Determination of Radiostrontium by Solvent Extraction WALTER C. JOHNSON, JR. Purex Analyfical Confrol, Chemical Processing Division, Hanford Atomic Products Operation, Richland, Wash.
b A rapid radiochemical procedure for the separation of radiostrontium from a mixed fission product matrix containing Ce-Pr144, CsI3', Ru-Rh106, ZrNbg5, PrnI4', and Ygo is described. The analysis can b e performed in 45 minutes. The rare earths and yttrium are preferentially extracted into 1.5M di(2-ethy1hexyl)ortho-phosphoricacid in toluene from a solution made 0.1M in nitric acid. After a pH adjustment to 9.5, the strontium is extracted into a 0.05M thenoyltrifluoroacetone solution in hexone and then stripped into 3.OM nitric acid, simultaneously decontaminating strontium from other major fission products, Strontium recoveries are presented as a function of p H along with a complete decontamination profile. Recoveries of 98.2% were obtained b y five different analysts with a standard deviation of 1.8%.
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Figure 1. profile
Gamma
decontamination
1 . Direct scan of sample
T
recovery program a t this laboratory has shown that during purification runs there is a need for a rapid and reliable method for the determination of total strontium in a mixed fission product matrix. Standard methods of analysis for strontium used in various laboratories are based on oxalate and fuming nitric precipitation techniques (6, 7 ) , ion exchange ( l a ) with selective elutants such as alpha ammonium hydroxyisobutyrate (9), ammonium lactate ( 5 ) , and ammonium citrate ( 8 ) or paper electrophoresis ( I O ) , which involves drying of the fission products. Although these methods are very accurate, they are time-consuming and are not applicable to the rapidity which is needed for process control of the strontium purification program. Solvent extraction techniques have been reported by Kiba et al. One report employed a 5y0 solution of cupferron-chloroform for extracting the rare earths and zirconium-niobium (4), and another utilized 0.05d1 TTAbenzene at a pH of 7 for removal of the rare earths and yttrium ( 3 ) . I n this paper, 1.521 di(2-ethylhexyl) ortho-phosphoric acid in toluene is used for the rare earth, yttrium, and zirconium-niobium decontamination. HE STROXTIUM
EXPERIMENTAL
Reagents. Di (2-e thy1hexyl) orthophosphoric acid (D2EHPA) was obtained from Van Waters and Rogers
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ANALYTICAL CHEMISTRY
2. 3. 4. 5.
Organic phase after first contact Aqueous phase ofter first contact Background Strontium fraction
Co., Inc., Seattle, Wash., and diluted to the desired concentration in toluene. The prepared solvent was then cleaned by three successive equal volume contacts with a 3% solution of hydrogen peroxide, according to the purification technique used by Britt ( I ) . All other reagents used were c. P. reagent grade and further purification was found to be unnecessary. Equipment. Clinical centrifuge, Node1 C. L., International Equipment Co., Boston, Mass., Gas-Flow Beta Proportional Counter, and Multi-channel Gamma Analyzer, R.I.D.L., Model 34-12, were used. Procedure for Total Strontium. A sample aliquot containing approsimately 1.0 X lo6 d./m. of strontium-90 is dispensed into a 2-dram vial containing 2 ml. of 0.131 nitric acid and a stainless-steel stir bar. TWOmilliliters of 1.5M D,EHPX in toluene are added and the mixture is stirred and maintained at a constant emulsion for 3 minutes, centrifuged, and the organic phase is discarded. The previous step is repeated two more times, with the addition of 100 pl, of 20Y0 hydroxylamine hydrochloride on the third extraction. After discarding the final organic phase, 1.5 ml. of the aqueous is transferred to a clean 2-dram vial containing 2 ml. of 0.9.11 ammonium acetate, (pH 9.5), 250 pl: of concentrated ammonium hydroxide, and a stainless-steel stir bar. The solution is stirred well to mix. Two milliliters of 0.05V thenoyltrifluoro-
