A COURSE in the USE of RADIOELEMENTS as INDICATORS* CHARLES ROSENBLUM University of Minnesota, Minneapolis, Minnesota
A series of experiments included in a course in Radioactive Indicators i s described. The experimats were designed to emphasize the use of radioelements as a practical tool in attacking various problems, rather than to illustrate methods of measuring the properties of radioactive radiations. The simple methods used to obtain the radioelements are likewise presented.
T
HE instruction of students in the use of radioelements as an aid in their research is the purpose of a course recently offered a t the University of Minnesota. A series of exercises was arranged to acquaint students with the various types of electroscopes used, with methods of preparation of radioactive sources, and with the utility of the classical experiments. Because the period of instruction was but one-quarter of the school year, only seven simple experiments were assigned. These experiments included measurements of the adsorption of thorium B, thorium C, polonium, and radium D by lead sulfate, the adsorption of radium E by glass, the emanating power of thermally aged barium sulfate, and the demonstration of radiocolloid (9) formation in the case of polonium. This arrangement illustrates both general methods of * Contribution from the University of Minnesota School of Chemistry.
attacking problems with the aid of radioelements, vie., the indicator method of F. Paneth (1)and the emanation method of 0.Hahn (2). For the alpha-ray measurements, an interchangeahlehead electroscope (3) with an open-door chamber was employed. Beta radiators were measured in a box electroscope of simplest construction (4). For determining the small amounts of radon, an emanation electroscope of the Lind type (3) was found satisfactq. SOURCES OF RADIOELEMENTS USED
The source from which the thorium B and C were obtained was a radiothorium preparation kindly presented by the University of Missouri for use in connection with a radiometric study of the aging of precipitates. The active deposit was collected on platinum foilsor wires attached to the negative pole of the customary collecting device (5). The activated platinum pieces were immersed in a saturated lead sulfate solution to form the stock solution of thorinm B and thorinm C. No attempt was made to separate the B from the C product, since a properly arranged set of measurements permits one to determine the relative amounts of both elements simultaneously present. The source of radium D (a lead isotope), radium E (a bismuth isotope), and radium F (polonium) was a number of old radon bulbs and capillaries which con-
tained radium D in equilibrium through radium E with radium F. The glass was pulverized and digested in concentrated nitric acid on a water bath; the filtered solution was then evaporated to dryness. The separation of the radioelements could next be made chemically (6). However, currentless electrochemical separations are much simpler and more rapid. Accordingly a procedure suggested by 0.Erbacher (7) was followed, with minor modifications. The residue was taken up in 5 ml. of 0.2 N hydrochloric acid, the solution was placed in a test-tube fastened to a slowly rotating platform (4 R.P.M.), and a clean, roughened silver foil, 1 an. X 1 an.,was immersed in the solution. Several hours of rotation gave very high yields of deposited polonium, especially when the solution in the test-tube was kept hot during the first half-hour. Because polonium tends to alloy with metals (8) it is difficult to remove the deposited polonium from the silver foil. Therefore the usual procedure of dissolving the silver in concentrated nitric acid was adopted. The silver was removed by precipitation with excess hydrochloric acid Tollowed by digestion (to coagulate) and filtration. The filtrate was evaporated to dryness; the residue was taken up with a small portion of concentrated bydrochloric acid and then diluted to a suitable concentration, in this case 0.01 N hydrochloric acid, to diminish radiocolloid formation (9). This constituted the stock solution of polonium. The solution remaining after removal of polonium by silver foil was evaporated to dryness and the residue was dissolved in 5 ml. of 0.05 N hydrochloric add. A second electrochemical separation then served for the extraction of radium E. A roughened, clean nickel foil was substituted for the silver; and immersion for less than an hour in the heated solution was sufficient. The radium E so obtained is relatively free from radium D. The nickel foil was washed with water and alcohol and then immersed for scveral minutes in concentrated nitric acid to remove the bismuth isoto~e. The residue obtained by evaporating this solution was dissolved in 1 ml. of 0.2 N hydrochloric acid and diluted to 0.002 N. This dilute acid solution served as the stock solution of radium E. The solution remaining after removal of radium E was evaporated to dryness, and the residue (chiefly radium D) taken up in a saturated aqueous solution of lead sulfate to form the stock solution of radium D. No attempt a t greater purification of these radioelements was made; nor was consideration given to the removal of mercury, which is frequently found in radon bulbs. These rehements require more elaborate chemical (6) and electrolytic (10) procedures. The radium-bearing solid used in the emanating power experiment was a sample of radium containing barium sulfate. The original radioactive solution from which the sulfate was prepared contained about 1.6 X 10-8 grams of radium and about eleven milligrams of barium chloride per ml. From 10 ml. of this solution, radioactive barium sulfate was precipitated by the addition of 0.1 N sulfuric acid. The precipitate was
