signal to noise ratio could be increased, linearity of response improved, and the amount of sample decreased. The measurements herein reported were completed in 3 hours which included 1li2hours for lamp warmup, 1 hour for analysis, and an additional 1 hour for data reduction. Thus, the method is relatively rapid. The current experiment consumed approximately 30 ml of sample in which the concentration was 10.00 pg per ml. Consequently, only a few hundred micrograms of Pb is required, and refinements could reduce the amount considerably. Application of the method to geochronology will probably necessitate preconcentration procedures and other techniques t o increase sensitivity. The geochronological promise of the method equals o r exceeds that of the Larsen (Pb-alpha) method ( 7 , 8 ) whereby only total Pb is measured. As of yet no experiments have been performed o n Pb204, but it is expected that differential atomic absorption occurs (7) E. S. Larsen, N. B. Keevil, and H. C. Harrison, Bull. Geol. SOC.Amer., 63, 1045 (1952). (8) H. W. Jaffe et a / . Lead Alpha age determinations of accessory minerals of igneous rocks. Bull. U. S. Geol. Surcey, 1097-B, (1959).
for this isotope as well. Its high cost has prevented investigation thus far, but it is hoped that experimental data can be obtained o n it in the near future. It should be noted from the data of Table I that the measurement of chemical Pb by atomic absorption can be in significant error when the proportions of isotopes in standards and unknowns are markedly different. Because such differences are not likely to occur in most P b analyses, no significant error would be expected o n this account. But AAS measurement of chemical P b in samples isotopically different from ordinary Pb by reason of geological or technological processes would be accurate only if, among other things, the proportions of isotopes in samples and standards are similar. RECEIVED for review February 14, 1969. Accepted May 28, 1969. The Robert A. Welch Foundation, Grant C-9 t o John A. S. Adams and John J. W. Rogers, made possible post doctoral studies in atomic absorption spectrometry at Rice University. Further financial assistance was provided by a grant of the Research Division, Brigham Young University, to the author for study in geochronology, 1967-69. W. K. Hamblin, Department of Geology, Brigham Young University made some additional funds available to the author.
Chemical Separation of Cerium Fission Products from Microgram Quantities of Uranium Ana Albu-Yaron,’ D. W. Mueller, and A. D. Suttle, Jr. Department o j Chemistry, Texas A&M Unioersity, College Station, Tex. 77843
THERE ARE a number of problems in earth sciences which may be elucidated if a n accurate method for determining very small quantities of uranium is developed. This paper describes an accurate, fast and reliable method for the separation of cerium which may subsequently be used to determine nanogram quantities of uranium in raw cores. It involves the fission of a portion of uranium, separation of an abundant fission product, in this case cerium which can be cleanly and quantitatively separated, and then counting the cerium activity. Data in the literature o n the radiochemical determination of cerium in fission product mixtures involves methods based on the insolubility of some cerous and ceric compounds, on ion exchanges processes and on solvent extraction. If precipitation alone is used, as many as nine precipitations may be required t o ensure a radiochemically pure product ( I ) . Because appreciable material is lost in these steps, the final oxalate precipitate must be weighed to obtain a chemical yield. Jain and Singh ( 2 ) reported a more rapid gravimetric determination of Ce(II1) by precipitation at p H 3-6 with diammonium-5, 5-indigo disulfonate. However, Th also forms a n insoluble complex in the same p H range. Recently many articles have appeared describing methods based on ion exchange processes for replacing a number of the On leave from the Hebrew University of Jerusalem, Department
of Inorganic and Analytical Chemistry, Jerusalem, Israel. (1) C. D. Coryell and N. Sugarman, Eds., “Radiochemical studies: The Fission Products,” Nat. Nucl. Energy Series, Div. IV, Vol. 9, Book 3, McGraw Hill, New York, N. Y.,1951. (2) B. D. Jain and J. J. Singh, Talaritn, 8, 648 (1961).