acetone (TTA) in hexone are added and the mixture is stirred and maintained a t a constant emulsion for 5 minutes, centrifuged, and 1 ml. of the organic phase is transferred to a clean 2-dram vial containing 2 ml. of 3.OM nitric acid and a stainless-steel stir bar. The mixture is stirred and maintained a t a constant emulsion for 5 minutes, centrifuged, and the organic phase discarded. A 500-p1. aliquot of the aqueous phase is mounted on a stainless-steel dish, dried slowly under a heat lamp, placed on an aluminum holder, and counted immediately on a beta proportional counter for total strontium. The remaining aqueous phase is held for a period of 7 to 10 days to permit yttrium-90 to grow back into the solution, if a strontium-90 measurement is desired. Determination of Strontium-90. One milliliter of the strontium fraction is transferred to a clean 2-dram vial after the yttrium-90 growth period (7 to 10 days). A nitric acid adjustment to 0.1X is made by adding 200 pl. of water and 300 pl. of 9.5-11 ammonium hydroxide. One and .one-half milliliters of 1.5M DzEHPA in toluene and a stainless-steel stir bar are added to the vial. The mixture is stirred and maintained a t a constant emulsion for 5 minutes, and then centrifuged. One milliliter of the organic phase is transferred t o a clean 2-dram vial containing a stainless-steel stir bar and 3 ml. of 6.0JI nitric acid. The solution is stirred for 10 minutes a t a constant emulsion, centrifuged, and the organic phase is discarded. One milliliter of the aqueous phase is mounted on a stainless-steel dish, dried slowly under a heat lamp, placed on an aluminum holder, and counted on the beta proportional counter with and without the 76.8 mg./cm.* and the 217 mg./cm.2 aluminum absorbers. RESULTS AND DISCUSSION
Prior work (2) shows that a good separation of the rare earths and yttrium from strontium was possible by making an acid adjustment to 0.1J1 with nitric acid and utilizing l . 5 X DlEHPA in toluene as the organic extractant. At this molarity the extraction of strontium is less than 0.3% Kiba et al. ( 4 ,showed that solvent extraction techniques could be used. I n their investigation, 5% cupferronchloroform was used as the extractant for decontaminating strontium from the rare earths and zirconium-niobium. The complete gamma decontaniination profile for this method is shown in Figure 1. In each case, the background
Figure 2. Effect of p H on extraction of strontium into 0.05M TTA-hexone
has been automatically subtracted. Spectrum 1 represents a direct gamma scan of the sample. The organic phase, after the first contact, is indicated in Spectrum 2, with the cerium shown to be quantitatively extracted and the zirconium-niobium only partially extracted. By making an additional contact, the zirconium was quantitatively removed. The addition of hydroxylamine hydrochloride increased the extraction of niobium which was evidenced by the 0.77-m.e.v. gamma energy peak. Whether the niobium was actually reduced to +3 valence, or a chlorocomplex was formed, rendering greater extraction, is not fully understood. However, addition of the hydroxylamine did enhance the distribution coefficient of niobium. Spectrum 3 represents the aqueous phase after organic contact. Ruthenium and cesium are shown to be quantitatively left behind confirming the low distribution ratios of these elements for this system. The spectrum indicates a decrease in peak height for ruthenium but this is mainly due to the Compton contribution from zirconium-niobium95, having been eliminated. Decontamination factors for the method were determined for the various isotopes and are presented in Table I. Calculations were based on the ratio of counts per minute taken in the initial aqueous phase to the counts per minute found in the strontium fraction. Feed material used for strontium recovery originates from fission product mixtures aged at least 120 days. Therefore, a separation from barium-140 is not needed. The method was evaluated using fission product concentrate from the Hanford Purex Process and on a synthethic mixed fission product solution, which contained accurately known quantities of cerium-144, promethium-
