THE STORAGE OF POLONIUM SOLUTIONS CHARLES ROSENBLUM
AND
E. W. KAISER
School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received November 9, 299.J
Polonium has been used but little as a radioactive indicator because, except in strong acid solution, it appears to be colloidally dispersed (18,19). This behavior is unfortunate, because the radioelement possesses ideal properties which qualify it for use as an indicator. Polonium is a strong alpha-radiator, and therefore produces an intense and easily measurable ionization. Because it is the last active member of the uranium-radium series, there is no need for multiple measurements or for delay until establishment of radioactive equilibrium with succeeding products. In addition, it is sufficiently long-lived to obviate the necessity of making decay corrections in most cases. Since polonium in true solution would constitute a valuable tool for a proposed study of crystalline precipitates, it was believed advisable to determine, if possible, useful conditions under which radiocolloid (23) formation by polonium is minimized. Elements like polonium, radium E, thorium B, etc., under suitable conditions of acidity, behave as though they were colloids. This conclusion is drawn from their dialyzability (18, 19), the small velocity with which they diffuse in solution (19, 20, 22), their centrifugability (2, 4, 24) and settling under gravity (16),their behavior in an electric field (8, 9, 10, 15, 19), and from the fact that radiograms (1, 11) of small quantities of their solutions apparently show the presence of large aggregates of radioactive atoms. Because it is uncertain whether true colloids are involved, the term “radiocolloid” (23) was coined. Opinions as to the cause of the phenomenon are not in accord. One explanation (19) views the effect as true colloid formation, the sol consisting of extremely insoluble products of the hydrolysis of salts of radioelements. Actually, those radioactive elements which would be expected to yield, upon hydrolysis, insoluble hydroxides and basic salts, do show a pronounced radiocolloid effect (11). Since the solubility products of these compounds of lead and bismuth are never exceeded in solutions containing only the radioactive isotopes of these common elements, it is difficult to see how such precipitates can be formed. Hence others (15, 24, 26) hold that the phenomenon is really a pseudo-colloid formation resulting from the adsorption of radioelements by traces of colloidal impurities, such as 7 97
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CHARLES ROSENBLUM AND E. W. KAISER
dust, alumina, and silica particles, suspended in the liquid media employed. This view is supported by the fact that it is almost impossible to prepare and store in the laboratory, water which is truly optically void (13, 25). Since the chief source of suspended silica is the walls of glass containers used for storage purposes, it occurred to the authors to study the behavior of polonium solutions kept in paraffined bottles. This procedure has the additional advantage of preventing adsorption of the polonium by the glass (5, 21) itself. Though by this simple expedient the radiocolloid effect could not be avoided entirely, it was found that polonium could be maintained in dilute acid solutions stored in paraffin for periods of time sufficiently convenient to permit its use as an indicator. The necessity of preparing fresh stock solutions for each set of experiments was thus eliminated. EXPERIMENTAL
The centrifuging method (12) of demonstrating radiocolloid formation, was used in these experiments. Paraffined, cylindrical copper cups (2.5 cm. in diameter X 7 cm. in height), closed by paraffined stoppers, were used for centrifugation. Into each cup was placed 16 ml. of a given polonium solution, and the whole rotated for various times a t a speed of 3000 R.P.M., with a radius of 22 cm. Then three 2.63-m1. samples of the supernatant liquid were carefully sucked off and evaporated in matched watchglasses on a water bath, prior to measurement in a Lind electroscope (17) equipped with an open door chamber. Samples of the uncentrifuged solution were taken as standards for comparison. All pipets used were paraffined. All experiments were done in duplicate. Electroscopic measurements were accurate to within f l per cent. In most cases, centrifuging was carried on for an