A Rapid Radiochemical Analysis for Americ um-241 MILTON H. CAMPBELL Chemical Processing Depf ., General Elecfric Co ., Richland, Wash,
b Americium(lll) is quantitatively separated from nitric acid solutions b y liquid-liquid extractiom with di(2-ethyl hexyl) orthophosphoric acid with no valence adjustment. Americium recoveries are presented as a function of acid molarity and extractant concentration. The effect of diverse ions upon the determination is presented, and complexants are recommended for improving yields in the presence of ferric and fluoride ions. Decontamination factors for plutonium, uranium, and neptunium are presented. Americium-241 yield for this analysis is 99.2% with a standard deviation of *O.8Y0. for production of transplutonium elements ( 3 ) and radioisotopic power sources for use in remote areas has created a substantial demand for americium-241. Since a significant source of this radioisotope is found in irradiated reactor fuel, a rapid radiochemical analysis for americium-241 in process stream samples was required. The chemistry of americium has been reviewed comprehensively in several publications (4, 7 ) . Americium in solution is commonlj. in the trivalent state. I n ' general, ion exchange techniques involving the amercium(II1) ion have been successful in separating this element from contaminants. These methods, however, a.re too time-consuming to be suited to process control. Oxidation-reduction and selective precipitation have a1.so been used to accomplish separations. I n a recently reported technique (!6), americium was oxidized to the sexivalent state by reaction with silver (catalyzed peroxydisulfate. After stabilizing the americyl ion with fluoride, the solution was decontaminated by successive lanthanum fluoride scavenges. Several solvent extraction techniques have been used exteiisivelj-. Thenoyl trifluoroacetone (TTh) has been employed as an extracta:nt ( 9 ) . However, separation factors from other alphaemitting elements are low unless an elaborate predecontarnination is used. Tri-iso-octylamine (6) has been used to separate actinide elements from lanthanide elements by preferential extraction from a diilute hydrochloric acid-concentrated lithium chloride soluPROGRAM
tion. Extraction of americium(II1) into di(2-ethyl hexy1)orthophosphoric acid, DBEPHA, has been reported by Peppard and associates (8). The extraction characteristics of plutonium and uranium in DPEHPA ( 2 ) indicate a potential liquid-liquid extraction m ith high separation factors. EXPERIMENTAL
Reagents and Equipment. Americium-241 was obtained as the oxide (.4mOz) from Oak Ridge Sational Laboratory. h weighed quantity was dissolved in 6 S nitric acid to prepare a 10 gram per liter solution. Di(2-ethyl hexy1)orthophosphoric acid, D2EHP.4, of a technical grade was adjusted to the desired concentration in toluene. The prepared solvent was then cleaned by three successive equal volume contacts with a 3y0 solution of hydrogen peroxide, according to the purification technique used b j Britt ( 1 ) . These scrubs destroyed oxidizing impurities and improved phase separation. .ifter the find contact, the organic phase was centrifuged and decanted into a storage bottle. Solvent cleaning was repeated every seven days; however, only one contact was made following the initial cleaning. Sample disks for radioassay were prepared by drying a sample aliquot on a i/*-inch passivated, stainless steel disk. The total alpha activity on the sample disk was counted in a pa. flow proportional counter with a 50% geometry. Sample disks were also analyzed for alpha energy spectra using a solid state surface barrier detector in conjunction with a multichannel analyzer.
Disposable stirring bar- for use n.ith magnetic st rers were made fi,om a str$ of IO-mil magnetic ,itainle.ss qtepl, 3 / 4 inch long and 1 '8 inrh wide. Ciiving thip strip a full twist lirovided good stirring characteristics. Procedure. Pipet a sample aliquot, having a maximum of 0.1 microgram americium, into a 2-dram \ial containing a stainless steel stirring bar. Make the aqueou.c phase U l J t o 1.5 milliliters volume after adjusting the nitric acid concentration to fall in the range: 0.05~lf-0.10A14.Pipet three milliliters of 50% D2EHP.i in toluene to the vial, then maintain the phases at a constant emulsion for five minutes using a magnetic stirrer. Sellarate the phases by centrifuging for two minutes. Transfer exactly two milliliters of thP organic phase to a clean vial containing a stainless steel stirring bar and one milliliter of 3 J HXO,. .igain maintain the phases at a complete emulsion for five minutes, then separate phasea 1)centrifuging. Draw the organic phase off with a micropipet and diqeartl i t . Withdraw 0.25 ml. of the aqueous ))has? by sparging through the thin sorganir layer before loading the pipet. 1Iount the sample on a 7, inch stainless steel disk; dry, flame, and count i t on an alpha proportional counter. DISCUSSION
According to prior work (8)) americium appears as a contaminant in rare earth fraction; hel)arated with 1I2EHPAL In this analysis, how.rver, the converse is true. I'remwe of rare earths normally found in aged reactor fuel does not interfere in this analysis
M "03
Figure 1 . Extraction of Am2" into D2EHPA as a function of nitric acid molarity (0.01 p g . of americium t a k e n ) VOL. 36, NO. 1 1 , OCTOBER 1964
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60 4.