precipitation steps (3-6). A large variety of ion exchange media as well as experimental conditions, have been studied for a greater specificity in separation of individual lanthanides. Separation was incomplete and repeated oxalate precipitations were required. Extraction methods developed to date for cerium are generally tedious (7, 8). Glendenin et al. (9) extracted cerium (IV) into methyl isobutyl ketone from strong nitric acid, forming a potentially explosive mixture. Separation from Zr, Nb, Th, and N p was incomplete, however, and it was necessary to effect final decontamination by several oxalate precipitations. Smith and Moore (10) report greater than 98% extraction of tracer level of radiocerium with 0.5M 2thenoyltrifluoro acetonexylene; however, Cl-, F-, and P04ainterfere. McCown and Larsen (ZZ) report the quantitative extraction of Ce(IV) from nitric acid as the chelate of 2ethylhexyl phosphoric acid into n-heptane followed by a
(3) F. H. Spedding, A. F. Voigt, E. M. Gladrow, and N. R. Sleight, J . Amer. Chem. Soc., 69, 2777 (1947). (4) E. C. Freiling and L. R. Bunney, ibid., 76, 1021 (1954). (5) A. P. Baerg and R. M. Bartholomew, Can. J. Cliem., 35, 980 (1957). ( 6 ) J. Alstad and A. C. Pappas, J . Zuorg. Nucl. Chenz., 15, 222 (1960). (7) C. F. Metz, G . M. Matlack, and G . R. Waterburg, Proc. Znterli Conf. Peaceful Uses A t . Energy, Geneca, 28, 441 (1958). (8) M. P. Menon and P. K. Kuroda, Nucl. Sci. Eng., 10, 70 (1961). (9) L. E. Glendenin, K . F. Flynn, R. F. Buchanan, and E. P. Steinberg, ANAL.CHEM.,27, 59 (1955). (10) G. W. Smith and F. L. Moore, ibid., 29,448 (1957). (1 1) J. J. McCown and R. P. Larsen, ibid., 32, 597 (1960). VOL. 41, NO. 10,AUGUST 1969
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Table I. Recovery of Cerium after Anion Exchange Step Recovery of Ce I11 ( x )in Concn of HCI eluate 15 ml Run loading s o h , M HCI wash soh 1 8 99 2 10 99 3 12.5 99 4 12.5 99
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Table 11. Coseparation of Fission Products on Anion Exchange Step Amount in eluate Concn of HCI 15 rnl HCI wash solution U NP Ru Zr-Nb loading soh, M 8 ... 80 ... 95 10 ... 45 ... 70 12.5 ... ... ... trace 12.5 ... ... ... trace
(z)
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second extraction from 9M HC1 for the separation of molybdenum and two perchloric acid fumings for the removal of ruthenium. F o r the use of cerium separations in the determination of uranium at submicrogram levels, most of the methods cited could not be satisfactorily applied. I n the present study a radiochemical procedure was developed which is rapid, gives clean separations of cerium from the fission mixture, and is adaptable to routine. On an anion exchange column C1- form equilibrated with concentrated hydrochloric acid, cerium is first quantitatively separated from all anions and cations of oxidation state IV, V, and VI. From the effluent containing the alkaline earth, rare earth, and alkali cations, cerium is isolated as Ce(103)4, mounted and counted. Recovery greater than 95 %, usually 99 %, can be obtained. EXPERIMENTAL Apparatus and Reagents. Analytical grade reagents were used throughout the work. For the cerium carrier, ammonium hexanitratocerate (dried at 85 "C for five hours) was used. The standard uranium solution was aqueous uranyl acetate (100 ppm). The ion exchange separation step was carried out on resin columns 0.9 cm i.d. X 5 c m long, which were filled to a height of 2.5 cm with Dowex 2 X 8 anion exchange C1- form, 50 X 100 mesh size, followed by 2.5 cm of Dowex 1 X 8 anion exchange C1- form, 200 X 400 mesh size. The flow rate through the ion exchange columns was 0.25 ml per min. The resins, after several cycles of C1--N03--C1- conversions, were finally equilibrated with concentrated HCl. The Texas A & M Nuclear Science Center Reactor (NSCR) was used for the irradiations. A standard solution containing 100 micrograms uranium was irradiated for 1-4 hours to fission U235 giving sufficient activity to readily measure. All samples were counted with a Victoreen SCIPP 400 pulse height analyser using a Harshaw Integral Line 3 X 3 inch solid NaI(T1) detector (6.8 % resolution). The sample was mounted in a beta absorber directly against the face of the crystal. The intrinsic efficiency varied from 46% to 50% depending upon the energy of the gamma ray used in the determination. Integration of peak areas was accomplished using the method described by Covell (12), and was done instrumentally in the SCIPP analyzer. (12) D. F. Covell, ANAL.CHEM.,31,1785 (1959).
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Procedure. To one ml of the uranium solution after irradiation, 100 ml of concentrated HC1 and 0.1-0.2 gram (exactly weighed) Ce(NH&(NO& were added. After dissolution, one drop of H202,30%, was added in order to reduce the cerium to the f 3 oxidation state. The solution was quantitatively passed into the top of the anion exchange column and was passed through the column with a flow rate of 0.25 ml/min after which the column was washed with 15 ml concentrated HCI. The effluent, including washing solutions, was evaporated nearly to dryness to remove the HCl. The residue was dissolved in water (1 5-30 ml). Twenty milliliters of concentrated "03 were added to make the solution 7-10M in H N 0 3 and enough solid NaBr03 (0.35 gram) to oxidize Ce to Ce(1V). After the dissolution of the NaBrOa, 20 ml of the precipitation solution (333 ml concentrated H N 0 3 100 grams K I 0 3 to one liter) were added with rapid stirring. The precipitate was washed by decantation (3-4 times) and o n filter paper (8-10 times) with washing solution (8 grams K I 0 3 and 50 ml concentrated HNC!3 to 1 liter) after which it was washed with a few ml of distilled water and finally with alcohol. The precipitate was dried for 15 minutes at 250 "C, weighed, and mounted for counting.
