Spectrophotometric Determination of Rhodium in Uranium-Rhodium Alloys ROSS D. GARDNER and ALVIN D. HUES 10s Alamos Scientific laboratory, University o f California, 10s Alamos, N. M .
b Rhodium in uranium-rhodium alloys can b e determined without separation by measuring the absorbance a t 520 mp of the red color formed by the addition of stannous chloride in hydrochloric acid. The procedure is rapid and only a few metals interfere a t concentrations equal to or less than that of the rhodium. One hundred determinations on 0.02 to 0.2 mg. of rhodium in the presence of 10 mg. of uranium showed a relative standard deviation for individual values of 0.870.
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methods for the determination of rhodium in uranium-rhodium alloys were investigated because of the need for a more rapid, sensitive method than the gravimetric procedure. The colorimetric methods available for rhodium were reviewed by Beamjsh and McBryde ( 2 ) , who discuss the advantages and disadvantages of several procedures including those using 2-mercapto-4,5-dimethylthiazole. 2-mercaptobenzoxazole, stannous chloride, sodium hypochlorite, and the color of the rhodium alone in strong acids. The use of stannous chloride as a color reagent for rhodium was recommended by Ivanov (3) in 1918. He used it for qualitative tests and for quantitative measurements by visual comparison n-ith standard solutions. Sandell (5) describes a procedure for rhodium, using stannous chloride, and more recently Ayres, Tuffly, and Forrester (1) and Smith (6) have proposed spectrophotometric procedures using the same color reaction. This work, which describes the determination of rhodium in uraniumrhodium alloys, is based on the stannous chloride method recommended by Ayres, Tuffly, and Forrester (1). Changes were made to prevent the interference of large amounts of uranium. OLORIMETRIC
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WAVE LENGTH, m p
-Rhodium (10 -y/ml.) --- Uranium (vi)( 2 0 mg./ml.) -.._ (1’4 ( 2 0 mg./mt.) . - .- Uranium Uranium treated according to procedure (20 mg./ml.) from rhodium chloride and standardized gravimetrically. Standard uranium solution was made by dissolving a weighed amount of pure metallic uranium in hydrochloric acid and hydrogen peroxide. Stannous chloride solution was made by dissolving 22.6 grams of analytical reagent grade stannous chloride dihydrate in 21 ml. of 12M hydrochloric acid and diluting to 100 ml. with distilled water. RECOMMENDED PROCEDURE
Absorbance curves were obtained with a Cary recording spectrophotometer, Model 11. Further absorbance measurements were made with a Beckman Model D U spectrophotometer using 1cm. Corex cells. Calibrated volumetric ware was used throughout. Standard rhodium solution was made
Dissolve a weighed sample of the uranium-rhodium alloy with concentrated hydrochloric acid and 30% hydrogen peroxide, using a hot plate to finish the dissolution after the initial reaction has subsided. Dilute the solution with water to volume in a volumetric flask of such size that an aliquot
ANALYTICAL CHEMISTRY
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Figure 1 , Absorbance spectra of rhodium and uranium
APPARATUS AND REAGENTS
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of 5 ml. or less will contain about 0.1 mg. of rhodium. Place such an aliquot in a 10-ml. volumetric flask with 1 ml. of stannous chloride solution and 2 mi. of 6:V hydrochloric acid. Dilute with water to about 8 ml., swirl the flask to mix the solutions, and place the flask in a steam bath for 30 minutes. Cool in running water to room temperature, add another 1 ml. of stannous chloride solution, and make up to volume with water. Mix thoroughly, let stand 15 minutes, and measure the absorbance of the solution against a reagent blank a t a wave length of 520 mP. Calibration Curve. Repeat the procedure with known amounts of standard rhodium solution plus a fixed amount of standard uranium solution corresponding t o the amount of uranium in the sample aliquots. Measure the absorbance a t 520 mu
against a reagent blank as before. Plot t h e absorbance as a function of t h e rhodium concentration or calculate a factor t o convert absorbance t o milligrams of rhodium. Slightly varying values were obtained with stannous chloride from different sources; therefore, the calibration curve should be checked each time a new stannous chloride solution is used. RESULTS AND DISCUSSIONS
Choice of Wave Length. Figure 1 shows absorbance curves obtained with a Cary recording spectrophotometer for rhodium(II1) and uranium(VI) solutions which had been treated according t o t h e procedure. This procedure reduces some of t h e uranium t o (IV). Figure 1 also shows absorbance curves obtained for untreated solutions of uranium(V1) and uranium(1V). The rhodium maximum a t 475 mp is in agreement with the findings of Ayres, Tuffly, and Forrester ( 1 ) . The use of this wave length, however, mould involve the subtraction of a sizable uranium blank, which would be complicated by the uncertainty regarding the degree of reduction of the uranium from (VI) to (IV). While the absorbance a t 475 n i w of the developed rhodium solution is stable with respect to time, temperature, and the concentrations of hydrochloric acid and stannous chloride ( I ) , Moore (4) has shown that the reduction of uranium with stannous chloride is very sensitive to these factors. By using a wave length of 520 mp (with some sacrifice in rhodium sensitivity) the absorbance due to uranium becomes very small. If all the uranium were reduced, its absorbance would be appreciable (Figure l), but under the conditions of the procedure not enough reduction takes place for slight variations t o be significant and a reliable blank can be determined for even a large amount of uranium. While reduction of the uranium may continue, a t room temperature it is so s l o ~that, after the first 5 minutes, samples showed no significant increase in absorbance in the next .hour of standing after color development. Effect of Diverse Ions. Table I shows t h e effect of certain ions a t 520 mp expressed in percentage recovery of 0.1 mg. of rhodium in the presence of 10 nig. of uranium. Of the metals tried, only other platinum metals, gold, copper, and molybdenum caused more than 1% error when present in concentrations equal to t h a t of t h e
rhodium. No study was made of methods of removal, because none of the metals which interfered seemed likely to occur to an appreciable extent in the alloys for which this procedure was developed. Ayres, Tuffly, and Forrester ( I ) suggest methods for removing most of these metals. Table I .
