Determination of Total Radiostrontium in Biological Samples Containing Large Quantities of Calcium Selective Precipitation with Potassium Rhodizonate A. L. BONl Savannah River Planf, E. 1. du Pont de Nemours &
b Improved recovery of total radiostrontium from biological samples containing large quantities of calcium has been attained through the use of potassium rhodizonate as a selective precipitating agent. An analytical procedure utilizing this method was developed, and optimum conditions for an 85% chemical recovery were determined. An over-all recovery of 72% (with a standard deviation of rrt9 at the 90% confidence level) was obtained for 2-gram bone samples.
T
method based on the differential solubilities of strontium and calcium nitrate for the determination of small quantities of radiostrontium (7) is limited by quantities of calcium present in excess of 10 mg. per sample. Varying amounts of calcium are carried over into the strontium precipitate and absorb the low-energy strontium-90 8radiation. Large amounts of calcium also result in the incomplete precipitation of strontium. TTeiss and Shipman ( 5 ) reported on the use of potassium rhodizonate as a selective precipitating agent for separating strontium from large amounts of calcium. Its use has been further developed a t this laboratory and applied to the analysis of biological samples containing large quantities ot calcium. Improved recovery of total radiostrontium has been attained through the use of this method. An analytical procedure was developed to determine the radiostrontium content of a variety of biological samples which contained too much calcium and/or phosphates for accurate assay by the generally accepted nitrate precipitation method (4, 6). A cation exchange resin was used to separate phosphates from sample solutions. Potassium rhodizonate was then used to separate calcium and strontium, based on the differential solubilities of calcium and strontium rhodizonate. Further decontamination of the strontium rhodizonate precipitate from other radionuclides was achieved by the fuming nitric acid treatment and reprecipitation of the strontium as the nitrate. Various quantities of different types of biological samples were examined, and the optimum conditions for an 85% chemical recovery were determined.
The over-all recovery, including selfabsorption, chemical loss, and counting efficiency, is 72% for 2-gram bone samples containing up to 1 gram of calcium (Figure 1'). A standard deviation of *9 a t the 90% confidence level is attained. EXPERIMENTAL
Carrier solutions are listed in Table I. All are made from reagent grade materials. Table 1.
HE
744
ANALYTICAL CHEMISTRY
Co., Inc., Aiken, S. C.
Carrier Solutions
(Reagent grade materials) Solution Carrier concn., Solute and ion mg./ml. solvent Fe +3 5 FeCl3.6Hz0 in HnO 1 cs + CSXOIin H20 Ce f 3 5 Ce(S 0 3 ) 3 . H 2 0 in LV H S 0 3 Ba +* 20 Ba(h'O& in H20 Sr +2 20 Sr(N0& in HzO Potassium Rhodizonate, 0.4% (4 grams per liter). TT7eigh out Eastman reagent grade rhodizonie acid dipotassium salt to make just enough solution for the samples to be ana-
220
lyzed. (The amount of reagent made up a t one time is limited because the solution is stable for only several hours.) Let the reagent stand 30 minutes prior to use. Ion Exchange Column Preparation. The ion exchange column is designed so t h a t it can be operated in either a n upright or inverted position (Figure 2). A small glass-wool plug is loosely placed in the closed end of the ion column. The column is filled with a water slurry of previously washed Dowex 50WX2 (100- to 200-mesh) cation resin. After any water present has drained to the level of the resin, a small glass-wool plug is placed in the detachable stopcock and the open end of the ion column is capped. The reservoir is placed on top of the column. Bfter inversion of the column, the reservoir is removed and placed on the opposite end. Sample Preparation. All samples must be free of organic material and dissolved i n 1N hydrochloric acid. lJ7.4TER. JTrater samples must be evaporated to dryness. The residue is dissolved in IN hydrochloric acid. BONE. Two-gram bone samples must be fired to a Iyhite ash. The ash is dissolved in concentrated nitric acid and evaporated to dryness (this step should be repeated for samples incompletely fired initially). The residue is dissolved in 5 ml. of 1 N hydrochloric acid.
