T h e variation of intensity of the LY line of uranium in the range of 1.5 to 3.0 mg. per ml. rvas of such magiiitude that a scintillation counter was required to obtain the required precision in these analyses. For routine analytical work, four pairs of counts are usually sufficient (Table 111). Voltage Regulation. T h e voltage regulator supplied m-ith t h e x-ray equipment is not capable of regulating t h e current required b y t h e instrumentation. \Then t h e x-ray basic unit is operated a t 50 k r . a n d 30 ma., i t alone draws the maximum rated load from t h e regulator. An additional voltage stabilizer was, therefore, necessary t o regulate t h e voltage input required b y the electronic panel. T h i s was a n important factor in ob-
L
taining t h e precision in counting of which t h e equipment is capable. Radiolysis. When a n aqueous solution is exposed t o certain types of radiation (gamma, neutron) gas is formed even though t h e solution is cooled. T h e formation of hydrogen and peroxides in t h e present case is caused by t h e radiolytic action of the x-rays. This can readily be proTed by placing a colorless solution of titanium sulfate in the x-ray beam and noting the development of the yellow color of titanium peroxide. -4 rough calculation shows that 1,000.000 roentgens of Yradiation are produced in 3 minutes a t 1 em., (path to the sample) The formation of hydrogen causes a slight increase in counting time, up to 1 second, on successive counts of the same solution if counting times are prolonged; lion-ever,
the ratio of intensity of uranium to intensity of strontium remains the same. LITERATURE CITED
(1) Birks, L. S., Brooks, E. J., ANAL. CHEW23, 707-09 (1951). (2) Pish. G.. Huffman. -4.A.. Ibid.. 27. \
,
1875-8 (1955).
,
I
’
(3) Silverman, L., Houk,
W. W.,North
American Aviation, Inc., Special Rept. 1788, Narch 15, 1957. (4) Silverman, L., Houk, W.IT., Taylor, W., Sorelco Reptr. 1, 118 (1954). (. 5,) Silverman. L.. ‘Moudv. L.. ANAL. CHEM.28, 45-7 (1956). ’ (6) Silverman, L., Moudy, L., Hawley, I)., Ibid., 25, 1369-73 (1953). RECEIVEDfor review January 28, 1957. Accepted August 22, 1957. Sixth Annual Conference on Industrial Applications of X-Ray Analysis, Denver, Colo., August 1957. Based upon studies conducted for the Atomic Energy Commission under contract AT-1 1-1-Gen-8.
Separation of Strontium from Calcium with Potassium Rhodizonate Application to Radiochemistry H. V. WEBS
and W. H. SHIPMAN
Chemical Technology Division,
U. S.
Naval Radiological Defense laboratory, San Francisco 24, Calif.
Strontium can b e separated from calcium with the organic precipitant, potassium rhodizonate. A single precipitation can separate 50 mg. of strontium, determined as oxalate, from calcium chloride in quantities up to 1 1 grams, with strontium recovery and calcium removal efficiency greater than 80 and 99%, respectively. The extent of recovery, which i s reproducible, is a function of the calcium, strontium, and rhodizonate molar ratios. The two procedures given involve simple and rapid manipulations and employ easily handled chemicals, in contradistinction to the conventional procedure which employs relatively large volumes of 75% nitric acid.
T
Hh + C P ~ R A T I O S of
milligram quantities of strontium from gram quantities of calcium frequently arises in the determination of strontium-89 and -90 radioisotopes in calcareous soil and biological specimens. The conveiitional wet procedure most often used in the radiochemical scheme is predicated upon the differential solubility of strontium and calcium in 75% nitric acid (3). O r i n g to the limited solubility of calcium in concentrated nitric acid, rel-
1764 *
ANALYTICAL CHEMISTRY
atively large volumes of the reagent and repeated precipitations are often necessary to effect separation. Because manipulations with this acid are both hazardous and time-consuming, an alternate procedure was sought. Salts of strontium, but not calcium. react in neutral solution with sodium rhodizonate to form a stable brown-red precipitate ( 1 ) . Use of this reagent to separate these elements was investigated. Preliminary experiments determined the dependence of precipitation upon calcium, strontium, and potassium rhodizonate concentrations. Procedures TI ere developed, and with the aid of a strontium-85 tracer the optimum conditions of separation and recoverv n-ere &:Lblislieci. INSTRUMENTS A N D REAGENTS
Strontium-85 ganinia activity \\-as determined in a scintillation n-ell counter. Potassium Rhodizonate Solution (Eastnian Kodak). Because the reagent is stable for only several hours, a 0.2% aqueous solution was freshly prepared just prior to use. Strontium-85 Tracer. The source of strontium-85 was a year-old sample of neutron-irradiated strontium nitrate.