401 20
N I
E 4
c Lu u L Lu Ll.
0
20
10
40
50
yield as a function of
D2EHPA
30
Percent DZEHPA (by Volume)
Figure 3. Americium concentration
(0.01 pg. of americium taken)
since americium is unique in being the only alpha-emitting radioisotope of the separated fraction. Alpha proportional counters have a high tolerance to beta-gamma activity, so none of the lanthanides would be counted. I n the extraction, plutonium, uranium, some zirconium, and the rare earths were found in the organic phase as well as the desired americium. The remaining fission products were retained in the aqueous phase for discard. The extraction behavior of americium as a function of nitric acid concentration is shown in Figure 1. Using these data, a nitric acid concentration between 0.05 and 0.10JI was selected to provide maximum americium extraction. The acid molarity of the stripping solution also influenced aniericium re-
Table I. Americium(lll) Decontamination with A 3M HNOB Stripping Solution
(0.01 p g . americium-241 taken) Overall decontamination Element factor 2 . 3 X 10' Pu(1V) uoz + 2 3 . 5 x 104 1 . 0 x 103 NpOz +
Table II. Effect of Diluent on Americium(ll1) Yield
(0.01 fig. ameririum-241 taken) Amz4' extracted, Diluenta 70 Toluene 99 4 Xylene 98 9 Benzene 98 5 Carbon tetrachloride 93 9 Shell spray base 91 6 The DZEHPA concentration in each diluent was 50%.
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ANALYTICAL CHEMISTRY
covery as would be expected by the extraction characteristics. Figure 2 presents this information. .iny acid concentration in the strip between 3 and 6W yielded quantitative recoveries. Three molar acid was selected for the strip concentration to provide the optimum separation from alpha emitters in the solvent. Overall decontamination from alpha emitters using the recommended acid stripping molarity is shown on Table 1. At stripping acid molarities higher than 3JI, lower decontamination factors were experienced. The D2EHPX concentration also had an effect on the americium yield as illustrated in Figure 3. X 50-vol. 70 solution of DZEHl".A was adopted for the separation. Extraction of nonradioactive cations could reduce the DZEHPA concentration available for extracting americium, so this high concentration was selected to provide a desirable excess. Moreover, the 50y0 concentration was low in viscosity, permitting quantitative solution transfers. Several organic diluents for the DBEHPX were tested in addition to the toluene. Table I1 shows that aromatic organic compounds were the best diluents tried. This method was evaluated using a standard which contained 2 pg. per liter of americium(III), 1.0 mg. per liter of plutonium(IV), and 2 grams per liter of T'Os-. Repetitive analyses of this standard showed an average recovery of 99.2%, and a standard deviation of &0.8%. .Alpha energy analyses of the mounts showed the activity was greater than 9970 americium-241, illustrating the quality of this separation. In addition to the satisfactory yield, time expended for the analysis was only 45 minutes, thus
satisfying requirements for process control. Most process streams containing americium would require dilution prior to analysis, because of the high level of radioactivity involved. .As a consequence of this dilution, the total cationic concentration of the sample would be less than 0.1.11. Waste solutions from an aniericium recovery process, however, would involve analysis for low level activities in high salt solutions. The effect of diverse ions on this determination were investigated with the results shown on Table 111. Generally, the anions did not seriously interfere in the analysis. The poor recovery in the presence of fluoride was Table 111. Effect of Diverse Ions on the Determination of Americium(Ill)
(0.01 pg. americium-241 taken) Americium-241 extracted, n Ion Molarity /C NO'5 0 97 7 2 5 99 4 0 5 96 0 0 25 96 6 c199 4 1 0 I1 0 99 4 F1 0 0 2 0 1 Crz0,-2 99 4 Cz04-2 0 02 99 4 0 25 94 9 A1 + 3 1 0 89 3 0 5 A1F -z 89 3 Fe +3 1 0 2 5 0 25 25 4 Ca +z 0 5 78 8 0 25 88 8 ?.Ig+% 0 5 95 9 Li 8 0 72 0 1 0 98 1 +
predictable, but addition of aluminum to complex the fluoride satisfactorily increased recoveries. I n general, high concentrations of cations reduced the measurable americium, although not to the extent that this extraction would be unsuitable for low level waste streams. Although the true effect of foreign cations could not be explicitly determined, the author believes reduced americium recoveries were not caused by loss 011 extraction, but rather by absorption of the alpha particle on the sample disk. Since a fraction of several cations did follow americium through the separation, they mould necessarily be present on the sample disk to serve as a partial barrier between the americium and the detector. Such inert solids were visually detectable in several cases, and always discernible by loss of resolution in the alpha energy spectra. Additionally, a second extraction with fresh D 2 E H P h yielded no further americium-241 activity in the organic phase. The ferric ion presented by far the most serious cationic interference. Al-
though complexing with oxalate or thiocynate did improve the americium yield, phase separation was too difficult to be practical. Fluoride satisfactorily complexed the iron a t mole ratios of the fluoride to ferrous ions between 1.3 and 1.6. Table IV shows the improvement in americium yields. As the ratio exceeded 1.6, fluoride appeared to interfere in the americium extraction. ACKNOWLEDGMENT
The author thanks C. ;i.Radasch for his technical assistance in the development of this procedure. LITERATURE CITED
(1) Britt, R. D., Jr., ANAL.CHEY. 33,
602 (1961).
