February, 19583 QUATERNARY HALIDES BY PAPER CHROMATOGRAPHY 75 Precise Determination of Plutonium by Differential Spectrophotometry BY G. PHILLIPS (Analytical Chemistry Group, A .E.R. E., Harwell, nr. Didcot, Berks.) A method is described for the determination of milligram amounts of plutonium by differential spectrophotometry with a precision (u) of & 0.05 per cent. The sample is dissolved in hydrochloric acid and maintained in the reduced state by the presence of hydroxylamine hydrochloride in the solution. The relative absorbancy of the sample is then measured against that of an accurately known standard solution at 5650 A. The optimum solution conditions to give highest precision have been selected and the effect of the presence of some foreign cations has been investigated.THE metallurgical investigation of plutonium alloys necessitates the development of accurate methods of analysis for that element. A gravimetricl and volumetric methods213 are avail- able, but are subject to interference from other elements. Spectrophotometry offers the possibility of determining the plutonium content of an alloy with the minimum of chemical pre-treatment, a factor of some importance when dealing with radio-toxic materials. To achieve absorptiometrically the accuracy normally required for major constituents of an alloy, a differential t e c h n i q ~ e * ~ ~ ~ ~ must be employed. Differential absorptiometry with use of a Spekker absorptiometer has been employed in the analysis of plutonium alloys by Atkins and Jenkins,' but it was considered that the use of a spectrophotometer offered the advantage of improved precision, e.g., uranium has been determined with a precision of 1 part in 1000.889 PRELIMINARY CONSIDERATIONS Aqueous solutions of plutonium may contain the metal in the ter-, quadri- and sexa- valent forms, or less commonly the quinquivalent form, the spectra of which all show the characteristic narrow absorption bands.1° Chemically it is convenient to work with the lowest valency that is relatively stable in acid solution in the presence of excess of reductant, e.g., hydroxylamine hydrochloride.Plutonium111 has narrow peaks at 5650 A (E = 35.5) and 6030 A (E = 35.0); the peak at 5650 A was selected for a number of reasons. Although previous investigators have usually worked at 6030 A, they have done so to permit the deter- mination of plutonium111 to be made in the presence of plutoniumIv and plutoniumv1.For the determination of total plutonium, it is not necessary to employ a wavelength that is characteristic of one valency state, and hence it is better to work at 5 6 5 0 ~ . From the instrumental point of view, the transmission of the optical system of the Beckman spectro- photometer is at a maximum at about 5000 A, and so narrower slit widths can be used, and this results in a greater intrinsic scale length. Finally, the peak at 5650 A has a lower tem- perature coefficientlo and is less subject to changes of anion concentration.ll It will be shown that Beer's law is obeyed up to absorbancies of 2.0, when the slit width is 0-34 mm. EXPERIMENTAL A series of standard plutonium solutions having concentrations of from 5 to 40 mg per ml was prepared by dissolving weighed amounts of the pure metal in dilute hydrochloric76 PHILLIPS : PRECISE DIETERMINATION OF Wol.a3 acid. Impurity analyses on the metal in other laboratories had revealed less than 0-1 per cent. total weight of all likely impurities. The metal was freshly cut in an atmosphere of argon and was weighed before appreciable surface tarnishing had occurred. The solutions were finally diluted to a known volume in molar hydrochloric acid and 5 per cent. w/v hydroxyl- amine hydrochloride at 25" C. By using aliquots from the standard solutions and diluting them to 2500ml with a molar solution of hydrochloric acid containing 5 per cent. w/v hydroxylamine hydrochloride, a series of calibration curves was prepared with use of reference standards of increasing concentration and by adjusting the slit width as necessary to achieve balance (see Fig.1). These cali- bration curves show marked deviations from linearity at the higher concentrations. By using methods described by Hiskey and Young, the optimum concentration of the reference solution to give greatest precision was calculated. The calculations