Qua nti ta tive RadiochemicaI Ana lyses by Ion Exchange Sodium and Cesium LEON WISH U. S. Naval Radiological Defense laboratory, San Francisco 24, Calif.
b In the quantitative radiochemical analytical procedure for flssion product radionuclides, the alkali metal, alkaline earth, and rare earth activities are adsorbed on a cation column. The alkali metals and rare earths are eluted together before the separation of the individual alkaline earths. To determine the sodium and cesium activities, a separation from the rare earths and from each other is necessary. By using solutions of sodium and ammonium chloride it was shown that sodium and cesium activities could be eluted individually from the cation column. The subsequent rare earth and alkaline earth separations were not affected.
I
the quantitative radiochemical analytical procedure for fission product mixtures (3-6), the alkali metal, alkaline earth, and rare earth activities are adsorbed on a Dowex 50 cation resin following the separation of silver, cadmium, and rhodium. The alkali metal and rare earth radionuclides are first removed together by elution with ammonium a - hydroxyisobutyrate. Finally the alkaline earths are eluted individually by varying the concentration and p H of the same eluting agent. Since there is considerable interest in the fission product cesium-137, and the neutron-induced activity sodium-24, it would be desirable to recover them from the cation column already separated from the rare earths and from each other. At the same time this would provide a decontaminated rare earth group which could be further analyzed for the individual nuclides. Bonner and coworkers (1, 9) have investigated cation exchange equilibria between Dowex 50 and some mono-, di-, and trivalent ions, and have set up a selectivity scale. Among the monovalent ions, lithium shows the smallest and cesium the largest affinity for the resin. Between lithium and cesium, in order of increasing affinity are hydrogen, sodium, ammonium, potassium, and rubidium ions. The latter two have selectivity coefficients very similar to cesium. On this basis, it would appear that ammonium ion would give the best chance of N
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ANALYTICAL CHEMISTRY
separating sodium and cesium from each other and from the rare earths, since its selectivity coefficient is about halfway between that of sodium and cesium and is considerably less than those for the di- and trivalent ions. The sodium on the resin would readily be replaced by the ammonium ion which would then also exchange with the cesium ions when present only in tracer amounts. This would also apply to the sodium tracer on the resin if macro amounts of sodium ions are passed through the resin column. Accordingly, solutions of sodium, ammonium, and cesium chloride were investigated as eluting agents for sodium22 and cesium-137. A procedure was developed for the determination of sodium and cesium in fission product mixtures. This was included in the sequential analytical scheme. EXPERIMENTAL
Solutions of sodium-22 and cesium137 in 0.1N hydrochloric acid were passed through a Dowex 50-X8, 200to 400-mesh resin column at room temperature with a flow rate of 0.2 to 0.3 column volume per minute. The column was 0.2 sq. cm. X 6 cm. The
L
Figure 1. Elution of sodium-22 and cesium-1 37 from Dowex 50X8 at room temperature Column
6 crn. X 0.2 sq. cm.
resin was washed with 0.1N hydrochloric acid and the eluting agent added. The eluent was collected in 50-ml. Lusteroid tubes which were transferred directly to a gamma-ray scintillation well counter for activity measurements. When samples of mixed radionuclides were used, a gamma ray pulse height distribution was obtained with a 2 5 6 channel analyzer as a measure of purity. RESULTS
The elution of sodium-22 with 0.1M ammonium chloride required 30 to 40 column volumes whereas that with 0.5M required only 6 column volumes. However a significant fraction of the cesium-137 was also eluted within this volume so that there would be no separation of the two. With 0.25hl reagent the sodium was recovered in 12 column volumes, but again there was a cesium overlap, although considerably reduced. Further reduction of the ammonium chloride concentration gave a better separation but increased the elution volume of the sodium to 20 column volumes. Sodium-22 was rapidly eIuted with 0.5M sodium chloride solution in 10 column volumes. However cesium-137 began to appear in the seventh volume SO that the separation was similar to that with 0.25111 ammonium chloride. With 0.25111 sodium chloride the sodium elution was made in 12 volumes, but the cesium did not elute until the 19th volume. The cesium was quantitatively recovered in the following 14 volumes. Since 0.5M ammonium chloride eluted the cesium more rapidly, a more efficient separation was achieved by using this reagent after the 0.25hI sodium chloride. Figure 1 gives the elution curve of a mixture of the two radionuclides using both the reagents. The yields from the separation of several mixtures are given in Table I. No cross contamination could be detected in any of the fractions with the gamma ray spectrometer. To determine if the alkali metal separations had any effect on subsequent rare earth and alkaline earth elutions with ammonium a-hydroxyisobutyrate (6), composite samples of sodium-22, cesium-137, calcium-45, strontium-85
barium-133, and cerium-144 were adsorbed on a cation column. Figure 2 shows the cation exchange separation of a mixture. The sodium and ammonipm chloride elutions did not interfere with the recovery of the cerium and alkaline earths. There was a slight increase in the cerium-144 tail and about 0.2% eluted with the calcium 45. Since cerium is the lightest of the rare earths, the procedure was investigated with lutecium, the heaviest, to determine if any would be eluted with the sodium and ammonium chloride solutions. None of the lutecium-177 was eluted. DETERMINATION OF FISSION PRODUCT CESIUM
Three samples of uranium-235 were irradiatcd with thermal neutrons in the Materials Testing Reactor at Arco, Idaho. The uranium was enclosed in aluminum foil to prevent the escape of the gaseous precursdrs of the cesium radionuclides from the surface. The aluminum and uranium were dissolved in hydrochloric and nitric acids. Aliquots of this solution were analyzed for cesium by the quantitative radiochemical procedure (3-6). The results are given in Table I1 as gamma counts per minute per fission (fissions determined from molybdenum-99 analyses). Since all the samples were analyzed only a few days after irradiation, the 13-day cesium-136 was a significant fraction of the total cesium gamma-ray activity. The first and third cesium fractions were allowed to
Table I. Yields of Sodium-22 and Cesium-1 37 from Dowex 50 Cation Column
Mixture 1 2 3 4 5 6
Figure 2.