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RESULTS AND DISCUSSION The conditions for the quantitative column separation as well as precipitation without contamination were studied. The results might be influenced by interfering factors. In the following discussion all estimates of these effects will be taken into account. Cerium Equilibrium Absorption. The distribution coefficient of cerium(II1) in hydrochloric acid solutions with those of other fission products are compared by Kraus and Nelson (13) for Dowex 1 anion exchange resin and by Bunney et al. (14) for Dowex 2. Cerium (111) did not show significant absorption by both Dowex 1 or Dowex 2 resins a t any normality of the acid. The behavior of U, Nb, Zr, Mo, and R u is similar to that of several other elements with absorption increasing with increasing hydrochloric acid concentrations. By comparing the distribution coefficients between Dowex 1 or Dowex 2 and hydrochloric acid, we can see that for uranium, protactinum, and zirconium the values are about the same. N b has a Kd greater o n Dowex 2 than o n Dowex 1 in high concentrations of hydrochloric acid and less in low concentrations. F o r ruthenium and molybdenum the adsorption is at minimum with Dowex 2 at concentrations at which it is a maximum with Dowex 1. The data suggest that cerium (111) should be separated from uranium, zirconium, niobium, and ruthenium in 8-12N HC1. F o r N p our results show maximum Kd between Dowex 1 and concentrated hydrochloric acid. Experimental Column Studies. Based o n the K d values given before, the exchange column separation experiments were performed o n a resin column filled with two equal beds of anion exchange resin Dowex 1 and Dowex 2. Table I gives the results for four column runs to test whether column contamination by Ce(II1) occurs. Cerium tracer was loaded into the column in 8N, lON, and concentrated HCl. The same amount was recovered in the loading eluate and wash of hydrochloric acid. The results of the experiments with a mixture of fission products are given in Table 11. All samples were loaded into a chloride resin column from different concentrations of (13) K. A. Kraus and F. Nelson, Intern. Conf. Peaceful Uses At. Energy, Geueea, 7 131 (1955). (14) L. R. Bunney, N. E. Ballou, J. Pascual and S. Foti, ANAL. CHEM.,31, 324 (1959).
Figure 1. Gamma ray spectra of cerium isotopes separated after 4 hours from the end of irradiation. One ml standard uranium solution (100 ppm) irradiated for 4 hours hydrochloric acid solutions between 8-12.5N. Only from very concentrated acid solutions is the separation from N p complete. N o N p was detected in the gamma pulse height distribution curve of the final cerium counting. The concentration of the hydrochloric acid also influences the Zr and N b adsorption. Therefore, it is necessary to work in very concentrated solutions of hydrochloric acid. For this purpose 20-30 ml of concentrated HC1 must be passed through the column before the loading of the fission mixture and the cerium carrier must be added as a n exactly weighed amount (0.1-0.2 gram) of Ce(NH4)a(N03)6to 10 ml concentrated HCl containing the fission mixture in order not to decrease the concentration of the solution. One drop of H ? 0 23 0 x ensures the reduction of cerium to $3 oxidation state and the isotopic exchange with the carrier. Cerium Iodate Precipitation. Cerium(1V) iodate is the only specific precipitate which has been proposed that is not subject to hydrolysis and it offers a ready means of separating cerium quantitatively from even large amounts of other rare earths. Precipitation from elution solutions as cerium iodate gives a solid source of stoichiometric composition, desirable for counting with complete decontamination of the remaining fission products. Cerium must be oxidized to the tetravalent state before precipitation because cerous iodate is soluble. Several oxidizing agents were considered in the literature. Schumann (1) has shown that oxidation proceeds rapidly with K B r 0 3 . Willard and Yu (IS) obtained satisfactory results with ammonium persulfate. We worked with N a B r 0 3 (10-15 times the weight of Ce) as oxidizing agent. It is necessary to remove the HCI completely by evaporation, so that none of the Br03- is consumed for the oxidation of Cl-. Also, solutions of Ce(1V) in hydrochloric acid are not stable. To reduce the coprecipitation of La and Y, the acidity of the solution was increased. The maximum concentration of "03 recommended in the literature (16, 17) is 3N because of the increasing solubility of Ce(103)4in greater concentrations. However, a high concentration of acid gives a better separa(IS) H. Willard and S. Tsai Yu, ANAL.CHEM.,25, 1754(1953). (16) K . Kimura, H. Natzume, and Y. Suzuki, Bu/iseki Kuguku, 6 , 719 (1957); Chem. Ahsfr., 53 293012 (1959). (17) N. H. Furrnan, "Standard Methods of Chemical Analysis," D. Van Nostrand Co. Inc., New York, N. Y. 1962, p 317.
(18) E. Staritzky and D. Cromer, ANAL.CHEM., 38,913 (1956). VOL. 41, NO. 10,AUGUST 1969
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