Quantitative Radiochemical Analysis by ion Exchange Anion Exchange Behavior of Several Metal Ions in Hydrochloric, Nitric, and Sulfuric Acid Solutions 1. R. BUNNEY,
N. E.
BALLOU, JUAN PASCUAL, and STEPHEN FOTl
U. S. Naval Radiological Defense Laboratory, San Francisco 24, Calif.
b Distribution coefficients have been measured for americium, cerium(lll), molybdenum, protactinium, niobium, ruthenium, strontium, thorium, uranium(VI), yttrium, and zirconium in the systems Dowex 2-hydrochloric acid, Dowex 2-nitric acid, and Dowex 2sulfuric acid. The acid concentrations ranged from 0.1N to concentrated. Strontium, yttrium, cerium, and americium had no significant adsorption in any o f the systems studied. A number of elements showed strong adsorption a t low concentrations o f sulfuric acid. Thorium, uranium(VI), and zirconium showed an adsorption maximum at approximately 8 N nitric acid.
I
x RECEKT YEARS anion exchange chro-
matography has found a wide range of applications in analvtical chemistry (11,12,16)and in radiochemical separations (3, I S , 17). The behavior of most elements in hydrochloric acid solutions in equilibrium with the strong base anion exchange resin Dowex 1 has been studied extensively (9). Less attention has been given to other anion exchange systems, although several systems have been studied less exhaustively. The purpose of the present work was to elucidate the anion exchange behavior of certain radioelements frequently encountered in radiochemical work and thereby to improve present radiochemical separations. The equilibrium behavior of americium, cerjum(III), molybdenum, niobium, protactinurn, ruthenium, strontium, thorium, uranium(VI), yttrium, and zirconium with the strong base quaternary amine resin, Dowex 2, with various concentrations of hydrochloric acid, nitric acid, and sulfuric acid was studied by a batch equilibration method. EXPERIMENTAL DETAILS
A large batch of Dowex 2 X 8, 200400 mesh, was further graded by settling through water. The fraction that settled at a rate of 1.5 to 3.0 em. per minute was selected, and divided roughly into thirds. Each third was converted to the appropriate form by passing the appropriate 6 N acid through 324 *
ANALYTICAL CHEMISTRY
the resin in a column bed. Each portion was rinsed with deionized water until a negative test for the anion was obtained. Each portion was then dried overnight a t 105' C. and stored over concentrated sulfuric acid in a desiccator. The experiments were carried out by equilibrating a weighed amount of resin in the appropriate form with a known volume of the desired acid in a screwcap glass container or a heat-sealed polyethylene tube for not less than 16 hours. The concentration of the metal ions varied from traces to about 0.04M. I n all the experiments no significant adsorption was observed on the container walls. N7hen radioactive tracers were available, measurement of the radioactivity in the liquid and resin phase after equilibration furnished the data to calculate the distribution coefficient, Kd (the amount of metal ion per gram of dry resin divided by the amount of metal ion per milliliter of solution), Where no tracers were available or a-emitters were used, the liquid phase was assayed before and after equilibration and the difference was assumed to have been adsorbed by the resin. Blank runs shoned no significant adsorption by the container walls. All the equilibrations were carried out a t room temperature in duplicate and the average of the duplicate results is reported. The average spread of the duplicate determinations was 15%, with the higher distribution coefficients having better precision than the lower ones. Molybdenum, thorium, and uranium were the only elements for which suitable radioactive tracers were not available. Molybdenum was determined spectrophotometrically with a Beckman DTJ spectrophotometer using the peroxymolybdic acid (19) complex a t a wave length of 400 mp. Thorium and uranium were determined colorimetrically with a Fischer photometer, using the colored complex formed with thoron ( I S ) and hydrogen peroxide (n, respectively. Niobium-95, zirconium-95, protactinium-233, and ruthenium-106 were determined by gamma counting in a well-type crystal scintillation counter. The niobium-95 and zirconium-95 were purified by the dibutyl phosphate (16) extraction method. Beta-ray absorption measurements showed the tracers to be of satisfactory purity. Protactinium233, which was purified by extracting
into diisopropyl ketone (14). showed satisfactory purity by decay and betaray absorption measurements. Ruthenium-106 was purified by distillation of the tetroxide (6), reduced with ethyl alcohol, and dissolved in 6 N hydrochloric acid. The hydrochloric acid was removed for the nitric acid and sulfuric acid equilibrations by evaporating with the appropriate acid. Strontium, cerium, and yttrium were determined by beta counting. The strontium-89 and cerium-144 were purified from mixed fission products by standard radiochemical procedures (2, 6) and the yttrium-91 by ion exchange chromatography (1). ilmericium was determined by alpha counting. Americium241 tracer was purified-by ion exchange chromatography (4,
m.
