Electrodeposition of Alpha-Emitting Nuclides from a Mixed Oxalate-Chloride Electrolyte Kenneth W. Puphal and Donald R. Olsen Health Services Laboratory, US.Atomic Energy Commission, Idaho Falls, Idaho A method is described for electrodepositing various alpha-emitting nuclides, singly or in combination, onto a stainless steel cathode. The nuclides are deposited as hydroxides to better than 98% in 50 minutes from an ammonium chloride-ammonium oxalate electrolyte. The optimum conditions were determined by studying the effects of time, current, electrolyte concentration, electrode spacing, pH, and the effect of hydroxylamine as a reducing agent. Additional studies include electrolyte variations with strong organic acids and a diverse ion study of various anions and cations. The use of a chelating agent reduced the interference of some cations and greatly improved the yield and precision of several nuclides. Fluoride markedly alleviated the interference caused by iron, aluminum, natural thorium and zirconium. However, the presence of even microgram quantities of the rare earths caused serious losses in the presence of fluoride. The procedure was developed with 239Pubut is applicable to thorium, protactinium, uranium, neptunium, americium, curium, and californium, as well as cerium, polonium, and bismuth. Five replicate depositions of 239Pugave an error of less than 2% standard deviation at the 95% confidence level with an average yield greater than 99%. The resolution obtained with 241Amwas 24 keV full width at half maximum peak height.
RELIABLE DETERMINATIONS of the alpha-emitting nuclides require well prepared sources of activity. Although these sources may be prepared by evaporation or sputtering, electrodeposition is the method most commonly used to obtain undegraded energy spectra and good chemical yields. Many of the electrodeposition methods currently used have serious drawbacks that often prevent their efficient and economical application to various types of samples. The deposition might require several hours, or the cathode material is restricted t o platinum for highly corrosive electrolytes such as chloride or sulfate. Because platinum is expensive and must be cleaned and a background determined before reuse, a new method was sought with a relatively short depositing time which uses a less expensive cathode material such as stainless steel. Platinum has a n additional disadvantage in that polonium is surprisingly difficult to remove satisfactorily for 1000minute spectra. The method should be capable of depositing the actinides from their different valence states with high chemical yields, and the deposited samples should permit good resolution of alpha energies. Since some preliminary investigations in this laboratory have shown that electrodepositions are extremely critical where interferences are concerned, a further requisite in the development of the method was t o show the effect of interferences on yield and resolution of alpha spectra. Roslyakov and Ezhova (1) describe a method for electrodepositing uranium from an alkaline oxalate medium in 15 minutes. However, their results were not reproducible in this laboratory. Earlier, Mitchell (2) obtained nearly quantitative (1) V. S. Rosllakov and M. P. Ezhova, SOC.Radiochem., 7, 621 (1965). (2) K.F. Mitchell, ANAL.CHEM., 32, 326(1960). 284
yields depositing various actinides from a n ammonium chloride-hydrochloric acid system onto platinum in 10 minutes. These results were reproducible but longer depositing times were required than claimed. Cathodic deposition of plutonium from an alkaline medium (3) often requires 2 t o 3 hours and a prior oxidation to the hexavalent state. The resulting deposits are not always free of extraneous material with yields often as low as 90 %. An additional disadvantage of alkaline deposition is that many nuclides which form insoluble hydroxides might not deposit on the cathode, but could precipitate and remain suspended in the electrolyte. Barring ( 4 ) reports a method for depositing from an ammonium nitratenitric acid system. Although a fairly good yield was obtained by this method, better yields with a shorter depositing time were desired. Bains (5) describes a method using low concentrations of chloride in an oxalate medium but his depositing time of 100 minutes is undesirably long. The experimental deposition of plutonium onto stainless steel from a n oxalate medium revealed excellent tolerance to significant concentrations of chloride without noticeable cathodic corrosion. The chloride-oxalate system presented combines the speed and reliability of a pure chloride system with the clean plates of a n oxalate medium, and permits the use of a relatively inexpensive stainless steel cathode. Several examples of the excellent resolution obtained by this method are shown in recent publications from this laboratory (6-8). EXPERIMENTAL Apparatus. All depositions were made with a n Eberbach Electro-Analysis apparatus modified t o maintain a constant preset current. The rotating platinum anode (Engelhard Industries, Inc., Carteret, N.J., No. 613) is 0.5 inch in diameter, 5.5 inches long, and is composed of pure platinum gauze supported by wire and stem of 95 platinum-5 % ruthenium. The stainless steel cathode disks were 1.875 inches in diameter, 20 mils thick, with a paper covered No. 8 polish finish (Salt Lake Stamp Company, Salt Lake City, Utah). The deposition cell is a glass joint 4 inches high with a Teflon ‘‘0”-Ring seal (Owens Illinois Glass Co., Toledo, Ohio, Kimble No. 33650) as shown in Figure 1. Either a 20-mm or 25-mm i.d. glass joint can be used interchangeably in the polyethylene collar shown in the figure, giving deposited areas of about 480 mm2 and 760 mm2, respectively. Other cell sizes require modification of the collar. The 20-mm i.d. joint was used for all the developmental work done in this paper. The brass base is jacketed for water cooling t o prevent corrosion of the cathode from overheating. One or two drops of water are placed between the cathode and the base t o aid in cooling. (3) M. L. Miller, US. At. Energy Comm. Rep., MDDC-468 (1946). (4) N . E. Barring, Actiebolaget Atometlergi (Stockholm, Sweden), Rep., AE-217, Feb. 1966. (5) M. E. D. Bains, U . K . At. Energy Auth. Rep., AEEW-R-292 (Dorset, England), 1 (1963). (6) C . W. Sill, Health Phys., 17, 89 (1969). ( 7 ) C. W. Sill and R. L. Williams, ANAL.CHEM., 41, 1624 (1969). (8) C. W. Sill and D. G. Olson, ibid., 42,1596 (1970).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
I
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Figure 3. Effect of current density on the deposition of plutonium
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Figure 1. Electrodeposition cell 100
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Figure 4. Effect of oxalic acid concentration on the deposition of plutonium
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Position the anode to stir approximately 4 m m above the cathode and electrodeposit the sample at 3.6 amperes (0.75 A/cmz) for 50 minutes. Adjust the flow rate of the cooling water so that the temperature of the electrolyte is 25 t o 50 OC throughout the deposition. Because of evolution of chlorine, the deposition must be performed in a fume hood. At the end of the deposition, add 2 ml of concentrated ammonium hydroxide to the cell, and with the current o n and the stirrer off, lower the cell away from the anode. Discard the elecThe alpha-emitting nuclides were electrodeposited from solutions of approximately 2 X 10 dpm and were counted for trolyte and rinse the plate with 0.1M ammonium hydroxide. 10 minutes by alpha scintillation as previously described (6) Dismantle the cell and carefully rinse the plate with 0.1M t o give a n error
