.06 4.44 4.51 4.37 4.65
Rh
Pt
104.0 104.0 99.6 100.5 104.7 103.0 100.0 100.0 99.6 99.6 100.6 100.6
101.0 99.5 99.2 98.0 99.5 102.5 102.8 99.2 96.9 98.5 95.4 101.5
Av. 101.4
99.5
the absorbance of the plutonium a t 665 mp. The observed increase was due to the formation of colloidal gold by the reduction xi-ith tin(I1) chloride. Zirconium and uranium present in concentrations approximately 15 times that of the rhodium or platinum did not change the absorbance measurements by more than 3%.
Table 11. Per Cent Rhodium and Platinum in a Plutonium Ternary Alloy Rh Pt Sample So. 1 2
3 4
5.18 5.12 5.05 5.17
3.91 4 83 4 84 4 95
Reliability. An indication of the reliability of this method was obtained hy the simultaneous determinations of rhodium and platinum in several plutonium chloride solutions (Table I). The amounts of rhodium and platinum used mere varied, while the plutonium was kept constant a t 100 mg. The standard deviation of metal found in both cases is 2.2%. The percentage of rhodium and platinum in a ternary plutonium alloy was determined. Four 100-mg. portions of the alloy mere dissolved and treated according to the recommc nded procedure (TL'blc 11). LITERATURE CITED
(1) Ayres, G. H., Tuffly, B. L., Forrester, J. S., AXAL.CHEJI.27, 174L2 (1955). (2) Gordon, C. L., Schlecht, W. G., Wichers, E., J . Research X a t l . Bur. Standards 33, 45'7 119441. G:, Gordon, (3) Wichers, E., Schleckit, W. ,_^, C. L., Ibzd., 33, 3bd ( l Y 4 4 ) . ^^
RECEIVED for review August 30, 1957. Accepted January 15, 1958. Kork done under auspices of the U.S. Atomic Energy Commission. VOL. 30, NO. 5, MAY 1958
913