Ion
Effect of Diverse Ions ITt . Taken, Found,“
Sitrate Sulfate Perchlorate Phosphate Titanium(1V) Iron( 11) Cobalt(111) Copper(I1) Chromium(111) Nickel(I1) Cerium(IV) Ruthenium Palladium Iridium Platinum Osmium Gold Molybdenum(V1) Vanadium(V)
Mg.
10 600 10 10 10 10 1 0 10 10 0 10 0 10 0 10 1 10 0 0 0 0 0 0 10 0 10 0 10
1 1 1 1
1 1 1 1 1 1 1 1
70
83 85 99 92 99 96 98 100 99 134 100 76 101 136 101 101 100 101 102 164 100 121 102 123 143 103 116 101 102
2 6 9 8 7 1 3 3 9 6 6 2 8 3 0 9 6 0 5 7 5 6 1 6 0 2 8 0 2
Uranium( IV) a 0.1 mg. of rhodium taken in each case. Range. Emphasis in this work was on 1% rhodium alloys; consequently t h e calibration curve prepared was made from d a t a obtained from solutions containing 10 mg. of added uranium and various amounts of rhodium. At this concentration t h e line went through t h e origin, indicating t h a t there was no appreciable absorbance due t o uranium. Determinations made in t h e presence of 100 and 200 mg. of uranium showed an absorbance due to the uranium of 0.003 and 0.007, respectively. Therefore, if more than 100 mg. of uranium is present in the sample solution, a correction should be made for the uranium. X o determinations were made with less than 0.02 mg. of rhodium or with more than 200 mg. of uranium. Reliability. I n determining t h e calibration curve, 20 determinations were made with each of the following amounts of rhodium: 0.0198, 0.0495,
0.0954, 0.1449, and 0.1909 mg. The uranicni concentration was kept constant a t 10 mg. Cells of 1-em. light path gave a n average absorbance of 0.286 per 10 p.p.m. of rhodium for the 100 determinations. The standard deviation for t h e individual values was 0.0023 absorbance unit or 0.870. I n the analysis of routine samples duplicate weighings are made from each sample and triplicate aliquots are taken from the solution resulting from each weighing, giving six values for each sample. The average relative standard deviation ithin each group of six for 12 samples recently analyzed is 0.43y0. A comparison of the results obtained for actual samples by this method and by a gravimetric method is shown in Table 11.
Table II.
Results for Uranium-Rhodium
Alloys
Rhodium Found, 70 GraviSample Spectrophotometric metric 1.09,1,11,1.09 1,09,1,10,1.09
1.10 1.09
1,08,1.07,1.07 1.09,1.09,1.09
1.10 1.09
3
1 03,1.04,1.03 1.04, 1 . 0 4 , 1 . 0 4
1.03 1.04
4
1.04,1.04,1.04 1.04,1.04,1.04
1.04 1,04
5
1.03,1.04,1.03 1.03,1.03,1.03
1.03 1.02
1
-7
The time required for a set of 12 to 18 determinations is about 3 hours. ACKNOWLEDGMENT
The authors express appreciation for the assistance received from M. E. Smith, W, H. Ashley, and C. F. hletz. LITERATURE CITED
(1) .&yres, G. H., Tuffly, B. L., Forrester, J. S.,k A L . CHEM.27, 1742 (1955). (2) Beamish, F. E., hfcBryde, W.A. E., Anal. Chim. Acta 9,349 (1953). (3) Ivanov. V. N., J. Russ. Phys.-Chem. hoc. 50, I, 460 (1918). (,4-,) Moore. R. L.. J. Am, Chem. SOC. 77, 1504’(1955). ‘ (5) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., P . 523, Interscience, Sew York, 1350.(6) Smith, hf. E., ~ A L CHEM. . 30, 912 (1958). RECEIVEDfor review March 30, 1959. ilccepted April 28, 1959. Work done under auspices of the U. S. Atomic
Energy Commission.
VOL. 31, NO. 9, SEPTEMBER 1959
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