Radiochemical Determination of Cerium in Fission L. E. GLENDENIN, K. F. FLY",
R. F. BUCHANAN, and E. P. STEINBERG
Chemistry Division, Argonne N a t i o n a l Laboratory, Lemont,
111. free cerium-144 t,racer and for cerium a t a concentration of about 1 mg. per ml. The results are shown in Figure 1, where t,he distribution of cerium obtained by counting equal aliquots of the organic and aqueous phases is plotted as a function of nit,ric acid concentration. Cerium is well extracted at. nitric acid concentrations above i M , the optimal range being 8 to 10M. The aqueous phase should be relatively free of chloride ion (not greater than 0.1M) t,o avoid reduction of bromate and cerium(1V). Sulfate ion also lowers the efficiency of extraction, although concentrations up to 0.5M can be tolerated without serious loss in yield. Separation from Other Elements. The elements with ratlioisotopes formed in appreciable yield in fission and likely t o be cstracted hy methyl isobutyl ketone are zirconium, niobium, and ruthenium. Other interfering elements which may be enrountered in fission product sources are thorium, uranium, and neptuniiim. The cerium procedure (as described) was tested for coseparation of these elements by using the following radioactive trawrs: zirconium-95-niobium-95 (equilibrium mixture), ruthenium-106, normal uranium with US1 (thorium-234) i n equilibrium, and neptunium-237. The results of these experimenh are summarized in Table I. These data indicate that, the separation from zirconium, niobium, and ruthenium is entirel?, adequate for fission product mixt.ures generally encountered. Separation from trivalent rare earths was shown to be excellent! not more t,han O.lcoof trivalent cerium-144 being extracted. For the remaining elements in Table I, however, modifications of the procedure may be required under certain conditions. For example. i n the slow neutron-induced fission of uranium-235 in normal uranium the neptunium-239 activity formed by neutron capture in uranium-238 exceeds that of fission-produced cerium for 2 t,o 3 weeks after irradiation. During this time t,he separation of cerium from neptunium (Table I ) would be inadequate.
'The work reported was undertaken to develop a simple and rapid radiocheniical procedure for the determination of cerium activity produced in nuclear fission. A method was deieloped based on solvent extraction of cerium(1V) with methyl isobut?1 ketone. Good separntion from the large number of elements encountered in fission product sources is aforded by the extraction procedure as well as a considerable saving in time and effort over the ceric iodate precipitation method in pre>ious use. Further applications of the new procedure are the preparation of carrier-free radioactive cerium and activation anal>sis for small amounts of cerium.
T
HE procedure currently emplo>.ed for the radiochemic:rl determination of cerium activity in fission (1, 4 ) is based on the insolubility of ceric iodate and involves a series of separationby precipitation. This procedure is consequently soniewh:it lengthy and tedious. It would lie of considerable advantage to the :inalyst faced with ninny determinations or with the isolation of short-lived cerium activities to have a more rapid proccdure without sacrificing accur:icy or radiochemical purity. I n drveloping such a procedure t h r solvent extraction of tetrnvalent cerium seemed most attractivc. Extraction of cerium( IT-) by ethcr from nitric acid solution was first, studied by I n m (61, aiitl ;ilso employed by Ckyder and Dodson ( 5 )and Bock and hleycr ( 2 ) . -4lthough cerium(IT-\ c a n be extracted bl- this mct,hod, the attack of ether by strong nitric acid and the osidizing agcnts required t,o oxidize ceriuni(II1) to cerium(1V) is a serious drair-luick. Warf (11) found t h a t cerium(1V) is extracted ti>tri-n-hutyl phosphate and that this solvent. is satisfactori1)resistant t o nitric acid and strong oxidants. The disadvantage in this case, however, i p that the separation from trivalent rare earths is n o t adequate ( 7 ) . Rothschild et al. (IO)observed t h a t thoriuni nitrate is extracted by methyl isobutyl ketone (hexone). .4 patent has been issued to Pitzer (9) for a procedure in nhicli cerium in trace concentration is extracted by methyl isobutJ.1 ketone from aqueous media cont,aining dichromate as the osidizing agent. In the present investigation the method of Pitzcr wai fouiid to be unsuc~cessf'ulwith cerium in macro conc8entr:itions. Good extraction of cerium hy methyl isobutyl ketoiir wr.~ol)t:rinrd in both trace and m:itro concentrations, 1iowevt.r. froin strong nitric acid solution using sodium (or potassium) bromate i3.u oxidant, and a procedure was dcvelopcd for the radiochemic:il determination of cerium Iiasrd on this method. I n tlie new proredure cerium( 111) cari,icr is oxidized to cerium(IT-) 111. sodium bromate in 9 J I nitric acid solution, c.ut,racted into niethJ.1 isobutyl ketone, back-extracted into Tvater containing hydrogcn peroside to reduce cerium(1T') to cerium(III), and precipitntrd as cerium oxalate for gravimet,ric determination of yield and activity measurements T h e extr:wtion step replaces sevcn p r e c i p h t i o n s used in the older procedure and results in coilsiderable saving of time and effort. The cerium is obtainpd i n high yield and radiochemical purity.