( 2 ) Campbell, 11. H., C. S . At. Energy ,Comm. Rept., HW-64619 (1960). (3) Chem. Eng. S e w s 41, S o . 24, 52
11963). ( 4 j Keenan, T. K., J . Chem. Educ. 36, 27 (1959). (5) Moore, F . L., ANAL. CHEY. 35, 715 (1963). (6) Zbid., 33, 748 (1961).
Table IV. Americium(lll) Yield as a Function of F-/FeC3 Mole Ratios
(0.01 9g. of americium-241 taken) F-/Fe Americium-241 mole ratio recovered, yo 0 25 4 0 4 54 9 77.3 0.8 87.3 1.0 91.9 1.3 91.9 1.6 87.3 1.9 (7) Penneman, R. A., Keenan, T. K., “The Radiochemistry of Americium and Curium,” NAS-NS-3006 (1960). (8) Peppard, D. F., LIason, G. W., Maier, J. L., Driscoll, W. J., J . Inorg. Sucl. Chem. 4 , 334 (1957). (9) Schneider, R. A , , Harmon, K. bl., USAECD-HW-53368 (1961). RECEIVEDfor review l l a y 1, 1964. Accepted July 31, 1964. 15th Pittsburgh Conference on Analytical Chemistry and rlpplied Spectroscopy, March 2 , 1964. Work performed under Contract KO. hT(45-1)1350 between the Atomic Energy Commission and the General Electric co.
Determination of Silver, Cadmium, Indium, Tantalum, Rhenium, Gold, and Rare Earths at Low Concentration by Neutron Activation and Radiochemical Analysis CHESTER
E. GLEIT,’
PHILIP A. BENSON, and WALTER D. HOLLAND
Tracerlab, A Division of Laboratory for Electronics, Richmond, Calif. IRVING J. RUSSELL Air Force Weapons Laboratory, Kirtland Air Force Base, N . Five types of organic filter media and high purity quartz were analyzed for Ag, Cd, In, Tb, Hot Er, Tm, Ta, Re, and Au. Organic matter was removed by reaction withe lectrically excited oxygen before neutron irradiation. After irradiation, radiocheimical separations stressing rigorous purification were utilized. A combination of beta-ray counting and gamma-ray spectrometry was employed to determine the activity of the separated (elements and to check sample purity. Concentrations as low as one part in 10” were measured. Large differences in traceelement concentration were observed. In the most extreme case, four sheets of I.P.C. grade 107 filter paper displayed a 30-fold variation in Ag and Au concentrations. A study of the sources of experimental error indicates that the technique is #capableof yielding quantitative data and that the observed variations are caused b y nonuniformity among the specimens.
A
M.
employing high intensity neutron sources permits accurate determination of over 30 elements in quantities of less than 10+ gram. By minimizing chemical treatment before irradiation and by using rigorous radiochemical decontamination procedures after the neutron bombardment, concentrations of less than 1 p.p.b. can be measured. As part of a program to determine the minimum blank correction which could be obtained in a study of the atmospheric concentration of trace elements, a high purity quartz and five types of organic filter media were analyzed for nine extremely rare elements. The combined requirements of specimen purity and elements of low abundance suggest the concentrations measured in this program may be the lowest ever determined by activation analysis. To achieve this goal, novel manipulation and incineration techniques were employed before irradiation to avoid CTIVhTION AKALYSIS
contamination of the filters from reagents or dust. To increase the sensitivity and accuracy of measurements of weak activities in the presence of large concentrations of other activation products, high specificity radiochemical separations followed by beta- or gammaray counting were used rather than a purely instrumental method based on gamma-ray spectrometry. Direct activation of large organic specimens is undesirable because intense neutron bombardment degrades organic compounds producing tars and volatile gases. The generation of gas in the sealed samples has been known to produce hazardous pressures. Furthermore, a large variation in neutron energy may occur throughout a bulky hydrogenous sample and lead to erroneous results. To obviate these difficulties, all samples were ashed bel Present address, Dept. of Chemistry, Sorth Carolina State, University of North Carolina, Raliegh, S . C.
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