-
-
- 100 POTASSIUM RHODIZONATE METHOD (47 SAMPLES) :
to Figure 1. Srg0added.
Recovery statistics
221 X
c./2 grams bone
VEGETATION (FOODSTUFFS). Saniples must be fired to a white ash and the ash dissolved or leached with 6N hydrochloric acid. The solution is evaporated t o dryness and dissolved in a miiiimum amctunt of 1N hydrochloric acid. URINE. Urine samples are first analyzed for gross heion products by precipitat,ion as the ammoniacal phosphate ( I ) . This precipitate is then dissolved in 5 ml. of lAr hydrochloric acid. NILK.Milk samples are first treated by a batchwise extraction technique using ion exchange iesin t o concentrate the strontium along with the calcium ( 2 ) . Ammoniuni hydroxide instead of S a O H is used in the neutralization of the milk. The leacn from the resin is evaporat,ed t o dryness and dissolved in 40 ml. of distilled wat'er. Potassium rhodizonate solution is added directly at' this point. Procedure. DilJte the prepared saniple wit,h a n equal volume of distilled water to adjust the normality to 0.5-1- hydrochloric acid. Pass this solution through a previously prepared Dowex 50TVX2 (100- to 200mesh) cation resin column. Add a 10-nil. 0.1N HC1 wash from the beaker to the column, followed by an additional 70 nil. of 0.1S HC1, and allow the column t o drair. completely. Discard the column efluent. Invert the column, elute with '70 mi. of 6 N HCI, and again allow th': column to drain completely. Add 30 mg. of strontium carrier t o the eluate, and evaporate to dryness completely a-ithout charring. Dissolve the residue in a minimum amount of distilled water (approximately 10 to 40 ml.) and determine the pH, which must be between 5 and 7 . If the pH is not 5 or greater, repeat the evaporation and distiolution in distilled water. Add 120 ml. of a 0.47, potassium rhodixonate so1:ition to the sample and let it stand 30 minutes. Centrifuge the sample in a 100-ml. centrifuge tube and discard the supernate. Wash the beaker and the rhodizonate precipitate in the tube with 30 ml. of distilled water, centrifuge, and again discard the supernate. Dissc'lve the rhodizonate precipitate in a minimum amount of 4-V H r o 3 and trantifer to the original beaker. Add 1 ml. of a 1-mg. per ml. cesium carrier solution and 1 ml. of a 5-mg. per ml. cerium carrier solution. Evaporate to dryness;. Dissolve the residle in 1 ml. of distilled water and transfer the solution to a 100-ml. round-bot.tonied centrifuge tube. Rinse the beaker wil,h 2 ml. of concentrated H S O I and add this rinse to the centrifuge tube. Rinse the beaker with a total of 40 ml. of fuming H S 0 3 . i n two equal portions and combine with the solution in the centrifuge tube. Cool the solutions under tap water and a1loTv to stand for 15 minutes. Centrifuge t'he solution and discard the supernate. T17ash the: nitrate precipitate n-ith 10 ml. of fuming HKOa. l g a i n centrifuge and discard the supernate. Dissolve the nitrite precipitate in 10 ml. of distilled water and add 1 ml. of a 5-mg. per ml. iron carrier solution.
Ground Glass Inner Jomt
(f
12/30)
II Ground Glass O u t e r Joint (f
Figure 2.