RESULTS AND DISCUSSION
The distribution coefficient vs. acid normality for the elements giving significant resin adsorption is plotted in Figures 1, 2, and 3 for hydrochloric, nitric, and sulfuric acids. These data clearly indicate numerous useful separation procedures for elements studied, and give some insight into the complex ion formation occurring in these mineral acid solutions. I n hydrochloric acid solutions strontium, yttrium, cerium(III), thorium, and americium did not show significant adsorption by the Dowes 2 resin a t any normality. Molybdenum showed the greatest adsorption betneen 0.1 and 1N hydrochloric acid (Figure 1). The shape of the molybdenum curve indicates that more than one species of molybdenum is being adsorbed by the Dowex 2 resin. The adsorption of ruthenium, originally in the IV state, goes through a maximum near 2 N ; niobium has minimum adsorption at 2h' hydrochloric acid. The slow rise in adsorption of niobium below 2N hydrochloric acid may not indicate anything more than physical adsorption of hydrolyzed niobium by the ion exchange resin. The behavior of protactinium is somewhat similar to that of niobium, although the minimum adsorption is somewhat higher. The notable feature of the uranium curve is the maximum adsorption near 8.V. The be-
1000
E
la
5
b.c1
Figure 1. Equilibrium adsorption in hydrochloric acid
pi
Figure 2. nitric acid
iuo,
Equilibrium adsorption in L1
havior of zirconium is similar to that of several other elements, with adsorption increasing with increasing hydrochloric acid concentrations. Figure 1 clearly indicates various separations possible in hydrochloric acid solutions. Thus the actinide elements thorium, protactinium, and uranium are sequentially separable by use of concentrated hydrochloric acid, 4 to 5.V hydrochloric acid, and 0 . 1 9 hydrochloric acid. Uranium can be separated from molybdenum with 0.1-Yhydrochloric acid; molybdenum can be r moved from the resin column with 6 to 1 0 s nitric acid (see Figure 2 ) . I n nitric acid solutions strontium, yttrium, cerium, americium, and niobium did not show significant adsorption by the D o v e s 2 resin at any acid concentration. I n Figure 2 molybdenum and ruthenium, originally in the IV state, stand out wjth the highest adsorption at O . l S , the adsorption decreasing with increasing acid concentration. At low concenti ations the adsorption of protactinium in nitric acid is much like that in hydrochloric acid. iit higher concentrations it increases, but more slowly than in hydrochloric acid. Zirconium, uranium, and thorium show a maximum adsorption near &l4 nitric acid. Nitrate ion concentration reaches a maximum in nitric acid near 8 M acid (8). This behavior is therefore t o be expected when nitrate ions enter into the complex form. However, the reduced adsorption could also be due to partial destruction of the resin in high concentrations of nitric acid. Numerou. qeparations can be obtained from nitric acid solutions. For euample, uranium, zirconium, and thorium can be separated by 8 N nitric acid elutions from a column bed, with zirconium eluting first, followed by uranium and then thorium. I n addition, ru-
thenium should be separated from all the other elements studied here by a pass of 1N nitric acid solution through a n anion resin bed. I n sulfuric acid solutions strontium, yttrium, cerium, and americium did not show any significant adsorption by the Dowex 2 resin a t any acid concentration. I n Figure 3 molybdenum, niobium, protactinium, uranium, and zirconium all showed decreasing adsorption by the resin with increasing sulfuric acid concentration. The sulfate ion may be a more effective complexing agent here than the bisulfate ion, as the ratio of sulfate to bisulfate decreases with increasing sulfuric acid concentration. If this is so, a solution in which the sulfate ion concentration is high, such as a solution of lithium sulfate, might be a useful complexing agent for anion exchange work in a manner analogous to that of lithium chloride (IO). Ruthenium, originally