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EXPERI3IEYTA L
Extraction of Cerium. For estraction tBestsboth terhnical grade and redistilled methyl isoliutyl ketone were used. Although the latter was somewhat. superior with respect to reaet,ivit>with nit,ric acid and oxidizing agents, t,he technical grade solvent was found t,o be satisfactory for estrartion of cerium, provided it is first equilibrated with nitric acid containing sodium bromate to remove reducing substances. The optimal nitric acid concnentration for the extraction was determined both for carrier-
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Figure 1. Effect of Nitric Acid Concentration on Extraction of Cerium by Methyl Isobutyl Ketone
59
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ANALYTICAL CHEMISTRY
60 Table I. Element
Coseparation of Various Elements with Cerium
70 Extracted
in ~ ~ Extract
Oxalate ~ % ikn Cerium First Second
T h e separation from uranium and thorium n ould also be inwfficient if cerium activity in low intensit;\- nere to be isolated from large quantities of normal uranium. I n this case the cerium would be contaminated both Tvith weighable amounts of uranium (causing a n error in the gravimetric determination of yield) and with the natural radioactivity of thorium-234. T h e additional separation from uranium, neptunium, and thorium required for such samples is readily obtained by a simple modification of the procedure to include extraction of these elements before oxidation and extraction of cerium. This is accomplished by adjusting the sample taken for analysis (containing cerium carrier) to a nitric acid concentration of 10.11 a n d a volume of about 10 ml , and extracting with 50 ml. of methyl isobutyl ketone for removal of uranium and neptunium, or with 50 ml. of tri-n-butyl phosphate for removal of thorium (8, 11). T h e solvents should be equilihrated with 10-If nitric acid before use. T h e methyl isobutyl ketone extraction removes 8670 of the uranium and 967, of the neptunium, and the butyl phosphate extraction removes more than 99% of the thorium with only a few per cent loss of cerium ( 8 ) . Repeated evtractions may be made if further purification is desired. T h e aqueous phase is then treated with 2 ml. of 2 M sodium bromate solution t o oxidize cerium(II1) to cerium(IT’), and the cerium is separated by the regular procedure. Reaction of Methyl Isobutyl Ketone with Nitric Acid. I n extractions of strong nitric acid solutions (6 to 12M) with methyl isobutyl ketone considerable amounts of nitric acid pass into the organic phase. It was observed that such solutions of nitric arid in methyl isobutyl ketone are unstable and will undergo a violent reaction after standing for a few hours. The methyl isobutyl ketone phases remaining after back-extraction with 5 ml. of water were observed t o react similarly but only after standing for about 3 days. It is recommended, therefore, that the methyl isobutyl ketone not be equilibrated with nitric acid until just before use and that i t be washed thoroughly with water (three times with an equal volume) soon after use. It is also recommended that nitric acid solutions which have been in contact with methyl isobutyl ketone be neutralized with ammonium hydroxide before storing or discarding. B y observing these simple precautions no difficulties with reactivity have been encountered. REAGEkTS
IIethyl isobutyl ketone Hydrogen peroxide, 30% Ammonium hydroxide, (hevone) Tri-n-butyl phosphate concentrated Sitric acid, 6J1, 9Jd, Oxalic acid, saturated and concentrated Ethyl alcohol, absolute Sodium bromate, 2 M Ethyl ether Cerium Carrier Solution (cerium, 10 mg. per ml.). Dissolve 23 grams of C.P. cerous nitrate in 1 liter of water. Standardize as follows: Pipet 5-ml. aliquots into 50-ml. centrifuge tubes. T o each add 1 ml. of 6M nitric acid and 15 ml of water. Heat just to boiling and add 15 ml. of saturated oxalic acid with stirring. Cool in a n ice bath for 10 minutes with occasional stirring. Filter on a weighed sintered-glass crucible with suction, transferring and washing with three 5-ml. portions of water. Wash three times with 5 ml. of ethyl alcohol, three times with 5 ml. of ether, and place in a vacuum desiccator. Evacuate for 2 minutes, release, and evacuate again for 5 minutes. Weigh as Ce2(C20a)s.10HzO. PROCEDURE
T o the aliquot (1 to 5 ml.) taken for analysis add 1 ml. (10 mg.) of standardized cerium carrier, 2 ml. of 2.V sodium bromate, and sufficient concentrated nitric acid to make the solution 8 to 10M in nitric acid. Transfer to a separatory funnel containing 50 ml. of methyl isobutyl ketone (which has just been equilibrated with 50 ml. of 9 M nitric acid containing 2 ml. of 2J4 sodium bromate) and shake for 15 t o 30 seconds. Withdraw the aqueous phase and wash the methyl isobutyl ketone phase twice with 10 ml. of 9 M nitric acid containing a few drops of 2 M sodium bromate. (Caution. Combine the aqueous phase and washings, and neutralize with ammonium hydroxide before discarding.) Backextract the cerium by shaking the methyl isobutyl ketone phase with 5 ml. of water containing 2 drops of 30% hydrogen peroxide.
