0
SOME CONVENIENT LABORATORY EXPERIMENTS IN RADIOCHEMICAL TECHNIQUES' CLYDE R. DILLARD and LINWOOD MORTON Morgan State College, Baltimore 12, Maryland
M A N Y universities and colleges, while recognising the need for instruction in nuclear chemistry on the undergraduate level, have found that this is prohibitively inconvenient. Only in those universities having facilities for producing radioisotopes is it possible to obtain activities at any desired time. Of course, it is possible to purchase isotopes, but in this case, careful advanced scheduling is required if they are to be delivered a t the right time and used t o the best effect. Moreover, purchased radioisotopes must have half-lives of a t least a few days which makes decay studies considerably drawn out. For instructional purposes, it is most desirable to use isotopes with half-lives such as to allow the completion of a decay curve in a single (3-hour) laboratory period. An alternative course to the purchase of radioisotopes is the use of radioactive series, a supply of suitable long-lived parent being kept as a self replenishing reservoir of a shorter lived daughter. Experiments in this category have been described by Booth,= Jones,' and Robinson and Cole.& This paper describes an extension of the experiment of Robinson and Cole, to embrace several important radiochemical techniques which could be conveniently included in the laboratory part of courses such i s physical chemistry, inorganic chemistry, and/or instrumental analysis. Lead-212 and bismuth-212 are separated by electrolysis from a solution of a thorium salt. These activities are used for instruction in (1) preparation of carrier-free activities; (2) use of carrier techniques in radiochemical separations; (3) study of laws of radioactive decay; (4) study of parent-daughter relationships. I n addition, instruction is provided in the techniques of handling long-lived radioactive materials in such a manner as to minimize contamiuation of the apparatus and laboratory. As an introduction t o the experiments described below, the students should become familiar with the naturally radioactive thorium (4n) series and the other naturally radioactive s e r i e ~ . Of ~ particular note is the "branched" decay of bismuth312 which affords an example of nuclear isomerism. The half-life of bismuth-212 (60.5 minutes) is sufficiently long to allow good .yields of activity without excessive haste,. -vet
short enough t o permit the determination of a decay curve in a single laboratory period. Moreover, its energetic 2.2 Mev. beta radiation requires no special countine: - techniaues. APPARATUS AND CHEMICALS
The importance of detailed planning of radiochemical experiments must he strongly emphasized. This is necessary because (1) the short-lived activities are rapidly decaying and undue delay in manipulations must be avoided; (2) the danger of contamiuation of apparatus and instruments by long-lived radioactive substances must be constantly guarded against. Consequently all apparatus must be assembled beforehand and its intended use carefully noted. Likewise as each piece of apparatus is used it should be disposed of in a manner which will permit its being identified as a source of contamination. The equipment required for this series of experiments is listed in Table 1. In addition, the usual counting equipment, Geiger-Miiller tube and scaling circuit, is required. Because of its greater efficiency in counting gammas, in the measurement of radiations from Ph-212, a scintillation counter is desirable but not necessarv. SEPARATION OF CARRIERTREE ACTIVITIES
A diagram of the apparatus used in the electrolysis of the thorium nitrate solution is shown in Figure 1.
9
Ammeter
Slide W i r e Rheostat Pt crucible Fi1. EL&lg.i.
Circ"it
About 20 ml. of 0.5 M Th(NOa)r in 0.1 N HNOI is placed in the 30-rnl. platinum crucible which serves as the cathode. The anode consists of a platinum foil electrode. Current is supplied by four 1.5-volt dry(1953). cell batteries connected in series and is adjusted to 0.1 'JONES,WILLIAM H., J. CEEM.EDUC., 34,406 (1957). 6 D ~ ~ F., ~AND~ R.L A. ~A ,~ ~ ~~~physicSI1 R T ~ ~, h ~ ~ i ~ t ampere ~ ~ , , by , means of the slide wire rheostat. The solution is electrolyzed for 5-10 minutes, then the anode is John Wiley & Sons, Inc., New York, 1955, p. 609. Based an an undergraduate research project by Linwoad Martan. Boom, A. H., J. CXEM.EDUC., 28,144 (1951). a ROBINSON, L. B., AND E. M. COLE,Am. J. Phys., 21, 469
238
JOURNAL OF CHEMICAL EDUCATION
removed and rinsed into a wide mouth bottle labeled "Thorium Risings." The solution in the crucible is poured into a bottle labeled "Electrolyzed Th(NOa)r" and marked with the date. The crucible is rinsed several times with distilled water, the rinsings being poured into the bottle provided for them. TABLE 1 Eaui~ment Special Apparatus Platinum crucible, 30 ml. Platinum foil electrode Ammeter, 0-2.5 amps. 4 dm cells. 1.5 volt Slide wire iheostat
Wash bottle Centrifuge tubes, 20 ml. Aluminum sample pans (planchets) Infrared heat lamo
N Bi(N03)3. Six drops of 18 M H2S04are added and the mixture is heated until dense fumes of SOa are given off. (Care! Avoid spattering.) One ml. of water is added and the mixture is transferred t o a centrifuge tube, washing in a11 of the precipitate with 0.1 M H2S04. After centrifuging, the supernatant liquid, which contains the bismuth, is decanted into another centrifuge tube and saved. The precipitate of PbS04 is washed twice with 0.1 M HzS04, combming the washings with the previous decantate. The precipitate is then dissolved in a few drops of concentrated HN03, and the solution is poured into a counting planchet, evaporated to dryness under a heat lamp, mounted and counted.
W. M. Bottle, G . S. labebeled, "Thorium Rinsings" W. M. Bottle, G. S. labeled, "Eleetrolvzed Th(NOd," . ... ...... ..
0.5 M Th(NO& in 0.1 M HNOI s o h 0.1 N Bi(NO& 0.1 N PblNO"),
0.1 M H 8 0 , Conc. HNOs, H9SO
