MEASUREMENT OF T H E ACTIVITY O F RADIOSULFUR I N BARIUM SULFATE RUSSEL H . HENDRICKS, LOREN C. BRYNER,‘ MOYER D. THOMAS, A N D J.lblES
0. IVIE Department of Agricultural Research, American Smelting and Rejhing Company. Salt Lakc: City,Utah Received March 8 , 1948
In dealing with weak radiation, such as that emitted by radiosulfur2 (0.120 M.E.V.), it is necessary to consider suchJactors as the extent of dilution of the act,ive material with the inactive carrier, the nature of the carrier, and the geometry of the deposit. Barium sulfate is in general the most convenient carrier, but it absorbs a considerable amount of the radiation. Cooley, Yost, and McMillan (1, 2) have suggested the use of lithium sulfate or elemental sulfur to minimize this effect. Further, they placed the deposits on metal surfaces. Libby (4) and Libby and Lee ( 5 ) have described a “screen wall” counter into which they introduced samples of radioactive barium sulfate before evacuating. Rather large amounts of material have generally been used for these measurements, since about 15 mg. per em.* of lithium (1) or barium sulfate ( 7 ) is needed to make the measured activity independent of the thickness of deposit. When the amount of active material available is small, this is a serious handicap. At the beginning of a study of the sulfur nutrition of plants, using radiosulfur, considerable difficulty mas encountered in measuring the activity of barium sulfate precipitates. For this work a University of California Geiger-Muller counter assembly (6) \vas used. The ionization chamber was a 25-mm. brass tube, having a mica window 10 microns thick a t the bottom and a central nichrome wire 0.12 mm. in diameter. The chamber was filled with argon (90 mm.) and alcohol (10 mm.). Absorption of the sulfur radiation by a mica window weighing 3 mg. per is said to be about 50 per cent.3 The active precipitates mere filtered on small paper disks, which mere then mounted in a holder and bropght to a fixed position close to the mica window. The results were extremely variable, particularly with small amounts of precipitate. For example, when identical preparations of active barium sulfate (2 mg. per cm.2) were filtered on disks cut from a single piece of high-grade paper, the counts varied over a range of 100 per cent from the lowest values. This was presumably due to the variable texture of the paper. The differences were especially evident in a comparison of the two sides of the paper. This variability mas largely eliminated when the barium sulfate was spread out on a polished brass disk and brought to the counter window on a suitable Assistant Professor of Chemistry, Brigham Young Gniversity. The authors are indebted to Dr. J. G. Hamilton of the Radiation Laboratory, Berkeley, California, for the radiosulfur used in this work. Information from Department of Chemistry, University of California. 1
460
470
R. E. HENDRI
1,. C. BKYSCR, M. D. THOMAS, A S D
J. 0. IVIE
support. Figure 1 is a diagram of the disk. The cylinder A was screwed into the seat B of the disk C, forming a liquid-tight cup. -2 weighed amount of barium sulfate mas placed in the cup, the lumps were broken up by the flattened end of a stirring rod, 1 ml. of ethyl alcohol mas added, and the solid was uniformly suspended in the liquid. When the solid had settled and the alcohol had evaporated (under a lamp), a uniform deposit was obtained on the disk. The cylinder was removed and replaced with a brass ring D before counting. The precipitate showed no tendency to “creep” with alcohol or to stick to the cylinder. With nater, creeping and sticking n w e troublesome. When variable amounts of radiosullur were precipitated in a constant total amount of baiium sulfate, the activity of the deposit was directly proportional to the amount of radiosulfur precipitated (figure 2, curve A). When increasing amounts of a uniformly active preparation of barium sulfate n w e added to the disk, curve B, figure 2, was obtained, which illustrates the absorption by the solid and indicates that maximum activity is reached at about 15 mg. per cm.2
r” FIG.1. Brass disk and detachable cup for preparation of barium sulfate for activity measurements,
When a constant amount of radiosulfur was thrown down with variable amounts of inactive barium sulfate, the actkity of the deposit decreased with increasing figure 3. weight of precipitate, as shoivn in curve 9, By extrapolating this curve to the zero abscissa, it is evident that very little activity is unavailable to the counter, if all the radiosulfur is present in a small amount of precipitate. For example, a given amount of radiosulfur showed about 88 per cent of its total act,ivity vhen it was dispersed on the 25-mm. disk in 10 mg. of barium sulfate. The total activity of the heavier precipitates could be readily determined by counting a 10-mg. aliquot (curve B, figure 3), multiplying by one-tenth the weight in milligrams, and adding 12 per cent (curve C, figure 3). The standard deyiation of the values in curve C is 3.5 per cent. This variability could be reduced somewhat by close attention to the voltage of the counter and frequent readings on the background and on a standard preparation. When the sulfate content of a sample is low, it is advisable to add enough sulfuric acid to yield a precipitate of at least 5-10 mg. Inactive barium sulfate could
ACTIVITY O F RADIOSULFUR I N BARIUM SULFATE
47 1
not be mixed with an active preparation to give a quantitative lowering of the count.