(Caution. Wash the methyl isobutyl ketone three times with 50 ml. of water before discarding.) Neutralize the aqueous phase by adding concentrated ammonium hydroxide ( 3 t o 5 ml.) until a precipitate just appears, and acidify with 1.5 ml. of 6 M nitric acid. Dilute the solution to a volume of 15 ml. with water, heat just to boiling, and add 15 ml. of saturated oxalic acid. Cool for 2 to 3 minutes with running water (or ice bath), centrifuge, decant, and wash the precipitate with 10 ml. of water. Dissolve the precipitate in 1 ml. of 6 M nitric acid (warming if necessary), and dilute with water to 15-ml. volume. Repeat the oxalate precipitation, centrifuge, and filter with suction on a weighed filter paper disk in a small funnel, transferring and washing with three 5-ml. portions of water. Wash three times with 5 ml. of ethyl alcohol, three times with 5 ml. of ether, and place in a vacuum desiccator. Evacuate for 2 minutes, release, and evacuate again for 5 minutes. Weigh the cerium oxalate to determine the yield through the procedure, and mount for measurement of radioactivity. T h e yield of cerium through the above procedure is usually about 80%) and the time required is approximately 1 hour. RESULTS AND DISCUSSION
T h e methyl isobutyl ketone extraction procedure was compared n i t h the ceric iodate precipitation procedure (3) by analysis for 282-day cerium-144 in 21 different samples of neutronirradiated uranium. T h e results obtained by the two procedures agreed to nithin 2y0 for every sample. T h e eytraction procedure was also employed for isolation of short-lived cerium activities from uranium fission. The separations were made shortly after irradiation, and the procedure was modified to include the preextraction with methyl isobutyl ketone for additional removal of neptunium-239. Decay curves showed the presence of the known 15-minute cerium-146, 33-hour cerium-143, and 33-day cerium-141 and confirmed the recent observations of Caretto and Katcoff ( 4 ) that no 1.8-hour cerium +. 4.5-hour praseodymium chain exists in fission. Extraction with methyl isobutyl ketone is also well suited for convenient isolation of carrier-free cerium activity from fission products. For this purpose, however, the extraction cycle should be repeated to obtain good separation from zirconium, niobium, and ruthenium (Table I). I n the course of another investigation farrier-free cerium-144 of high purity was prepared by this method and examined in a mass spectrometer. No trace of other rare earths was observed. Another application for ~1hich the new procedure should be highly satisfactory is the determination of small amounts of cerium b v radioactivation. The senqitivity of this method is proportional to the neutron flux and to the cross section for activation. I n the case of cerium as little as 10-7 gram may be determined with a n accuracy of about 5 % by counting the 33hour cerium-143 produced in a 1-day irradiation a t a thermal neutron flux of 2 X 10’2 per sq. cm. per second (generally available in nuclear reactors). LITER4TURE CITED
(1) Ballou, N. E., “Radiochemical Studies: The Fission Products,” C. D. Coryell and K . Sugarman. eds., Kational Nuclear Energy Series, Div. IV. Vol. 9, p. 1673, New York, McGraw-
Hill Rook Co.. 1951. (2) Bock, R., and Meyer, K . H., Chem. Ing. Tech., 25, 141 (1953). (3) Boldridge, W.F., and Hume, D. N., “Radiocheniical Studies: The Fission Products,” C. D. Coryell and N. Sugarman, eds., Sational Nuclear Energy Series, Div. IV, Vol. 9, p. 1693, S e w York, McGraw-Hill Book Co., 1951. (4) Caretto, A4.A., and Katcoff, S.. Phys. Rev., 89, 1267 (1953). (5) Gryder, J. W., and Dodson, R. W., J . Am. Chern. Soc., 73, 2890 (1951). (6) Imre, L., Z . anorg. u . allgem. Chem., 104, 214 (1927). (7) Peppard, D. F., Faris, J. P., Gray, P. R., and Mason, G. W. J . Phys. Chem., 57, 294 (1953). (8) Peppard, D. F., private communication. (9) Pitwr, E. C., U. S. Patent 2,615,798 (Oct. 28, 1952). (10) Rothschild, B. F., Templeton, C. C., and Hall, N. F., J . Phys. & Colloid C h ~ m . 52, , 1006 (1948). (11) Warf, J. C., J . Am. Chpm. Soc., 71, 3257 (1949). RECEIVED for review September 14, 1954.
Accepted October 12, 1954