in the IV state, and thorium go through a minimum near 2N sulfuric acid and rise again, and thorium shows a maximum adsorption near 18h' sulfuric acid. This behavior of thorium might indicate t h a t a bisulfate complex is being adsorbed b y the resin, as the concentration of bisulfate ion is a maximum at about 20N (8),but it could also be due to degradation of the resin a t high acid concentrations. The resin darkened noticeably a t high sulfuric acid concentrations. Among the numerous separations indicated to be possible by Figure 3 is that of americium and thorium from uranium, protactinium, and zirconium. All these elements except americium will be removed from 0.1N sulfuric acid solution by a Dowex 2 resin bed. Subsequent isolation of thorium can be accomplished with a 2 to 4N sulfuric acid solution. The same procedure can be used if, instead of americium, a separa-
ni2s04
Figure 3. Equilibrium adsorption in sulfuric acid
tion of rare earths from thorium, protactinium, uranium, zirconium, niobium, molybdenum, and ruthenium is required. I n addition t o the examples of separations given for each of the three acids, other useful separations based on the combined use of the acids are readily apparent. For example, niobium retained on a resin column from a strong hydrochloric acid solution could be removed from many elements on the column by elution n-ith 8N nitric acid. As dictated by the particular requirements of given experiments, many other separations are possible. Hoivever, the adsorption functions indicate the possible methods. Comparison of the results for hydrochloric acid solutions in this work and the extensive studies in the literature on Dowex 1 resin should provide a basis for selection of the more appropriate resin for a given separation. Protactinium, zirconium, and uranium(V1) in hydrochloric acid solutions adsorb on Dom-ex 2 to approximately the same extent as with Dowex 1. Siobium adsorbs more strongly on Dowex 2 than on Dowex 1 in high concentrations of hydrochloric acid and less strongly in low concentrations. lIolybdenum(V1) in hydrochloric acid solutions behaves in an extremely different manner with Dowex 2 than n i t h Dowex 1. The adsorption is a t a minimum with Dowex 2 at concentrations a t which it is a maximum ivith Dowex 1. Ruthenium behaves in a similar manner with the two resins in high hydrochloric acid concentrations, but a t low concentrations its adsorption increases VOL. 31, NO. 3, MARCH 1959
325
with Dowex 1 and decreases with Dowex 2. Thorium in nitric acid and uranium(V1) in sulfuric acid show resin adsorptions with Dowex 2 similar to Dowex 1. Uranium(V1) in nitric acid shows a different adsorption function with Dowex 2 than with Dowex 1, although the magnitude of the maximum adsorption is similar. ACKNOWLEDGMENT
The authors thank S. W. Alayer and E. C. Freiling for helpful discussions and suggestions. LITERATURE CITED
(1) Bunney,
L. R., Freiling, E. C., McIsaac, L. D., Scadden, E. M., Xucleonics 15, No. 2, 81 (1957). (2) Burgus, W.H., Engelkemeir, D. W., “Radiochemical Studies. The Fission
Products,” K.N.E.S., Vol. IV, p. 9, Paer 291, ed. by C. D. Coryell and N. gugarman, McGraw-Hill, New York, 1951. (3) Campbell, E. C., Nelson, F., Phys. Revs. 91,499A (1953). (4) Diamond, R. M., Street, K., Jr., Seaborg, G. T., J . Am. Chem. SOC.76, 1461 (1954). (5) Glendenin, L. E., “Radiochemical
Studies: The Fission Products,’J N.N.E.S., Vol. IV, p. 9, Paper 236, ed. by C. D. Coryell and N. Sugarman, McGraw-Hill, Yen- York, 1951. (6) Ibid., Paper 260. ( 7 ) Hacke, O., 2. anal. Chem. 119, 321 (1940).
(8) Harned, H. S., Owen, B. B., “Physical Chemistry of Electrolytic Solutions, Reinhold, NeIY York, 1950. (9) Kraus, K. A., Nelson, F., “Anion Exchange Studies of the Fission Products,” Paper 957, Session 52, International Conferences on the Peaceful Uses of Atomic Energy, 1955. (10) Kraus, K. A., Kelson, F., Clough, F. B., Carlston, R. C., J . Am. Chem. SOC.77,1391 (1955). ( 1 1 ) Kunin, R., McGarvey, F. X., ANAL. CHEM.26, 104 (1954).