Elution from Dowex 50-X8
Column 6 cm. X 0.2 sq. cm.) volume/3 mln.
flow rate column
Na” (%) 99.3 100.6 99.7 99.8 101 .o 101.2 Av. 100.3 f .9
* 1.0
Table 11. Cesium Activities from Neutron-Irradiated Uranium-235 y C.P.M./Fissiona X 10-e Sample, CS’J’ CS’M Ut86
1
decay for 10 half lives of cesium-136. The residual activity (assumed to be cesium-137) was subtracted from the original total to give the cesium-136 which is given in Table 11. The cesium-137 was also determined in a 256-channel gamma analyzer immediately after separation by summing up the pulses in the high energy gamma ray peak. The analyzer was crosscalibrated with the scintillation well counter so that the cesium-137 activity is given in the first column as determined by the g a m m a spectrometer, but in terms of the scintillation counter. A comparison of the first two columns shows very’ little difference in the spectrometer method from the decay method which takes considerable time depending on the age of the sample.
Cs*” (%)
100.2 100.0 100.0 100.9 99.1 100.2 100.1
2
y Spec. 1.41 1.42 1.40 1.39
Decay 1.37 1.38
y
3
8
1.41 1.42 1.40 i 1.41 f .03 .02 Fissions baaed on Mom.
Decay 2.48 2.47
y
2.46 2.48 2.47
*
.@2
LITERATURE CITED
(1) Bonner, 0. D., J . Phye. Chcm. 59, 719 (1955). (2) Bonner, 0. D., Juniper, C . F., Rogera, 0.C.,la., 62,260 (1958). (3) Wish, Leon, ANAL. CHBM.31, 326 (1959). (4 Zbid.,32, 920 (1980). Zbid., 33,53 (1961). RBCBIVED for review January 19, 1961. Accepted March 13, 1981.
Radiochemical Determination of Total Rare Earths by Liquid-Liquid Extraction J. J. McCOWN and R. P. LARSEN Chemical Engineering Division, Argonne National Laboratory, 9700 South Cass Ave., Argonne, 111.
b A rapid and quantitative radiochemical method for the determination of rare earth activities produced in nuclear fission is based on the extractability of the rare earths with bis(2ethylhexy1)orthophosphoric acid. The extraction procedure affords excellent separations from the other fission elements as well as a considerable saving in time and effort.
P
for the radiochemical determination of rare earth activities in fission product mixtures depend either on precipitation, or a combination of precipitation and ion exchange, for separating the rare UBLISHED PROCEDURES
earths from other fission products. Those methods utilizing only precipitation (1, 4, 10) are designed to determine the total rare earth activity present in a sample, whereas the methods involving an ion exchange separation procedure (8, 3, 7) generally separate the individual rare earth activitiea one from another. In every instance four or more precipitation steps are required for separation purposes and these are followed by a final oxalate precipitation to determine a chemical yield factor. Such methods require 2 or more hours to complete duplicate analyses. Solvent extraction methods for rare earth separation as reviewed by West (9) are generally used in separating
individual rare earths from ores and sands. Tracer studies (6, 8) have shown that both tributyl phosphate and dibutyl phosphate extractions can be used in separating the yttrium group of rare earths from the lanthanum group. Theae solvent systems have definite possibilities in radiochemical analysis applications but there is no evidence that any investigation has been made. Peppard et al. (6)demonstrated the effectiveness of the hydrochloric acidbis(2-ethylhexyl)orthophosphoric acid (HDEHP) extraction system for separating yttrium-90 and lanthanum140 from their parent radio nuclides, strontium-90 and barium-140. In the VOL. 33, NO. 8,
JULY 1961
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