John C. Sheppardl and Reilly C. Jensen
San Diego State College San Diego, California
II
A radiochemistry experiment
Some Chemical Properties of Neptunium
The purpose of this undergraduate-level radiochemistry experiment is to examine the chemistry of neptunium. Part of the experiment repeats classical chemical studies of Seaborg and Wahl ( I ) , and part of the experiment involves the study of ion exchange and solvent extraction techniques currently used to isolate neptunium. By doing such experiments with neptunium-239 the chemistry of an actinide element can he obtained without the hazard associated with an alpha emitter. Through additional reading (t,5) and classroom discussion the student can learn about the analogous chemistry of the other actinides. Neptunium-239 is conveniently produced by neutron irradiation of uranium-238. To emphasize the nuclear transformations involved in the production, the student should calcolate the amount of neptunium formed during the irradiation from the weight of uranium, the neutron flux, and the irradiation time. A microcurie of neptunium-239 is quite sufficient for each student. Since it is desirable to allow the short-lived fission products to decay for one or two days before permitting students to begin the experiment, it is not necessary to have a reactor in the immediate locality, provided that convenient shipping arrangements can be made. The irradiated uranium is dissolved in a few milliliters of 4 11.1 nitric acid-0.01 M iron(I1) chloride-0.02 M sulfamic acid solution. Hexanitrato neptunium(1V) ion is separated by either absorption on a Dowex-l column and elution with 0.25 M nitric acid (4, 5) or by extraction with an equal volume of lo'% (v/v) trilanrylamine in xylene (6). The neptunium-239 is removed from the trilaurylamine solution by equilibration with 2 ml of 5 M perchloric acid. The extraction procedure is faster and easier for the student although excellent fission product decontamination is obtained by either method. The student confirms the radiochemical purity by following the decay of a separated sample for two weeks. After the neptunium-239 has been separated, lo4 c/m aliquots are treated with Fez+, MnOal-, CrzO?, and Ce4+ in order to selectively form neptunium(1V) Present address, Hanford Laboratories, General Electric Co., Richland, Washington.
or nept,unium(VI). Each oxidation state is then treated with 10 mg of lanthanum and 1 ml of hydrofluorir acid, and the amount of eoprecipitation determined by counting the precipitate. The amount of coprecipitation with bismuth phosphate is determined in n similar manner. To reduce contamination problems and to avoid filtering and mounting, the separated precipitates may be slurried into counting tubes and counted in a well-type scintillation counter. This procedure speeds up the experiment considerably. By judicious selection of additional oxidizing and reducing buffers (1) and noting which oxidation state is coprecipitated, the formal neptunium(1V)-neptunium(V1) potential may also be obtained as an optional part of the experiment. The distributions of neptunium(1V) and neptunium(VI) between carbon tetrachloride solutions of tributyl phosphate and nitric acid and between hexane and nitric acid are studied next (7). These experiments are conveniently done on a milliliter scale with stoppered centrifuge tubes and a. little shaking. Additional studies involve the determination of the distribution ratio as a function of the nitric acid concentration. Also the study of the extraction of neptunium(1V) with methyl isobutyl ketone or with xylene solutions of some tertiary amine, such as tri-n-octylamine, can be carried out. The time required for this experiment depends upon the number of precipitations and distributions done, but usually two three-hour laboratory periods are sufficient. Additional time is required for the determination of lhe half-life of neptunium-239. Literature Cited ( 1 ) SEABORG, G . T., AND WAHL,A. C., "The Transuranium Elements," edited by SEABORG, G. T., AND MANNING, W. M.. McGraw-Hill, New York, 1949, paper 1.6, p. 25. ( 2 ) KATZ,J. J., AND SEABORGI, G. T., "The Chemistry of the Actinide Elements," John Wiley and Sons, New York, 1957. (31 HINDMAN. J. C.. J. CHEM.EDOC..36. 21 (1959). , , (4) RYAN,J.'L., U: S. Atomic Energy Commission document, HW-59193, Sept., 1959. ( 5 ) SHEPPARD, J. C., AND RECRER, G. V., unpublished documents. (6) KEDER, W. E., SHEPPARI), J . C., AND WILSON,A. S., J . Znorg. and Nucl. C h m . , 12, 327) (1960). (7) SEABORG, G. T., AND WAHL,A. C., ibid., pp. 252-8.
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Volume 40, Number I, Jonuory 1963
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