12/30)
Ion exchange column design
To this solution, add 1 ml. of concentrated ",OH. Centrifuge and transfer the supernate to a clean 100ml. round-bottomed centrifuge tube. TVash the precipitate with two 5-ml. portions of 3iv NH40H and combine these wash solutions with the original supernate. Heat the combined supernatant solutions to boiling in a water bath and add 5 ml. of saturated ammonium oxalate solution with stirring. Heat the solution again to boiling, remove it from the water bath, and allow it to stand 5 minutes with occasional agitation. Centrifuge the solution and discard the supernate. Dissolve the oxalate precipitate in a minimum of 4 N Hn'O3 and transfer the solution to a fiamed 11/2-inch stainless steel planchet. Evaporate the solution under an infrared heat lamp and count in a beta counter as soon as possible. DISCUSSION
Theory. Strontium reacts with potassium rhodizonate in a neutral solution t o form a stable reddishbrown precipitate. Calcium competes
with strontium to form a soluble calcium rhodizonate complex. The optimum conditions for precipitation depend upon the calcium, strontium, and potassium rhodizonate concentrations. The best recoveries are obtained by maintaining a potassium rhodizonate-to-strontium molar ratio of 5 and a calcium-to-strontium molar ratio of less than 50. Calcium Content. As the calcium content of various samples increases, the amounts of strontium carrier and potassium rhodizonate must be increased accordingly to maintain optimum conditions for the precipitation. Table I1 lists the various amounts of potassium rhodixonate and strontium carrier needed in mater samples containing increasing amounts of calcium. Since less than 1% of the initial calcium is carried by the method, the amount of strontium carrier now becomes the limiting factor because of its absorption of the low-energy strontium-90 p-radiation. Decontamination Factors. The decontamination factors for the VOL. 35, NO. 6, MAY 1963
745
II.
Table
>90%
Reagent Quantities for Chemical Recovery
(Water solutions containing various amounts of calcium) Potassium rhodizonate Strontium (4grams/ carrier (20 Total calcium, liter), mg. /ml.) , grams ml. mg. 0-0.2
40 80
0-0.5
10
20
120 150 210
0-0.7 0-0.9
0-1.15
30 40
50
rhodizonate precipitation alone, listed in Table 111, are very poor for most of the common radionuclides. However, the addition of chromium and cesium substantially increases their respective decontamination factors. Decontamination factors greater than 1 X l o 3 mere obtained for all of the common radionuclides other than barium by following the rhodizonate precipitation with fuming nitric acid, iron hydroxide, and oxalate precipitations (Table 111). The incorporation of the standard barium chromate precipitation does not substantially raise the barium decon-
Y.
Remnv*a
80
1 ,o~ I
I I
I
I
Decontamination Factors
Potassium rhodiaonate precipitation CarrierCarrier free added Induced Activities 10 300 2 ...
CrK1 cow Fe69 Zna5
1 1
Zr-NbOE
Gel4', Ce-Pr14' (R.E.',
1 1
...
100
103
1
Table IV.
Sample Urine, ml.
500 1000 1500 Bone, g. 1 2
Milk, ml. 500 Foodstuffs, g. 500 Vegetation, g. 10
Optimum Reagent Quantities
Potassium Strontium rhodizonate, carrier, ml. mg. 80 120
120
20 30 30
so
20
150
30
120
30
120
20
120 ~.
30 _.
Table V.
0
...
...
Fission Products 500 ... 10 ...
I131 Ru 103,106
...
...
20
ANALYTICAL CHEMISTRY
40 EIUtrlOnl Velum.
Potassium rhodizonate nitrate precipitation . . precipitation CarrierCarrier free added
+
...
> 103 ... > 103 >io3 >io3 >io3
... ... ...
>io4 >io3 >io3
...