The purity of the tracer was estahlishcd by gamma spectral analysis. Strontium Solutions. 4 solution of strontium-85 tracer and strontium chloride carrier (Baker, reagent grade) n as prepared to give 3000 c.p.m. per nil. and 12 mg. per nil. of strontium, determined as the oxalate. T o avoid high counting rates, another solution of stable strontium m s prepared for diluting the active carrier solution when quantities of the carrier exceeding GO nig. n-ere desired. Standard Cnlciuni Solution. Calcium chloride (Raker, reagent grade) was dissolved to a specified volume 11-ith distilled water; the concentration of this solution (3.9731) n as determined by titration n ith standardized (ethylenedinitri1o)tetraacetic acid. Appropriate dilutions of the stork solution lyere mndc as required. All other chemicals n-ere of c i t h reagent grnde or C.P. quality. CONCENTRATION RELATION
Dependence of precipitation upon the calcium, strontium, and precipitant concentrations was determined in a preliminary experiment. Systems prepared to contain 1-arying concentrations of the cations to be separated m-ere ti-
trated with the rhodizonate reagent until a definite precipitate appeared. Figure 1 clearly indicates that a t a given concentration of rhodizonate, precipitation can be effected in the presence of greater quantities of calcium as the strontium concentration was increased. Furthermore, a t a constant calcium concentration less precipitant was required as the strontium content was increased, PROCEDURES
Two procedures nere studied by recovery experiments to establish the optimum concentration of rhodizonate to produce a favorable yield. The range of calcium concentrations n hicli could be conveniently handled in the presence of no more than 50 nig. of strontium carrier (as oxalate monohydrate) was also investigated. The restriction on carrier iveight was imposed by the anticipated application of these procedures to the direct determination of strontium-00. (Under the counting conditions adopted a t this laboratory the beta radiations of the nuclide are self-absorbed and scattered with a loss of 2570 when the final precipitate, strontium oxalate monohydrate, is 40 mg. The maximum alloilable correction for thwe factors n a e arbitrarily limited to this value.)
10
0
20
POTASSIUM
nil. of 1Oyooxalic acid, and 5 nil. of 307, hydrogen peroxide. The niixture was placed on a hot plate for 10 minutes to metathesize it to the oxalate. The oxalate precipitate \vas collected on a weighed paper disk by filtration and nashed with 20 ml. of hot nater. After being dried in the oven a t 110" C. for 20 minutes, the precipitate \Tas n-eighed and its gamma activity n-as counted. The neights of strontium and calcium were computed from these data. (The error attributable to \I eiahing ant] counting Tvas generally Gss than 257.) Modilfed. After addition of the iliodizonate reagent to t h e calciumstrontium solution, enough acetone \vas introduced t o create t n o separate phases. T h e mivture as allowed t o stand for 30 minutes, then t h e upper acetone layer was decanted. T h e aqueous phase was centrifuged a n d t h e supeinatant discarded. T h e precipitate n a s stirred for 2 or 3 minutes n i t h 45 nil. of absolute ethyl alcohol, centrifuged, and washed with a similar volume of 11-ater. After isolation and solubilization of the rhodizonate with acid, the sample \\as nietathesized, weighed, and counted as in the direct procedure.
Table I. Recovery of Strontium by Direct Method at Different CalciumStrontium Molar Ratios cium Cal- c/a of Original
cat
Added
A,
g.
2 2 1 0 0
0 0 2 4 4
stron-Quantity Retiurn
sr+ +,
iiolar
mg.