indicate that the optimum concentration is in the region of 8 mg per ml and it is seen that, at slit widths greater than 0-35 mm, the beam band width is greater than the absorption band width at 5650 A with consequent loss of precision. An accurate calibration graph was next prepared with use of a reference standard having a concentration of 8.23 mg per ml and a series of solutions of various concentrations up to 13-90 mg per ml.All these solutions were made to volume in molar hydrochloric acid and 5 per cent. w/v hydroxylamine hydrochloride at 25" IfI 0.2' C. This procedure is known to give plutoniumII1. Concentration of plutonium. mg per ml Fig. 1. Absorbancy results for solutions of plutonium111 The cell compartment of the spectrophotometer was also maintained at this temperature during absorption measurements. Concentration of plutonium, mg per ml . . . . 8-23 9.49 10.27 11.12 12.84 13.90 Relative absorbancy (l-cm cells and slit width of The results were as follows- 0.34mm) . . .. .. .. .. . . 0.000 0.180 0.296 0.421 0.659 0.820 Absolute absorbancy . . .. .... . . :La240 1.420 1.536 1.661 1.899 2-060 From the results it is seen that when a reference standard containing 8-23 mg of plutonium per ml is used the concentration of an unknown sample in the range 8-23 to 13.90 mg per ml can be determined with a precision (a) of kO.O!i per cent. It was convenient for working purposes to calculate a calibration factor from the values given above. The calculation was as follows- A Concentration of plutonium, mg per ml . . 1.26 2.04 2-89 4.61 5-67 Total = 16.47 A Absorbancy a . .. .. . . 0.180 0.296 0.421 0.659 0.820 Total = 2.376 Calibration factor = - = 6.93 mg per ml per unit of absorbancy. 16.47 2.376 EFFECT OF VARIATION OF CONDITIONS- to decide how critical slight variations might be. The solution conditions used were selected more or less arbitrarily and it was necessaryFebruary, 19581 PLUTONIUM BY DIFFERENTIAL SPECTROPHOTOMETRY 77 Temjwatzlre-From previous work7 it was known that the slope of the calibration curve would decrease with temperature.This was confirmed and the rate of change was deter- mined by measuring the difference in absorbancy between two standard solutions, each at the same temperature, over the range 16" to 35OC, the results being as follows- Temperature, "C . . .. 16 21 25 30 35 Relative absorbancy , , +0.304 +Om294 +0.287 +Om279 +0.273 The absolute absorbancy was 1-5 and the results gave a slope of -0.0016 units of absorbancy per "C. In terms of absolute absorbancy this is equivalent to an error of 0-1 per cent. per "C. It is therefore necessary, for highest accuracy, to measure absorbancy differences within +06" C of the temperature used for preparation of the calibration curve. Hydrochloric acid concentration-A series of solutions of identical plutonium concen- tration, but containing different concentrations of hydrochloric acid, was prepared.By using the solution that was molar in hydrochloric acid as the reference standard, the other solutions were compared differentially with it. No systematic trend was observed, the variations in relative absorbancy being of a random nature. The results were as follows- Concentration of hydrochloric acid, M . . 1.0 0-56 0-75 1.25 1-50 2.0 Relative absorbancy . . .. .. . . O*OOO +0*002 -0.004 0.000 +0.001 -0.003 The absolute absorbancy was 0.765 and the precision (a) of the results was k0.2 per cent.Hydroxylamine hydrochloride concentration-The effect of variation of the concentration of hydroxylamine hydrochloride was tested by the same differential technique as described above. Again only random variations in absorbancy difference were observed, the results being as follows- Concentration of hydroxylamine hydrochloride, yo w/v 5 1 2-5 5-0 7.5 10.0 Relative absorbancy . . .. .. .. . . 0-000 - 0.002 - 0.002 - 0.001 - 0.001 + 0-002 The absolute absorbancy was 0.765 and the precision (a) of the results was k0.2 per cent. Nitric acid concentration-In the separation of plutonium from uranium as described by Atkins and Jenkins,' there is the possibility of some nitric acid being present in the plutonium fraction. The effect of added nitric acid on the relative absorbancy of a number of solutions was tested.It was found that moderate concentrations of nitric acid, e.g., 3 M , had no effect on the relative absorbancy measured. Concentration of nitric Relative absorbancy The results were as follows- acid, M . . .. . . 