>104 >104 >loa >io4
> 104 3
tamination factor. However, a homogeneous precipitation of barium chromate (3) increases the decontamination factor for this nuclide t o 20. Phosphate Interference. In many biological samples, large phosphate concentrations precipitate from a solution at a p H greater than 4. These phosphate precipitates carry calcium. The minimum p H required t o obtain the optimum separation and recovery of strontium from calcium using a potassium rhodizonate precipitation is 4. Furthermore, large quantities of phosphate carry strontium on the discarded iron hydroxide precipitate in the fuming nitric acid method. The
Recovery Data
Fuming nitric acid Potassium rhodiaonate Recovery,. Std. dev. Recovery,' Std. dev. Sample % (90% C.L.) % (90% C.L.) Bone (2 g. wet weight) 11 f3 72 5 9 Urine (1.5 liters) .. .. 76 *12 Vegetation (10 g. wet weight) *. .. 76 *4 Foodstuffs (500 g. wet weight) .. .. 67 20 Includes self-absorption losses in counting.
746
I
0
Table 111.
I
,
bo
80
rnl
Figure 3. Strontium elution from phosphate removal column
phosphate ion is removed rapidly by passing the sample, dissolved in 0.5N HC1, through a cation column of Dowex 50WX2 (100- to 200-mesh) resin. The calcium, strontium, and other cations are quantitatively adsorbed on the resin, and the phosphate remains in the effluent. The cations are quantitatively removed from the resin by inverting the column and using 50 ml. of 6N HC1 as an elutriant (Figure 3). The column eluate must be evaporated completely to dryness to obtain total precipitation of strontium as the rhodizonate. After evaporation, the residue containing the calcium and strontium is easily dissolved in distilled water before precipitation with potassium rhodizonate. Although p H adjustment can be used as an alternative to evaporation of the eluate from the cation resin column, experience a t this laboratory indicates that when large numbers of routine samples are involved, more accurate results are obtained by evaporating the eluate. Iron Interference. Iron precipitates readily as the rhodizonate and reduces the strontium recovery considerably. If iron occurs in milligram quantities, i t must therefore be removed before the rhodizonate precipitation. I n general, foodstuffs are the only samples with a high enough stable iron content to cause interferences. Iron can be removed by passing the sample, in concentrated hydrochloric acid, through Dowex 21-K anion exchange resin. The iron is strongly adsorbed on the resin, while the strontium passes into the effluent. The effluent is evaporated to dryness and dissolved in 1N hydrochloric acid before proceeding with the normal procedure. A hydroxide precipitation cannot be used to remove iron before the phosphate removal step, since the
presence of phosphate \>ill coprecipitate strontium as the ammoniacal phosphate above p H 7 . Applications. The method is readily adaptable t o the determination of total radiostrontium in water, urine, bone, foodstuffs, vegetation, and milk samples. Various amounts of these samples were studied using the general method. The optimum quantities of reagents required for the various types of samples studied are shown in Table IT’. It was necessary t o increase the potassium rhodizonal e and the strontium carrier concentrtttion for biological samples over water samples with comparable concentrrttions of calcium, because biological samples contain I
higher concentrations of other competing elements. Recoveries, which include losses due to self-absorption by strontium carrier, are listed in Table V for all types of samples studied. The general procedure is applicable to larger samples containing higher concentrations of calcium and phosphate, if the quantities of cation resin, strontium carrier, and potassium rhodizonate needed to obtain a satisfactory recovery are determined. LITERATURE CITED
( 1 ) Boni, A. L., Health Phus. 2., 186 ”
, (1959): (2) Butler, F. E., “SrsoMonitoring at the Savannah River Plant,” E. I. du Pout dc Nemours & Co., AEC Research and
Development Rept., DP-473, 20, 21 (May 1960). (3) Gordon, L., Salutsky, ?VI. L., Willard, H. H.,“Precipitation from Homogeneous Solution,” Wiley, Kew York, 1959. (4) Murthy, G. K., Jarnagin, L. P., Goldin, A. S., J . Dazry Sci. 42, 1276 (1959). (5) Weiss, H. V., Shipman, U’. H., ANAL. CHEM.29, 1764 (1957). (6) Whitney, I. B., ed., C . S. At. Energy Comm., Health and Safety Laboratory, R. & D. Rept., NYO-4700, E-38-01-01 to E-38-01-21 (February 1960). (7) Willard, H. H., Goodspeed, E. W., IND.ENG.CHEV.,A y . 1 ~ .ED. 8, 414 (1936). RECEIVEDfor review October 29, 19G2. Accepted February 13, IIKj3. Information developed during work under contract AT(07-2)-1 with thr 1.. S.Atomic Energy Commission.