147, yttrium-90-91, strontium-90, cesium-137, ruthenium-103-106, and eirconium-niobium-95. Average recoveries of 98.2% were obtained on the standard solution by five different analysts with a standard deviation of 1.8%. Mount purity was determined by gamma energy analysis, and beta proportional analysis using aluminum absorbers. I n each case the activity was greater than 99% strontium-90. I n addition to the q u a n t i t a tive recoveries, the time required for each analysis was only 45 minutes, thus satisfying the requirements necessary for process control. Applications of this method have been used routinely for determining strontium in samples from the sulfate and oxalate precipitation flowsheets for the recovery of crude strontium, and on samples taken from the di(2-ethylhexy1)orthophosphoric acid flow-sheet used for the purification of strontium-90 (11). Effect of pH on Extraction of Strontium. Results of this study are shown graphically in Figure 2 . Controlling the p H between 9.0 and 9.5 provided quantitative extraction of strontium, Ammonium hydroxide was used for making the p H adjustments. Sodium hydroxide was found to form salts on the dish after drying, making the mount unsuitable for subsequent beta counting because of the absorption interference of the salts. Stripping of the strontium into nitric acid was found to be quantitative at any concentration. Three-molar nitric acid was adopted for the method. Maximum extraction was also obtained by varying the TTA concentration between 0.02 and 0.1531. Above 0.251, white solids formed a t the interface, rendering erratic results. Effect of Nitric Acid Molarity on Stripping Yttrium. The yttrium fraction after the growth period, is used t o determine the strontium-90 content. Quantitative extraction of yttrium was obtained, but stripping the yttrium fraction mas another problem. Figure 3 shows the effect that nitric acid concentration and volume ratio have for maximum stripping of yttrium. Contact time must be at least 5 minutes; less than 5 minutes gave yields of approximately 5y0 less. The use of ab-
HNOj
M
Figure 3. Effect of “ 0 3 on stripping yttrium from 1.5M DzEHPA-toluene, yttrium-88 used as tracer
0 Organic/aqueous 0
volume ratio ‘ 1 3 Organic/aqueous volume ratio 111 Contact time 10 minutes
sorbers in counting the strontium and yttrium fractions is valuable in determining mount purity. ACKNOWLEDGMENT
The author thanks G. T. Furner for assistance in development of this procedure. LITERATURE CITED
(1) Britt, R. D., Jr., ANAL.CHEM.33,602
(1961). (2) Johnson, W.C., Jr., Campbell, AI. H., U.S . At. Energy Comm. Rept., HW-SA3549 (1964). (3) Kiba, T., Nizukami, S., Bull. Chem. SOC.Japan 31, 1007 (1958). (4) Kiba, T., Ohashi, S., Maeda, T.,Ibid., 33, 818 (1960). (5) Knapstein, H., Z. Anal. Chem. 175, 225 (1960). (6)Kuroda, P. K., Arino, II., Tulunta 11, 343 (1964). (7) Metz, C. F., Waterbury, G. R., “Treatise on Analytical Chemistry,” I. M.Kolthoff, P. J. Elving, eds., 1st ed, Part 11, Vol. 9, p. 337, Wiley, New York, 1962. (8) Nelson, F., Kraus, K. A., J. Am. Chem. SOC.77,801 (1955). (9) Roberts, F. P., U. S . At. Energy
Comm. R e p t . , HW-69511 (1961). (10) Shukla; S. K., Lederer, ?*I., Anal. Abstr. 10, 2303 (1963). (11) Specifications and Standards, StronTable I.
Decontamination from Various Elements for the method
Isotope Ce-Pr-144 Pm-147 Zr-Nb-95 Ru-Rh-106 Y-90 CS-137
Decon tamination factor 2 . 7 x 104 2 . 6 x 104 2 . 2 x 104
x 2.1 x 3.9
104
2 . 4 x 105 10‘
tium Purification at the Strontium Semiworks, U . S. At. Energy Comm. Rept.,
RL-SEP-20 (1966). (12) Stanley, C. W., Kruger, P., Sucleonics 14, 114 (1956).
RECEIVED for review December 27, 1965. Accepted April 11, 1966. Division of Nuclear Chemistry and Technology, 150th Meeting, ACS, Atlantic City, N. J., September 1965. Work performed under Contract N o . AT(45-1)-1350 between the Atomic Energy Commission and the General Electric Co. VOL. 36, NO. 8, JULY 1966
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