Quantitative Determination of Strontium-89 and Strontium-90 in W a t e r JACOB K001' Radiochemisfry Group, Reactor Cenfrum Nederland, The Hague, The Netherlands
b A simple procedure has been developed for the quantitative determination of radioactive strontium in water which may b e rich in calcium. Strontium-89 and total radiostrontium may b e obtained in about 5 hours, and strontium-90 may b e estimated b y allowing the precipitate to stand overnight and separating yttrium-90. The sensitivity i s better than one tenth of the internationally recommended maximum permissible concentration. The method is specific for strontium and suitable for large-scale, routine application. Radioactive barium may also b e determined.
T
HE INTERNATIONALLY recommended maximum permissible concentration (MPC) in drinking water of the fission product pair, strontium-90 and yttrium-90, is 8 x 10-7, pc. per ml., several thousand times less than that for most of the other dangerous fission products (1). That for strontium-89 is 7 X 10-6 pc. per ml., which is still more than 10 times less ( 1 ) . Thus, their estimation is of considerable importance, especially in connection with problems of waste disposal and water pollution. A method for determining these isotopes in large-scale, routine applications should be rapid, sensitive, independent of possible variations in water composition, and accurate to a t least 90%. Because none of the published methods was considered satisfactory this procedure was developed.
SEPARATION
OF STRONTIUM
The entire procedure may be divided into three principal parts: (1) concentration, (2) separation and purification of total strontium, and (3) determination of total strontium activity and, finally, determination of strontium-90 separately. Concentration. The precipitation method recommended by Hahn and Straub (3) was selected for concentrating the radioactive strontium. Filter the sample if it contains solid particles. (It may be advisable to check that no strontium is adsorbed on the 1 Temporary address, Instituut voor Kernphysisch Ondersoek, Ooster Ringdijk 18, Amsterdam,Netherlands. 532
ANALYTICAL CHEMISTRY
solid. If adsorption losses occur, the solid residue may have to be analyzed separately.) To a 3-liter sample of water, add 30 mg. of strontium carrier and 15 mg. of barium carrier. Cover and heat to boiling, then add, for each liter of water, 12.5 ml. of concentrated ammonia followed by 12.5 ml. of 1M sodium carbonate. Keep just below the boil for about 15 minutes, then allow the precipitate to settle. Discard the supernatant liquid and centrifuge the precipitate in a 50-ml. centrifuge tube, again discarding the supernatant liquid. Separation and Purification of Strontium. REMOVAL OF CALcIuar. Dissolve the precipitate, which contains the carbonates of calcium, strontium, barium, and magnesium as well as ferric and aluminum hydroxides, in a minimum amount of concentrated nitric acid added dropwise. Dilute to 5 ml. with water, centrifuge, and pour the solution into a clean centrifuge tube. Add 6.2 ml. of fuming nitric acid (99.6%; density, 1.52). Cool under a tap, remove the nitric acid vapors, and centrifuge. Dissolve the precipitate in 5 ml. of water and repeat the precipitation again. For samples that contain little or no calcium, magnesium, iron, aluminum, etc.-for example, those obtained by diluting a solution of highly radioactive fission productsone nitrate precipitation is sufficient to remove the interfering elements. After the last precipitation, add 5 ml. of a 1 to 1 mixture of alcohol-ether to the centrifuged precipitate to remove excess acid. Centrifuge again. REXOVALOF BARIUM. Barium is removed according to a procedure given by Glendenin @), based on the insolubility of barium chloride in concentrated hydrochloric acid-ether solutions. Dissolve the precipitate of mixed strontium and barium nitrates in 4 to 5 ml. of water. Add 25 ml. of a hydrochloric acid-ether mixture (5 to 1). Stir for 2 minutes while cooling under running water. Centrifuge, then pour the supernatant liquid into a centrifuge tube containing 15 mg. of barium chloride in 3 to 4 ml. of water. Precipitate the barium chloride, then repeat this step once more. The second and third barium chloride precipitations are done simply by stirring the mixture while cooling. The hydrochloric acid-ether mixture added a t first is sufficient for all three precipitations. If a barium determination is desired, the precipitates may be recovered for counting as barium chloride (2). Final Purification
of Strontium.