0.0002 0.001 0.002 0.004 0.01 0.04 0.08 0.2 0.6 1.3 3.2 . . 0.000 0.000 - 0.005 + 0.005 - 0-004 0.000 - 0.002 - 0.001 + 0.006 + 0.010 0.000 The absolute absorbancy was 0.765 and the precision (a) of the results was k0-33 per cent. It should be noted that, in the tests to determine the effect of variation of concentrations of hydrochloric acid, hydroxylamine hydrochloride and nitric acid, the concentration of plutonium in the solutions used was unavoidably lower than is required for highest precision.INTERFERENCE BY ADDED CATIONS- The effect of adding cations likely to be met in the analysis of plutonium alloys was tested in the same manner as was used for the solution conditions. Reference standards and proposed solutions of identical plutonium concentrations were used, the absolute absorbancy being 1-5, and various amounts of the second cation were added to the prepared solutions. The standard and sample solutions were then adjusted to volume and compared differentially. From this it is seen that excess of thorium, uranium, calcium and cerium (cerous) can be present in the solution without greatly affecting the precision of the plutonium determination. Iron added as ferric chloride interferes seriously, even at ratios of iron to plutonium as low as 0.1.Greater amounts can be tolerated, up to a ratio of 1.0, by the inclusion of 5 ml of a 5 per cent. w/v solution of stannous chloride in the 25 ml of alloy solution. Aluminium causes a slight decrease in absorbancy at ratios of 1.5 and greater. This was observed to be due to a slight turbidity formed on the addition of zirconium (as the oxychloride) to the solution. ANALYSIS OF PLUTONIUM - THORIUM ALLOYS The results are given in Table I. Zirconium increases the absorbancy of a plutonium solution, It has been shown that excess of thorium does not interfere in the differential spectro- photometric determination of plutonium. Accordingly, the plutonium contents of some plutonium - thorium binary alloys were determined, without separation, by direct comparison78 PHILLIPS PRECISE DETERMINATION OF [Vol.83 of the absorbancy of the alloy solution in molar hydrochloric acid and 5 per cent. w/v hydroxylamine hydrochloride against that of a plutonium standard of concentration 8.30 mg per ml. The results are shown in Table 11. TABLE I EFFECT OF ADDED CATIONS Ratio of added cation Added cation tonium to plu- Thorium 0.80 1.55 3.90 Precision (u) when no definite Relative trend is absorbancy observable, % f0.15 - 0.001 - 0.004 + 0-003 Iron (as Fe*+) 0.13 $0.012 0.63 +0.020 2.5 +0.085 6.3 +0-213 Iron (in 1.6 +0.007 presence of 3.2 +0.028 stannous 4.9 +0*026 chloride) 6.5 +Om122 Aluminium 1.5 - 0.020 3.8 - 0.038 5.3 - 0.050 Ratio Precision (u) of added when no cation definite to plu- Relative trend is Added cation tonium absorbancy observable, Uranium 0.4 - 0.002 1.0 +0*005 2.0 +0-004 4.0 - 0.003 8.0 +0.004 Zirconium 1-0 3-0.020 6.0 +0*050 6.0 +Om065 % f0.31 k0.17 Cdcium 1.2 - 0.004 2.4 - 0.001 6.0 +0-002 G2rium 1-7 3.0.002 3.4 - 0-001 8.6 0.000 } 0.08 TABLE :I1 ANALYSIS OF PLUTONIUM - THORIUM ALLOYS Relative Relative Absoliite absorbancy of concentration of concentration of Plutonium Thorium Alloy solution plutonium, plutonium, .in alloy, in alloy, Total, 1 +0.017 +om12 8-45! 96.0 rt: 0.1 3.9 f 0.2 99-9 No.mg per ml mg per ml Yo % Yo 2 + 0.050 + 0.35 8-65 94.8 & 0.1 4.8 f 0-2 99-6 3 + 0.300 + 2.08 10.38 94-2 f 0.1 5.8 f 0.2 100.0 4 + 0.639 + 4.42 12-75! 67.3 rfi 0.07 32-8 f 0.1 100.1 5 + 0.034 + 0.24 8.54: 42.8 f 0.04 57.4 & 0-1 100.2 METHO:D REAGENTS- Hydrochloric acid, s$.gr.1.18-Analytical-reagent grade. Hydroxylamine hydrochloride-Analytical-reagent grade. Hydroxylamine hydrochloride solution, 5 per ceiat . w /v-Dissolve 50 g of the hydroxylamine Dilute with distilled hydrochloride in distilled water and add 90 ml of the hydrochloric acid. water to 1 litre. APPARATUS- Beckmavt DU spectro$hotometer-For highest precision, the temperature of the water circulated through the cell chamber block must be thermostatically controlled. A more even temperature throughout the cell chamber is attained if excess heat from the lamp housing is prevented from reaching it. This can be done by circulating