(12) Kunin, R., McGarvey, F. X., Ind. Eng. Chem. 47,565 (1955). (13) Lindner, M., “Radiochemical Procedures in. Use at the University of
California Radiation Laboratory, (Livermore),” UCFU-4377 (1954). (14) Meinke, W. W., “Procedures Used in Bombardment Work at Berkeley,” UCRL-432 (1949). (15) Samuelson, O., “Ion Exchangers in
Analytical Chemistry,” Wiley, New
York, 1953. (16) Scadden, E. M., Ballou, N. E., AXAL.CHEM.25, 1602 (1953). (17) Stevenson. P. C.. Hicks. H. C..’ ‘ Ann. Rev. N&leur Sd 3, 221 11953). (18) Thomason, P. F., Perry, M. A., Byerlv, W. M., ANAL.CHEW21, 1239 (i949j: (19) W~sh,L., private communication. (20) Wish, L., Freiling, E. C., Bunney, L. R., J . Am. Chem. SOC.76, 3444 (1954).
RECEIVEDfor review May 23, 1958. Accepted December 15, 1958. Division of Analytical Chemistry, Symposium on Radiochemical Analysis, Fission Product Analysis, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
Qu a ntita tive RadiochemicaI An a lysis by lon Excha nge Anion Exchange Behavior in Mixed Acid Solutions and Development of a Sequential Separation Scheme LEON WISH
U. S. Naval Radiological
Defense laboratory, Son Francisco 24, Calif.
b A method for rapid separation and determination of neptunium, plutonium, uranium, zirconium, niobium, and molybdenum isotopes in mixed fission products is evolved from Dowex 2 anion exchange equilibrium data on these elements in hydrochloric, hydrochlorichydrofluoric, and nitric acids. The fission products sample in concentrated hydrochloric acid is added directly to the resin column, and no tracers, carriers, or prior separations are required. The activities are eluted sequentially and determined directly in a y-ray scintillation well counter or a multichannel y-ray spectrometer. The yields are quantitative and the purity is equivalent to that obtained from the standard radiochemical procedure for molybdenum, neptunium, and Plutonium. The zirconium and niobium fractions may contain y-ray impurities which are easily resolved by y-ray spectrometry. The uranium is contaminated with the tellurium-1 32iodine-1 3 2 pair and usually requires further purification.
T
quantitative determination of radionuclides in products of neuHE
326
ANALYTICAL CHEMISTRY
tron irradiations of uranium and other heavy elements usually requires yield determinations, because decontamination procedures result in significant losses. A measured quantity of a different isotope of the element to be determined is added and if there is suitable isotopic exchange, the yield of the unknown will be the same as for the known. Alpha-ray tracers, neptunium237, uranium-233, and plutonium-236 are used to determine the radionuclides of these elements-e.g., neptunium239, uranium-237, and plu tonium-239. Milligram amounts of the stable elements are required for zirconium-95, zirconium-97, niobium-95, and molybdenum-99. These analyses are usually rather lengthy, especially those for the heavy elements uranium, neptunium, and plutonium, which require electrodeposited samples for suitable alpha counting. I n the past few years, ion exchange separations have materially simplified some radiochemical procedures and increased the over-all yields for many isotopes. Also, y-ray spectrometry has become increasingly useful for qualitative and quantitative analysis and for detection of contaminants. Complete
y-ray spectra can now be obtained in a short time with a 256-channel analyzer. The efficiency of y-ray detection has been sharply increased with scintillation sodium iodide-thallium crystal counters, and gamma counting of liquid samples is now precise and accurate. Connally (2) gives a good review of the instrumental methods of y-ray spectrometry. It would be extremely advantageous to add any fission product mixture directly to a n ion exchange column and elute the desired radionuclides quantitatively and sequentially with sufficient purity, thus eliminating the need for tracers and carriers. The eluates containing these gamma-emitting isotopes could then be transferred directly to a scintillation well counter and spectrometer for activity and purity measurements, so that decay measurements i ~ o udl be eliminated. ,4 sequential separation method for uranium, neptunium, and plutonium (16) utilized hydrochloric acid and nitric acid as eluents. However, the eluted uranium was grossly contaminated with zirconium and required further processing. It was necessary to separate ‘the molybdenum activity by precipitation prior to ion exchange, as it