A final purification is necessary t o remove lanthanum-140 which might be present due to decay of barium-140. Add 5 to 10 milliliters of water to the purified acid-ether solution of strontium in a beaker or porcelain dish. Evaporate, preferably to dryness, then dissolve the residue in a few milliliters of water and add 1 drop of nitric acid. Add 5 mg. of ferric ion as the nitrate, and a little hydrogen peroxide (30%) dropwise until no more gas evolution takes place. Heat to boiling. Precipitate ferric hydroxide by adding concentrated ammonium hydroxide, then filter the solution. Heat the filtrate to boiling and add an excess of 1M sodium carbonate to precipitate strontium carbonate. Note the time. Filter the solution on a tared filter paper in a demountable glass filtering apparatus. Dry the precipitate by twice pulling through a few milliliters of alcohol, then of ether. Determine the weight of strontium carbonate. The precipitate is now ready for counting. COUNTING
If the strontium-90 constitutes less than a few per cent of the total strontium content, the strontium-89 content of the original water sample may be obtained, within the limits of experimental accuracy, by beta counting, using an end-window Geiger-Muller tube. Because of the relatively high energy of the strontium-89 beta rays, self-absorption corrections are negligible, as the sample thickness is 5 to 10 mg. per square cm. Absorption losses in air and the counter window may also be neglected. After corrections for geometry and chemical yield are applied, the absolute amount of strontium-89 present is immediately obtained. Beta absorption measurements were performed to see if a direct determination of strontium-90 would be possible. The half-thickness value, dIi2, for absorption of strontium-89 in aluminum was 92 mg. per square cm.; however, there was a slight upward bending a t the first few milligrams. With the counting setup used here and without absorption corrections, this resulted in an initial activity which was 4 to 5% below the measured value when a straight line for dl,* = 92 mg. per square em. was drawn through the points a t greater absorber thickness. However, the dllr value for strontium-90 appears to be 24 mg. per square
cm. Therefore, it should be possible to detect this isotope by absorption measurements if it is present in amounts above a certain lower limit, which was found to be about 10%. Thus, after back-extrapolating from the measured absorption curve, if an initial activity is found nhich diffcrs by more than 10% from the measured activity, 10% or more of the radioactive strontium is strontium-90. Application of corrections for absorption in air and counter windoFv would allow an accurate estimate of the strontium-90 content. However, a reasonably fair estimate of the relative strontium-89 and -90 contents may be obtained even without these corrections. If a more accurate determination of strontium-90 or the determination of much smallrr quantities of this isotope is necessary, it is done by isolating the yttrium-90 growing into the final strontium carbonate precipitate from its parent strontium-90 according to the respective half lives of 64.5 hours and 28 years. Therefore, count the neighed strontium carbonate precipitate from the purification steps to obtain the total amount of strontium present. After counting determine a few points of the aluminum absorption curve and look for the presence of strontium-90. Calculate the strontium-89 content. To obtain the strontium-90, let the strontium carbonate precipitate stand oT.ernight or longer, then dissolve it in a few drops of nitric acid and dilute with several milliliters of water. Add 15 mg. of yttrium carrier and precipitate yttrium hydroxide by adding ammonium hydroxide. S o t e the time, then centrifuge. Dissolve the precipitate again in a fen- drops of nitric acid. Add 15 mg. of strontium carrier and again precipitate yttrium hydroxide. If the final -ttriuin precipitate still contains appreciable strontium-89 activity, the addition of strontium carrier and precipitation of yttrium hydroxide niay be repeated one or more times. Centrifuge the yttrium hydroxide precipitate, then dissolve it in a few milliliters of 2 to 3 S nitric acid. Precipitate yttrium fluoride by making solutions 331 in potassium fluoride by adding 30 to 40% fluoride solution. Filter on a tared filter paper in a demountable filtering apparatus. Remove the final traces of moisture by twice pulling through a little alcohol, then a little ether. Weigh the precipitate and count. Calculate the strontium-90 content using the gron-th curve for yttrium-90. The purity of the yttrium-90 sample may be checked by half-life measurements. I n a11 cases tested, it was found to be 64.5 hours, which agrees with the literature value. CALCULATIONS
Strontium-89. On semilog paper plot the values of the measured activi-