purified by digestion, and the filtered product was thermally aged before use, by maintaining it for several hours a t a temperature of 300°C. EXPERIMENTS
The adsorption of thorium B by lead sulfate was measured by adding 2.5 ml. of the stock solution to a gram of aged lead sulfate in a paraffined container. After shaking for an hour, the solid was separated either by centrifuging for five minutes or by permitting it to settle by standing undisturbed for seven minutes. Then 5-ml. portions of the supernatant liquid were slowly evaporated in round-bottomed copper dishes on a water bath, and their alpha-ray ionization (m divisions per second discharge of the electroscope leaf) compared with that of a standard prepared by diluting a quantity of the stock solution with an equal volume of saturated lead sulfate solution. The beta radiation from thorium B is extremely soft, and its measurement must be effected through the intense ionization produced by its decay product thorium C. Therefore if measurements are made about nine hours after performance of the experiment, in which time radioactive equilibrium between thorium B and thorium C is assured, the ionization produced by the residue in the dishes permits calculation of the fraction of thorium B left. This delay is necessary because the ionization produced by the alpha particles emitted by thorium C is measured in the electroscope. In order to determine the adsorption of thorium C itself, the dishes are measured also immediately after evaporation of the 5-ml. samples. It is of interest to recall that a knowledge of the thorium B adsorption permits calculation of the specific surface (11) of lead sulfate. The adsorption of polonium was measured in the same manner as was the thorium B adsorption, except that the stock polonium solution was used and watch glasses were substituted for the copper dishes. Since polonium is measured directly by its own alpha radiation, the electroscopic measurements could he made immediately after evaporation of the samples of supernatant liquid. The centrifuge method of E. L. Harrington (12) was used to demonstrate radiocolloid formation by polonium. A portion of the acid stock solution of polonium was neutralized, shaken for an hour, and then centrifuged a t 2000 R.P.M. for a half-hour, after which time samples of the supernatant liquid were evaporated and compared as to alpha-ray activity with the stock solution. Radium E adsorption by glass was shown by shaking a portion of the stock solution in an unparaffined bottle for an hour, and comparing the beta-ray activity of samples of remaining solution with that of the original solution. The procedure for the determination of the radium D adsorption by lead sulfate was identical with that for measuring thorium B adsorption, except that the stock solution of radium D was used, and that the
ionization was measured with a beta-ray electroscope. Because the beta radiation from radium D is extremely soft, the element is measured indirectly by the intense beta radiation characteristic of its disintegration product, radium E. Consequently two measurements separated by an interval of several days were made. Here, too, it is possible to calculate the specific surface of lead sulfate. The emanating power determination consisted of two parts. It was necessary to know the radium content of the solid; then the portion of the total radon which escaped by outward diinsiou had to be found. For the radium analysis, the standard potassium bisulfate fusion method (13) was employed. Details of this well-known method need not be repeated here. The escaping radon was determined by sealing a weighed amount of barium sulfate in a glass tube with constricted ends for a time sufficient to insure the maximum outward diffusion of the emanation. The radon was sucked through the usual purifying train into an evacuated emanation electroscope for measurement. In this course, the students sealed their tubes several weeks before the radium analyses were performed, and then both determinations were made
on the same day. Because the methods of determining radium by the emanation method are so widely known (14),it is not necessary to describe in this presentation the details involved in the emanating power experiment. LITERATURE CITED
p. m.
Ref. (4). p. 71. I. CURIE-JOLIOT, J. chim. phys.. 22, 471 (1925). 0.ERBACHER AND K. PAILIPP,Z. Phyxk, 51, 309 (1928); Z. physik. Chem., 150A, 214 (1930). G. TAWNN, Z . Elektrochem., 38, 530 (1932). F. PANSTH. Kolloid-Z., 13, 1 (1913). A. KORYEZEE, I, chim. phys., 30, 130 (1933). 0.ERBACHER. Z. physik. Chem.. 156A, 142 (1931). F. PANETH.Z. Ekktrochem., 28, 113 (1922). F. PANEi w W. ~ VORWERK, Z. physik. Chem., 101,445 (1922). E.L. HnnnINGToN. Phil Mag.. [7],6,685 (1928). H . H. BARKER, 1.Ind. Eng. Chem., 10,525 (1918). S. C. LIND,ibid., 7 , 1024 (1915); Bureau oj' Mines Bulktin. No. 212, Part V. p. 173 (1923).