Ratio 30 50
146 2 877 466 224 324
56 39 36
covered in Precipitate S r i + Ca++ 88 8 4 '3 793 809 815 829
2ti 2 1 1 0
2 1
RECOVERY STUDIES
Optimum Concentration of Potassium Rhodizonate. A series of re-
30
40
50
60
R H O D I Z O N A T E 0.2 PER CENT (ml.)
.
Figure 1 Concentration conditions required for strontium rhodizonate precipitation in presence of calcium
Direct. Appropriate volunic)s of calciuni a n d radioactive strontium solutions, not greater t'han 50 ml., were mixed in a beaker and treated with to t h e potassium rhodizonate solution optinlunl conditiolls for separating st,rontium, precipitate \,,.as separated by centrifugation. ~h~ insoluble rhodizonate was washed once \vith 45 Inl. of \yat,er and the precipitate was dissolved in a small quantity of concentrated hydrochloric acid (1 or 2 nil.), T o this solutio11 mere added 20 nil. of concentrated ammonium hydroxide, 5
recovery of strontium in t h e presence of 0.4 t o 2 grams of calcium ion was determined a t a rhodizonate-strontium ratio of 4. I n each case t h e actual recovery was within 2% of t h a t anticipated from t h e calcium-strontium molar ratio (Figure 2 ) . T h e d a t a (Table I) indicate t h a t , for a n SO% recovery of strontium under the aforementioned restriction, this procedure was effective in media n hicli contained up to 0.4 gram of calcium ion.
covery experiments \vas coiiducted by t h e direct procedure in ,\-hi& t h e collcentration of rllotlizonate varied as t h e calciLIIn-strontium lllolar ratio rras maintained constant. T h e optimum recovery \vas achieved at, a rliodizonate-strontium molar rat'io of 4 (Figure 2). Increasing this ratio b!: a factor of 2 or 4 did not improve t h e recovery. T h e results also dio\v t h a t t,he recovery of strontium was depressed as t h e calcium-strontium ratio was increased from 15 t o 50. Recovery of Direct Procedure. T h e
Recovery of Modified Procedure. T h e results of a series of recovery experiments b y t h e modified procedure are shown i n Table 11. I n one series t h e precipitation of 16.8 and 22.4 mg. of strontium ion in a matrix of 0.8 t o 4.0 grams of calcium ion was studied TI ith a rhodizonatr-strontium molar ratio of 2. T h e recoveries ranged from 65 t o 85% and were related t o the calcium-strontium molar ratio. Duplicate analyses were consistently reproducible. An average of 3 mg. of calcium ion coprecipitated a t the lower carrier level and slightly more when 22.4 mg. of strontium ion was used. ilnother series of recovery experiments was performed in which 2.0 t o 8.0 grams of calcium ion was separated from 22.4 mg. of strontium ion a t a rhodizonate-strontium molar ratio of 4. The recovery of strontium from 2 and 4 grams of calcium was 10 to 15% greater when the rhodizonate-strontium molar ratio was increased from 2 to 4 (Table 11). Concomitantly, a t the higher concentration of the precipitating agent more calcium coprecipitated (8.4 to 20.3 mg.). Purity of Strontium Separation. Amounts of calcium normally coprecipitating with strontium (4 t o 32 mg. of calcium ion) were added t o 22.6 nig. of strontium ion and treated by the direct procedure n i t h a 2 to 1 molar ratio of rhodizonatc-strontium t o determine t h e purity of separation by a second precipitation. T h e recovery of strontium was greater t h a n 90%. I n media which initially contained 4 to 8 nig. of calcium ion, deVOL. 29, NO. 12, DECEMBER 1957
* 1765
tectable quantities of calcium were absent in the final precipitate. A small quantity of calcium ion (0.8 mg.) followed strontium when the initial calcium ion concentration was 16 mg.; there was somewhat more (3.3 mg.) in a medium which contained 32 mg. (Table 111).
The precipitation of strontium in a medium which contained the products of a 1-year-old fission mixture revealed that the rare-earth radionuclides and zirconium-95 were carried b y the insoluble rhodizonate. Metathesis of the strontium rhodizonate t o the carbonate, followed by acid solubilization and an
I
/
Iv1
I
I
60
Table II.