ties of the strontium carbonate sample in counts per minute along the logarithmic axis, and plot the absorber thickness in milligrams per square centimeter (up t o 250 mg. per square em.) along the linear axis. Draw a curve through the points. Extrapolate the linear part with a half-thickness value of about 92 mg. per square em. back to zero absorber thickness. If the measured value mithout absorber is not more than 4 to 5y0 higher than the extrapolated value, all the activity may be ascribed to strontium89. If the measured zero absorber value is more than 5% higher than the extrapolated value, the activity due to strontium-89 must be taken equal to 105% of the extrapolated value. The excess value indicates the presence of relatively large amounts of strontium90. From the strontium activity found in this way (A8g) and expressed in counts per second, calculate the amount of strontium439 present in microcuries per milliliter (C8g) of the original u-ater sample as follom:
css=
1.37
x
10-5
x
IZ
X ?os X e
(1)
n here
n = iolume of 15-ater sample, liters 211, = neight of strontium carbonate, mg. e = efficiency of counter The constant, 1.37 X lo+, is based 3n the use of 30 mg. of strontium carrier. and niust be changed correspondingly if other amounts of carrier are used. The decay of strontium-89 (half life = 52 days) becomes important only if the activity of the sample is measured several days after separation. If it is desired to use the result obtained a t this point as a criterion-for example, for possible disposal of radioactive Tvaste-simply apply a safety limit by assuming that 10% of the activity found is due to strontium-90. Strontium-90. Obtain the strontium-90 content of the original water sample from the measured activity of the yttrium fluoride as follows. Curve I, Figure 1, represents the radioactive decay of yttrium-90 after its separation from strontium-90 according to the procedure. The horizontal axis shon s the time between separation and counting [ ( t o - &)I, while the vertical axis represents the per cent of yttrium left a t the time of counting ( p J . IVith this curve the measured activity, [An(Yl1, expressed in counts per second, may be corrected for radioactive decay. Curve 11, Figure 1, is the growth curve of yttrium-90 after separation of the purified strontium carbonate. The time difference between this moment of separation (to) and the moment of separation of yttrium-90 (t,) is plotted us. the amount of yttrium-90 growing into the sample. The latter is expressed as a per cent of the amount of strontium-90 initially present ( p p ) .With this curve the activity, corrected for the decay of yttrium-90, may be corrected €or the partial growth of yttrium-90.
After these tn-o corrections are applied, use the same type of calculation as for strontium-89 to obtain the concentration of strontium-90 (C ) initially present in microcuries per milliliter in the water sample: C80
0.35
= -
w.
A,(Y) 1 x p___ , ~ p , ~ n ~ w , ~ e (2)
where w , = weight of yttrium fluoride, mg. The constant in this equation, 0.35, is based on the use of 15 mg. of yttrium carrier, expressing p , and p , in per cent, and calculating yields on the basis of the formula, YF3.1j4 H20. If desired, the amount of strontium-90 found may be used to correct the amount of strontium-89 calculated previously. DISCUSSION
Purification of Strontium. I n the purification step of other strontium procedures, an attempt is usually made to recover the strontium quantitatively, generally by precipitation with fuming nitric acid. This necessitates a separate treatment for the removal or later determination of calcium, or it involves the use of impure strontium samples and the application of corrections for the calcium present Such corrections are sometimes rather arbitrarily chosen. This difficulty niay be overcome by applying the results obtained by Sunderman and Meinke ($) on the coprecipitation of alkaline earth nitrates as a function of the nitric acid concentration. The data in Table I show that, nith a suitable nitric acid concentration, almost complete removal of calcium may be obtained in two successive precipitations and still more than 60% of the added strontium carrier is recovered. In the proceciure given the nitric acid concentration chosen is 65% (by weight), and the yield is approximately 70%. Tracer experiments with calcium-45 showed that, after two precipitations, less than 0.2% of the calcium (from original amounts up to 1 gram) was present in the nitrate precipitate. Separate experiments m-ith strontium-89 tracer indicated that the amount of strontium left in solution after one precipitation arnounted to 10 to 12%, independent of calcium concentration. I n the removal of barium, three successive precipitations were found to be sufficient to remove all the barium activity from the final strontium solution. No difference betmeeen background counting rate and sample could be detected in trace experiments using several thousand counts per minute of freshly purified barium-140. A second series of tracer experiments with strontium-89 showed that only a few per cent of the strontium initially VOL. 30, NO. 4, APRIL 1958
533