cooling water through a metal coil welded underneath the lanip housing. Thermostatically controlled water tank maintained at 25" & 0.20" C-It is convenient to circulate water from this tank through the cell chamber block.Beckman l-cm cells-These must be identifiable and always used in the same position and in the same orientation in the cell carrier. One cell and position is reserved for the reference solution and corrections are applied for any differences in absorbancy due to the sample cells. The correction for each sample cell is determined experimentally by filling both standard and sample cell with the reference solution and comparing the two differentially.February, 19581 PLUTONIUM BY DIFFERENTIAL SPECTROPHOTOMETRY i !I SAFETY PRECAUTIONS- The general type of facilities necessary for the safe handling of plutonium in laboratories have been described elsewhere.12 In this work, plutonium samples were dissolved and the temperature of the solutions was thermostatically controlled in a “glove box.” The spectro- photometer itself was partly enclosed in a “glove box,” in such a way that solutions could be transferred to the cell compartment without exposing them to the open laboratory.PROCEDURE FOR DETERMINING THE CALIBRATION FACTOR- Weigh a number of plutonium samples that on dissolution will provide a series of standard solutions of various concentrations within the range 8 to 14 mg per ml. Place each sample in a beaker together with sufficient distilled water completely to immerse the metal. Add hydrochloric acid drop by drop as the reaction proceeds, but do not allow the reaction to proceed too vigorously. On nearing completion, add sufficient hydrochloric acid to make the solution molar in acid.Warm the solution for a few minutes, cool it and then transfer it to a calibrated flask and add sufficient solid hydroxylamine hydrochloride to make the solution 5 per cent. w/v. Thermostatically control the temperature of the solution for 1 hour and then adjust it to volume with hydroxylamine hydrochloride solution. By using the most dilute solution as the reference standard and balancing the instrument at a slit width of 0-34 mm, measure the relative absorbancy of each solution. Make any necessary cell corrections and calculate the calibration factor as shown on p. 76. PROCEDURE FOR DETERMINING PLUTONIUM- Carry out a preliminary assessment of the concentration of plutonium by direct spectrophotometry.To do this, prepare a dilution of the solution to give an absorbancy of less than 1.0. Knowing the approximate concentra- tion of plutonium in the sample, select a standard the concentration of which is nearest to that of the unknown. Measure the difference in absorbancy between the unknown and the reference standard and calculate the concentration of plutonium by using the calibration factor. Should the reference standard selected be more concentrated than the sample solution, balance the instrument on the more dilute of the two solutions and measure the relative absorbancy, which will be negative. I acknowledge valuable discussion with Mr. E. N. Jenkins, under whose general guidance this work was carried out. Dissolve the sample containing plutonium as before. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. REFERENCES Westrum, S. F., in Seaborg, G. T., Katz, J. J., and Manning, W. M., Editors, “The Transuranium Elements,” National Nuclear Energy Series, Volume 14B, The McGraw-Hill Book Co. Inc., New York, 1949, paper 6-57. Koch, C. W., in Seaborg, G. T., Katz, J. J., and Manning, W. M., Editors, op. cit., paper 17.4. Milner, G. W. C., and Woodhead, J. L., Analyst, 1956, 81, 427. Hiskey, C. F., Anal. Chem., 1949, 21, 1440. Hiskey, C. F., Rabinowitz, J., and Young, I. G., Ibid., 1950, 22, 1464. Hiskey, C. F., and Young, I. G., Ibid., 1951, 23, 1196. Atkins, D. H. F., and Jenkins, E. N., in preparation. Bacon, A., and Milner, G. W. C., Ibid., 1956, 81, 456. Susano, C. D., Menis, O., and Talbot, C. K., AnaZ. Chem., 1956, 28, 1072. Connick, R. E., Kasha, M., McVey, W. H., and Sheline, G. E., in Seaborg, G. T., Katz, J. J., and Hindman, J. C., ila Seaborg, G. T., Katz, J. J., and Manning, W. M., Editors, op. cit., paper 4.4. Dunster, H. J., and Bennellick, E. J., Atomics, 1955, 6, 312. Received July loth, 1957 Manning, W. M., Editors, op. cit., paper 4.20.