I
I
I
1
I
I
I
J
Recovery of Strontium and Calcium by Modified Method at Different Calcium-Strontium and Rhodizonate-Strontium Molar Ratios
Added Rhodizonatestrontium molar ratio
0.8
Sr + +, mg. 16.8 16.8 22.4
2.0 2.0
22.4 22.4
Ca + +, g.
0.8
2 2 2 2
2.0
4
4.0 4.0 8.0
2
4 4
Calciumstrontium molar ratio 104 260
78
22.4 22.4
390 580
22.4
Table 111. Separation of Strontium from Residual Calcium by Direct Method a t Rhodizonate-Strontium Molar Ratio of 2 yo of Original
Added, Mg. Ca++ Sr++ 4 22.6 8
22.6 22.6 22.6
16 32
Quantity Recovered in Precipitate Sr++ Ca++ 90.0 0.0 96.0 0.0 92.4 5.0 93.4 10.3
Interferences and Radioactive Decontamination. Under t h e conditions of analysis, t h e following ions are precipitated b y potassium rhodizonate (9): Cd++ Ag Bi+++ Zn++
: g+ cu++ +
Hg++
1766
uoz
+
+
Ba++
ANALYTICAL CHEMISTRY
% of Original Qu-antity Recovered in Precipitate
Sr++ 82.4 f 0 . 1
705fO4 85 3 7844~08 8453~06 69 8 819+21 6 3 4 i 2 0
Ca++ 0 . 5 i 0.05 02zkoo
0 6 02*005 1 3 + 0 1 0 1 0 5 1 0 1 O2fO05
iron scavenge (4, effectively decontaminated strontium from the interfering nuclides. DISCUSSION
The conditions for strontium rhodizonate precipitation are defined b y the rhodizonate, calcium, and strontium concentrations. T h a t calcium competes with strontium for rhodizonate to form a soluble calcium rhodizonate complex is evident from the incremental quantities of strontium or rhodizonate required for precipitation as the calcium concentration is increased. The observation that the insoluble strontium rhodizonate is readily dissolved upon addition of a soluble calcium compound to a suspension of the precipitate emphasizes further the competitive aspects of the system. The direct procedure does not produce an 80% yield of strontium if the calcium-strontium molar ratio exceeds
50. When separation is performed preliminary to radiochemical determination, it is recommended that the direct procedure be applied in situations where the calcium ion content is not greater than 0.4 gram. I n this range the required quantity of strontium carrier does not preclude direct measurement of soft beta radiations-vie., strontium-90. Indeed, a t this calcium level the direct method is preferred, because application of the acetone procedure in the lower calcium range necessitates the use of inordinately large volumes of acetone to expedite separation of the aqueous and acetone phases, The competitive effect of calcium is markedly altered in favor of strontium precipitation by treating the system with acetone. Addition of acetone reduces the aqueous phase to a volume governed by the quantity of salts in solution. B y virtue of the reduced aqueous volume the effective ionic concentration of calcium is diminished, thereby probably limiting its competitive behavior and favoring precipitation of the insoluble strontium rhodizonate. The modified procedure allows an eightfold increase over the direct method in the quantity of calcium separable from strontium. Further, if strontium recovery of the order of 60 to 65% is tolerable, a calcium-strontium molar ratio of 800 can be handled readily and 99.8% of the calcium is renioi-ed in a single treatment. The coprecipitation of small amounts of calcium was not inuestigated. The quantity of this element which follow strontium appears to be a function of the molar ratio of precipitant and strontium. However, a single rhodizonate precipitation reduces the calcium concentration to a level of noncompetitiveness, whereby reprecipitation of strontium by the direct method is possible a t a lower rhodizonate-strontium molar ratio. Under such conditions purity of separation is achieved.
LITERATURE CITED
(1) Feigl, F., Mikrochemie 2 , 187 (1924). ESG.CHEV., (2) Feigl, F., Suter, H., IXD. A N ~ LED. . 14,840 (1942). (3) Glendenin, L. E., Paper 236, Sational Nuclear Energy Series, Div. IV, 9 , hlcGraw-Hill, S e w York, 1951. (4) Punderman, D. N., Meinke, 11'. IT, .kXAL. CHEM. 29, 1578 (1957).
RECEIVEDfor review March 26, 1957. Accepted August 5, 1957.