present is carried by the barium chloride. Obviously, two precipitations are sufficient to obtain a strontium-free barium sample. The loss of strontium in the three barium chloride precipitations amounts to 10 to 20%. The final purification of strontium from lanthanum-140 can be accomplished easily by scavenging the strontium solution with ferric hydroxide. Tracer experiments showed that the removal of Ianthanum-140 was complete. From a solution containing about 2000 c.p.m. of lanthanum-140, a strontium carbonate sample resulted which showed no definite deviation from the background counting rate. The same results were found when the ferric hydroxide precipitates were checked for activity in experiments where strontium-89 was used as a tracer. The over-all yield of strontium in the final precipitate is about 60%. The sample for total strontium may be obtained about 4 to 5 hours after starting the analysis. Purification of Yttrium. Experiments with strontium-89 tracer showed that after two yttrium hydroxide precipitations only 20.03% of the strontium was present in the yttrium sample. Yttrium-90 tracer experiments indicated a loss of only about 0.5% of the yttrium in the strontium filtrate. Finally, it appeared that the conversion of yttrium hydroxide into yttrium fluoride is essentially quantitative. RESULTS
Accuracy. Results of duplicate tests agreed t o within a few per cent except in one case, where a deviation of 10% was found. Thus, although it is usually much better, a minimum accuracy of 90% may be assumed. An inaccuracy of 10% is usually sufficient for this type of analysis where safety factors of 5 to 10 are always used. Similar results were obtained in a number of experiments when the purified strontium carbonate ryas dissolved, added as tracer to the starting solution, and the complete procedure was repeated. The specific activity of the twice-purified strontium carbonate was the same as that of the once-purified sample to within about 10%. Interferences. As the procedure Table 1. Nitric Acid, Wt. 70
was developed for the determination of radioactive strontium content in river water (specifically, the Lek River), a number of experiments were carried out to investigate the influence of impurities present in the river water. Ordinary Amsterdam tap water, which is rich in calcium, was used to develop the procedure. Addition of more calcium did not interfere. Anions which are dominant in the Lek Lyater and which might form insoluble strontium compounds were added to tap water in amounts far above (2 to 20 times) the known maximum content in river water. The anions were added as the sodium or potassium salts. The results are given in Table 11, where results are also given for samples of Lek River water and ditch water which contained a large excess of iron and organic matter. All the experiments were carried out with added strontium-89 tracer. For the river water and ditch water, the solutions were filtered to remove solid matter after addition of tracer and several hours' stirring. This procedure was adopted only after the following experiments had shown that it did not invalidate the results.
Yield Data of Alkaline Earth Nitrate Precipitations"
% Carried Ba 100 15 . 3 100 3 . 6 86 b 3 . 3
Sr
80 100 =t1 . 7 70 98 f 1 . 4 60 81 1 4 . 2 a All data from Sunderman and Meinke (4).
534
ANALYTICAL CHEMISTRY
Ca on Ba
Ca on Sr
27 b 2 . 2 2 . 4 10 . 3 0 . 9 zk 0 . 0 5
51 z k 3 . 2 11 1 2 . 3 2 . 6 zk 1 . 0
When mixed fission products were added to several liters of river water, which was then stirred and filtered, the solid residue contained approximately 30% of the total gross beta activity. A strontium determination carried out on the filtrate gave a specific activity of 67 c.p.m. per mg. Khen the fission products were similarly added to tap water and it \$as analyzed, the specific activity was 66 c.p.m. per mg. These results mere confirmed by counting the river aater before and after filtration in a liquid counter and by repeating the experiment with strontium-89 added. In this case no activity could be detected in the solid residue, while the results in the liquid counter before and after filtration were 23 and 22 c.p.m., respectively.
Sensitivity for Strontium-90. The minimum amount of strontium-90 which may be determined by this method depends principally on the volume of v-ater sample, time allowed for decay of strontium-90, and counter efficiency and background. The effciency of the counter used in these experiments n-as about 157, and the background was about 30 c.p.m. Assuming that a sample of 3 liters is used and that complete results are desired 1 or 2 days after sampling, the following lower limits representing approximately 5 c.p.m. above background may be easily attained: Lower Level of Detection
Piights Standing
0.05 x n i x 0.02 x n i p c 0.013 X MPC
1 2 4
Sensitivity may be correspondingly increased by increasing the sample
size, lowering the background rate. or increasing the counter efficiency.
Table II.
ACKNOWLEDGMENT
The author wishes t o acknowledge the hospitality of the Instituut voor Kprnphysisch Onderzoek, which is carrying out part of the research program of the Foundation for Fundamental Research of Matter (F.O.11.) under financial wpport of the Netherlands Organisation for Pure Scientific Research ( Z . K . 0 . ) . Thanks are also extcnded to A. H. IT. Aten, Jr., for his interest in the work and t o 9. A. I. S . Roelrijk, who provided data on the salt content of the river water and carried out a few tests independently. The author is indcbted to the Director of Reactor Centrum Sederland for perniission to p-ihlish this paper.