WEIGHT
Ba 504,Mg.
FIQ.2. Activity of barium sulfate precipitates. Curve .4, different amounts of radiosulfur precipitated in a constant amount of barium sulfate; curve B, different amounts of a uniform mixture of radiosulfur in barium sulfate.
In the course of a series of measurements of samples derived from plant material, the activity of a reference deposit 'was determined a t frequent inter-
4i2
R. H. HESDRICKB, L. C. BRYKER, 31. D. THOlf.IS, A S D J . 0 . IVIE:
vals. Over a period of 6 months there were accumulated one hundred and eight,y-nine of these i-alues, starting at 3.1 counts and ending at 0.7 count, above a background of about 0.5 count per second. A plot of these data against the time gave a typical decay curve and the half-life calculated by the met,hod of least squares was 87.1 days, xith a standard deviation of 1.2 days. Thi? value agrees me11 with the half-life of 88 i 3 days reported by Iianien (3). I n spite IO
0
Mq. Ba 3 0 4 FIG.3. Activity of radiosulfur in barium sulfate. Curve A, a constant amount of radiosulfur precipitated in different amounts of barium sulfate; curve 5 , activity of 10-mg. aliquots of the corresponding samples in curve A ; curve C, values in curve B times onetenth the weight in curve A plus 12 per cent.
of the low activity of this sample, the correlation coefficient of the best-fit curve il-as 0.969, and the standard deviation of the count data from this curve lyas 6 per cent. Jn another series of measurements, pieces of mica of seven different thicknesses vere placed between an active deposit and the counter mindom. A smooth typical absorption curve was obtained with a correlation coefficient of 0.99. Extrapolation by the method of least squares to zero thickness of mica indicated
DISTRIBUTION EQUILIBRIA BETWEEN MOLTEN METALS AND MOLTEN SALTS
473
that the absorption by the mica window (3.0 mg. per cm.? of the ionization chamber was 49.5 per cent, with a standard deviation of 4.2 per cent. This confirms the value mentioned earlier. SUMMARY
The measurement of the activity of ‘radiosulfur in barium sulfate precipitates can be carried out quantitatively when a definite weight of the solid is deposited uniformly on a polished brass disk and brought to a fixed position with respect to a thin counter window. A convenient arrangement is described for accomplishing this purpose, and the variability of the results is defined. The half-life of radiosulfur is observed to be 87.1 f 1.2 days, and the absorption by the mica window (3.0 nig. per om.*) is 49.5 f 4.2 per cent. REFERENCES
COOLEY, R. A , , AND YOST,D. M.: J. Am. Chem. SOC. 62, 2474 (1940). COOLEY, R . A , , YOST,D. M., A N D MCMILLAN, E.: J. Am. Chem. SOC. 61, 2970 (1939). KAMEN,MARTIND.: Phys. Rev. 60,53741 (1941). LIBBY,W. F.: Phys. Rev. 46, 196 (1934). ( 5 ) LIBBY,W. F., AND LEE, D. D.: Phys. Rev. 66, 245 (1939). (e) MULLEH,RALPHH.: Ind. Eng. Chem., A n d . Ed. 13, 745 (1941). (7) VOOE,H. H.: J. Am. Chem. SOC.61, 1032 (1939).
(1) (2) (3) (4)
DISTRIBUTION EQUILIBRIA BETWEEN MOLTEX METALS AXD RfOLTEN SALTS, WITH REFERENCE TO THE STABILITY OF ISTERMETALLIC COMPOUNDS IN T H E MOLTEX STATE E. HEYMANN, R. J. L. MARTIN,
AND
M. F. R. MULCAHY
Chemistry Department, University of Melbourne, Melbourne, Australia Feceived August 27, 1948
Distribution equilibria of a metallic element between a molten metal phase and a molten salt phase have been used to investigate the nature of solutions of metals in molten salts (previously called “pyrosols”). The result of these investigations is clearly in favor of an atomic (and not a colloidal) solution (11, 12). In the case of solutions of cadmium in cadmium chloride, measurements of the magnetic susceptibility have shown that cadmium atoms may unite with cadmium ions to form Cd:’ (“subchloride”; see reference 6). By investigating the deviations of such distribution equilibria from ideality it is possible to obtain qualitative information about the activities in binary metallic mixtures and about the existence of intermetallic compounds in the molten state. The variety of systems is rather restricted, because metals are generally soluble only in their own salts (except silicates), and because pure distribution