hour. Polonium was obtained from a number of old radon bulbs, after digesting the crushed glass in acid, by currentless electrochemical deposition (5, 6, 7) on silver. The silver was dissolved in concentrated nitric acid, hydrochloric acid was added to precipitate the silver, and, after removal of silver chloride by filtration, the filtrate was evaporated to dryness in a Pyrex beaker on a water bath. The polonium source, from which all other solutions were subsequently prepared, was obtained by taking up the residue in cold 1.015 N nitric acid. Boiling the acid solution to accelerate solution was found to yield a large amount of centrifugable polonium, and was therefore undesirable. In this fashion, 125 ml. of a stock solution containing 100 E.S.U. of polonium was prepared and stored in a paraffined bottle. The solutions used in the centrifuging experiments were formed by diluting this source with twice-distilled water, and then stored in large paraffined containers. This water was used shortly after the second distillation, to avoid long standing in glass receivers. More active solutions were made by evaporating a portion of the source, and
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STORAGE O F POLONIUM SOLUTIONS
taking up the residue in a solution of the proper acid concentration. The centrifugability of the polonium was measured a t various times after the preparation of solutions. Results are expressed as per cent of polonium left in solution after centrifugation. Approximate polonium quantities are given in electrostatic units. RESULTS
The acid concentration and polonium content of solutions studied, as well as the results of centrifugation experiments, are summarized in table 1. TABLE 1 Centrifuging of polonium solution,s COMPOSITION BOLETION
Nitric acid concentration
Polonium content ~ . 8 . ~ . / 1 6 .ml. 1
N
1 2 43
5
{
I I
6
8 7
{
1.09 0.114 0.0196 0.0196 0.0108 0.00534 0,00534 0.00534 0.00534 0.00534 0.00534 0.00534 0.00059 0.00059 0.00059
0.06 0.06 0.07 0.07 0.48 0.07 0.07 0.07 0.07 0.07 0.07 0.59 0 .os 0.08 0.80
A Q E OF SOLUTION
DURATION OF CENTRIFEGING
POLONIUM LEFT
days
hours
per cent
45 45 10
13 22 7
7 7 12 39 45 9 11 11 8
1 1 3 2 1 0.5 1 3 1 1 1 1 1 2 1
98.8 97.3 98.0 98.2 95.2 100 100 100
99.4 99.8 98.2 96.2 96.5 97.9 96.3
From these data, it is evident that polonium solutions can remain practically uncentrifugable for long periods of time. Even the 0.00059 N acid solution shows little indication of radiocolloid formation after a week of standing. This is especially significant, because A. Korvezee (14) has recently reported a region of maximum centrifugability near this acid concentration. That this is an improvement over storage in uncoated glass vessels can be illustrated simply by following the behavior of solution 5 kept in a common glass container for six days. After this period, the activities of the centrifuged and uncentrifuged solutions were measured in the usual fashion. By comparing these electroscope discharges with samples of the original solution kept in paraffi, both adsorption by glass walls and
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CHARLES ROSENBLUM AND E. W. KAISER
radiocolloid formation could be determined. Using 200 ml. in 250-ml. bottles, it was found that 8.4 per cent of the polonium had been removed from solution by the glass walls, and of the polonium remaining in solution, 20 per cent was removed b y centrifuging for one hour. It seems definite that glass containers are undesirable as stock bottles for polonium solutions. The disadvantage in using glass is further illustrated by preparing radioactive solutions with the use of inactive acid solutions which had been shaken gently for several days in common glass bottles. A 0.00059 N nitric acid solution was employed for this purpose. After the dilute acid had been transferred to paraffined flasks, portions were centrifuged for an hour and a half, and half of the supernatant liquid separated from the remaining liquid. Then three polonium solutions were made by diluting fresh solution 8 with measured volumes of uncentrifuged acid, the supernatant liquid, and the residual liquid. These solutions were shaken in TABLE 2 Radiocolloid formation in solutions prepared from glass-stored acid
1
COMPOSITION AQB OF SOLUTION
ACID SOLUTION EMPLOYlD
Uncentrifuged acid.
......................
Supernatant liquid.. Residual liquid.