Sample Tap water
River water Ditch xi-ater
Influence of Impurities on Recovered Activity
Impurity Added, Mg./Liter
Specific Activity of SrCOs, C.P.U./ M g .
Total -4ctivity (10-2 pc.)
c1-;300 Sod--, 30 SO4--, 70 YO,---, 9 Sios--, 14 .. . .
39.8 37.7 41.1 40.4 41.7 36 2 36 9 39 7 Av. 3 9 . 1 i.5 5
101.7 96 4 105 0 103 3 106 5 92 5 94 3 101 4 99 9 & 5 %
LITERATURE CITED
J . R~diol.,Suppl. 6 (1955). ( 2 ) Glendenin, L. E., in “Radiochemical Studies: The Fission Products,” C. D. Coryell, N. Sugarman, eds., T’ol. 3, pL 1460, McGrav--Hill, Sew York, 1901. (1) Bi.it.
(3) Hahn, R. B., Straub, C. P., J . Am. Water W o r k s Assoc. 47, 335 (1955). (4)Sunderman, D. S . , Meinke, \V. TT’., A x . 4 ~ .CHEX 29, 1578 (1957).
RECEIVEDfor reviex April 5, 1957. Accepted Kovember 22, 1957.
Apparatus and Technique for Multiple Tests by the Confined-Spot Method of Colorimetric Analysis Application to Field Estimation of Nickel and Copper J. HOWARD McCARTHY, Jr., and ROLLIN E. STEVENS
U. S.
Geological Survey, Denver, Colo.
b The confined-spot method of colorimetric analysis is generally applicable to the semiquantitative estimation of traces of ions in solution that form colored precipitates or otherwise alter material on a confined area of reagent paper. For precise results, the rate of flow of test solutions through the reagent paper must be reproduced on successive runs, so that the same proportion of reaction products i s collected each time on the confined spot. A simple apparatus gives a controlled rate of flow and makes possible the analysis of many samples a t one time with good reproducibility. This apparatus utilizes the increasing pull, resulting from the gradual lowering of the water level in a tank, to draw a sample solution through a confined area o f reagent paper a t a reproducible rate. Application to the rapid determination of nickel with dimethylglyoxime and of copper with rubeanic acid provides a method for determination of as little a s 0.06 y of nickel and 0.03 y o f copper. The coefficients of variation are 14 and 22%, respectively. This method has wide applicability in more rapid and convenient determination of trace amounts of many metals.
T
HE USE OF SPOT TESTS for detecting ions in solution has long been known as a sensitive and specific technique. It is extensively used in modern qualitative analysis and consists essentially in bringing reagent and test solution together on paper or other porous medium. Feigl’s book (3) is perhaps the best known compilation of such methods. These techniques serve n-ell the needs of qualitatiye testing, but their application in determining the quantity of a n ion is limited by the fact that the insoluble reaction products form in a n area of indefinite extent. The sensitivity of the spot test can be increased and the measurement of quantities made more definite by any modification that confines the reaction products in a given area. Hahn (4) attained greater sensitivity by applying the test solution to the reagent paper through the fine tip of a capillary tube, the insoluble reaction products concentrating a t the point of entry of the test solution into the paper: Clarke and Hermance ( 1 ) used a similar technique to obtain quantitative results. Yagoda (8) first proposed the use of a confined spot in quantitative spot testing. Areas of definite size on reagent paper 17-ere confined by an impermeable
barrier of paraffin. ;1 measured drop was placed on the confined area containing reagent and the solution drawn through the paper by suction from below. X o attempt mas made to control the rate of flow of the test solution through the confined spot. I n making quantitative spot tests with large volumes of solution, Clarke and Hermance ( 2 ) collected the reaction products on a confined area of a reagentpaper disk, held between apertures in a special clamping assembly. They recognized that results would differ with differing rates of flow of the test solution through the confined spot, and they were able to control this rate of flow with the large volumes used (200 to 1000 nil.) by merely adjustiilg a stopcock. Stevens and Lakin (8, 7 ) describe an apparatus that controls the rate of f l o ~ of small volumes (less than 1 nil.) of test solution through the confined spot by the pull of a column of water of definite height passing through a fine-bore capillary. The rate of flow is particularly important for estimating small quantities of a n ion; the sensitivity of a particular test is increased by orders of magnitude using a slower rate of flow.. K i t h this instrument (calleda chromograph), as Tell as with the apparatus described here, the sensitivity of the spot VOL. 30, N O . 4, APRIL 1958
535
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