Radiocheimica I Dete r mina ti o n of Act inium in Uraniuim MiII Effluents HENRY G. PETRON“
and ROBERT J. ALLEN2
lonics, Inc., Cambridge, Mass.
b The determination of actinium in uranium mill effluent!; requires an accurate, sensitive method, free of interference from the host of impurities, both stable and radioactive, likely to be present. The method described is sensitive to low concentrations, less than 60 d.p.m. per liter, i:j free from interference from likely Contaminants, and has a relative error of about *5%. The radioassay of actinium is accomplished by following ihe in-growth rate of alpha-emitting daughters.
D
the development of a procedure for the determination of radium-223 in uranium mill effluents, the observed data proved conclusively that a long-lived precursor of radium223 was present in acid mill effluents. Mill effluents analyzed a t six-month intervals demonstrated little or no decay of radium-223, despite its 11.6 day half life. From 1;hese data, it was obvious that radium-223 was being generated by, and was in equilibrium with, 22-year actinium-227. A survey of the literature disclosed no procedure adequate for the determination of actinium in a mixture as complex as a uranium mill effluent. Besides varying amounts of uranium and uranium daughttm, large concenURING
Present Address, New York University Medical Center, N. Y. * Present Address, Technical Operations, Inc., Burlington, Mass.
trations of iron, aluminum, manganese, vanadium, molybdenum, calcium, magnesium, copper, and zirconium are often present. At least half of the mill solutions examined contained lanthanum, cerium, praseodymium, and neodymium, although the rare earth content never exceeded 50 p.p.m. To allow an accurate radioassay of actinium, the final sample must be substantially free of solids, so that the presence of even small amounts of the lighter rare earths greatly complicates the analytical problems. Actinium decays 98.8y0 by beta emission, of very low energy (0.046 m.e.v.), and 1.2% by alpha emission. To determine actinium accurately by beta counting is difficult, for even traces of solid matter cause serious absorption losses. However, actinium decays into a series of alpha-emitting nuclides, and measurement of the in-growth of alpha activity is a n excellent means of estimating the actinium content of the samples. For those laboratories equipped with beta liquid-scintillation equipment, beta counting may be feasible. The rate of in-growth of alpha activity from actinium can be calculated by use of the Bateman equation. The
calculated alpha activity associated with 1000 d.p.m. of actinium, as a function of time, is given in Table I. The calculated activity includes all alpha emitting daughters of actinium. A sample of pure actinium was prepared from pitch-blende. The rate of in-growth of alpha activity was observed for three aliquots. After aging for a known period, three separate aliquots were analyzed for thorium-227, by the method of Petrow, Sohn, and Allen (S), to establish the actinium concentration. The results obtained by thorium analysis agreed within 1% with the results determined from the alpha-count rate of the freshly purified actinium. The observed rates of ingrowth, when normalized to 1000 d.p.m. of actinium, were in nearly perfect agreement with the calculated rate of in-growth. These data indicated the usefulness of the technique for the assay of actinium. They also indicate that the 3.92-second radon-219 does not escape from the counting plate. Several means of concentrating actinium present in the mill effluents were investigated. Lanthanum oxalate carried actinium quantitatively if precipitated a t pH 2 from a solution
Table 1. Calculated In-Growth of a-Activity from Purified Actinium Time, days 0 1 2 3 4 5 6 7 8 9 1 0 d.p.m. of CY 12 58 110 165 225 289 360 429 507 586 668 1000 d.p.m. of Ac
VOL. 35, NO. 6, MAY 1963
747