{I
-~
I N I 0,00059 0.00059
1
POLONIUM LEFT
0.077 0.077
dags
per cent
2 8 2 2
81.9 72.5 93.0 39.5
paraffin for two days, and centrifuged for one hour in the usual manner. Table 2 shows the results of these experiments. The “uncentrifuged acid” solution clearly shows the radiocolloid forma‘ tion, accompanied by its characteristic increasing centrifugability with age (3). On the other hand, the “supernatant acid” solution is practically free from the disturbing phenomenon. I n accord with the last observation is the fact that the “residual acid” solution shows the greatest loss on centrifuging. Apparently the dilute acid, during its contact with glass, had become contaminated with impurities which promote the centrifugability of polonium. ’
DISCUSSION
Objections might be directed toward the use of such small amounts (0.06 to 0.8 E.s.u.) of polonium in studying the radiocolloid effect. These are unfounded, because the above experiments unquestionably point to the loss of polonium on centrifuging our weak solutions, unless paraffined containers are used. Furthermore C. Chamit? and M. Hai’ssinsky (3j have
STORAGE OF POLONIUM SOLUTIONS
801
demonstrated the phenomenon using 0.05 E.S.U. The stability of our solutions can be explained best by the care taken to avoid contact with uncoated glass walls. Even this precaution is insufficient to eliminate the effect in 0.00059 N acid after two weeks of storage. Experiments of this type unfortunately are chiefly empirical, and do not explain the mechanism of radiocolloid formation. Probably glass alone is not responsible. Dust particles too will produce the effect, as was shown by I. E. Starik (21). We have found that as little as 25 mg. of laboratory dust, shaken in 16 ml. of solution 5 for ten minutes, will cause a separation of 87 per cent of the polonium after centrifuging for only ten minutes at 1600 B.P.M. This, together with colloid-producing impurities in the nitric acid used, probably accounts for the small losses found in table 1. SUMMARY
1. The use of glass vessels for the storage of polonium solutions is undesirable. 2. Coating storage containers with paraffin eliminates, to a conaiderable extent, the disturbing radiocolloid effect in polonium solutions. 3. Polonium solutions which are as dilute as 0.00059 N in nitric acid may be used for more than a week if stored in paraffined vessels. More concentrated acid solutions may be kept for longer than a month and a half. REFERENCES
(1) CHAMII~, C.: Compt. rend. 184, 1243 (1927). C.,AND GUILLOT,M.: Compt. rend. 190,1187 (1930). (2) CHAMII~, , AND HAISSINSKY, M.: Compt. rend. 198, 1229 (1934). (3) C H A Y I ~C., ~ ~ ,AND KORVEZEE, A.: Compt. rend. 192,1227 (1931). (4) C H A M I c., (5) CURIE,I.: J. chim. phys. 22, 471 (1925). O.,AND PHILIPP, K.: Z. Physik 61,309 (1928). (6) ERBACHER, O.,AND PHILIPP, K. : Z.physik. Chem. 160A,214 (1930). (7) ERBACHER, T.:Le radium 10, 250 (1913). (8) GODLEWSKI, T.:Kolloid-Z. 14, 229 (1914). (9) GODLEWSKI, (10) GODLEWBKI, T.:Phil. Mag. [6]27, 618 (1914). (11) HAHN,O.,AND WERNER,0. : Naturwissenschaften 17, 961 (1929). E. L.: Phil. Mag. [7]6,685 (1928). (12) HARRINGTON, W.: Z.physik. Chem. 87, 257 (1914). (13) KANGRO, A.: J. chim. phys. 30, 130 (1933). (14) KORVEZEE, (15) LACHS,H.:Kolloid-Z. 21, 165 (1917). M.: Physik. Z. 23, 318 (1922). (16) LACHS,H., AND WERTENSTEIN, (17) LIND,S. C.: J. h d . Eng. Chem. 7,406 (1915). F.:Wien. Ber. [2a]121,2193 (1913). (18) PANETH, F.: Kolloid-Z. 13, 1, 297 (1913). (19) PANETH, M.:J. chim. phys. 31,211 (1934). (20) SERVIGNE, (21) STARIK,I. E. : Z. physik. Chem. 167A, 269 (1931). (22) VON HEVESY, G.: Physik. Z. 14,49 (1913). (23) VON HEVESY,G.: Wien. Ber. [2a] 127, 1787 (1918). (24) WERNER,0.: Z. physik. Chem. 166A,89 (1931). (25) WOLSKI,P.:Kolloidchem. Beihefte 13, 137 (1920). (26) ZSIGMONDY, R.: Communication to F. Paneth, cited in reference 19,p. 304.
