Roberl P. Pinnell Scripps, Pitzer and Claremont Men's Colleges Claremont. California 91711
I
I I
M a n y radiochemistry experiments describing half-lifc determinations require isotopes which are either relatively expensive, not readily available, or both. Uranium salts, on the other hand, are inexpensive and can serve as a source of a number of useful isotopes, present through radioactive decay. Described below is an experiment involving student separation of one of these isotopes from a uranium(V1) salt and the subsequent determination of its half-life. A brief listing of pertinent properties of the uranium238 decay series is given in t.he table (1). Thorium-234 Isotopes Observed in the Experiment gration Parent nuclide
Mode of disintegration
*nu
particle
1"Th
9, particle
2"'"Pe.
***pa
0 psrt$le B nartde
Daualrter nuolide
Half-life
.a4Th
4 . 5 X 10'
'J'Pa
+ '"'-Pa
eqj
mu
Y".
24.1 days I . I S min. 6 . 7 hr
energy
(Me") 4.20 0.20 2.32 1.2
is a contaminant present in a concentration high enough to be conveniently separated and is an isotope of half-life amenable to determination within the duration of a semester or quarter. The energy emitted in its decay, however, is too small to be observed on most radiation detectors and advantage must be taken of the secular equilibrium existing between thorium-234 and its protactinium daughters after short aging periods (1). The protactinium isotopes decay within a relatively short half-life and with emitted particle energies adequate for detection with inexpensive Geiger-Killer counters. The experiment is based upon the observation that the decrease in activity of a sample of thorium-234, in secular equilibrium with protactinium-234, will reflect the decay rate of the thorium. Separation of the thorium from the parent uranium salt has been carried out most efficiently using ionexchange techniques (2, 5). An earlier article in THIS JOURNAL (4) described the application of this method to the preparation of a sample of thorium-234 and its subsequent use as a source for the "milking" of protactinium-234. Unfortunately, the experiment described is limited to use in smaller classes because of the limitation of the number of milkings possible with the small amount of thorium isolated. The procedure re~ortedhere involves each student in individually separating the thorium from the parent uranium and determining its half-life, in situ, on the ionexchange column. Activity measurements are carried out over a period of six weeks and may be taken at any time during a laboratory period, thus minimizing
Determination of the Half-life of Thorium-234 A laboratory experiment the demands for a large number of radiation detectors and allowing students to proceed simultaneously with other experiments. The Experiment The ion-exchange column consists of a piece of gla% tubing 10 mm a d . by 200 mm in length. The lower end is fitted with a plug of glsss wool and a flow control constructed from a. short piece of plastic tubing and s. screw clamp. The upper end of the tube is joined by plastic tubing to a funnel. Approximately 2 g of Amberlite 120-H ion-exchange resin (medium porosity, 20-50 mesh) is washed into the tube a3 a. slurry. Conversion to the acid form is ensnred by passage of 5 ml of 2 M hydrochloric acid through the column followed by s. 10 ml distilled water rinse. Ten milliliters of saturated and filtered uranyl acetate solution' are then passed through the column s t a flow rste of one to two ml/min. Absorbed uranyl ion is separated from the strongly absorbed thorium(1V) ion by elution with 60 ml of 2 M hydrochloric acid. Complete elution of the uranyl ion can be confirmed by treating small aliqnots of the eluant with 3 M potassium ferrocyanide solution; trace amounts of uranyl ion in the solution are evidenced by the formation of the reddish-brown color of uranyl ferrocyanide (6). Upon completion of the elution, the funnel is removed, sufficient water added to msintsin uniform resin immersion during the counting period, and the tube stoppered at both ends. The resin is then allowed to stand for a t lesst two days to ensure that decay equilibrium is established hetwaen the thorium and its decay daughters. Activity counts are taken of the background and the top onethird of the resin column. Since the emitted particles have a relatively short penetration range, it is important that the student be able to reproduce the geometry of the counting assembly from one period to the next. A simple, but adequate, arrangement consists of butting the counting tube against a block of wood with a hole, slightly larger than the diameter of the counting window, bored into its face. A V-shaped groove cut into the face and crosqing the aperture serves to hold the ionexchange column in a reasonably reproducible position relative to the counting tube. A wax pencil line on the column, aligned with a fixed point in the block, is also helpful in ensuring that the same segment of the resin is scanned during each counting period.
Results A representative student plot is shown in the figure. The half-life of thorium-234, calculated from this graph, is 23.8 + 2.3 days. A sample of six determinations yielded a mean value of 24.7 + 2.6 days. Considering the simplicity of the equipment used in the experiment this is a respectable result when compared to the literature value of 24.1 days. The following sources of determinate error have been
' The solubility of UO2(C,HzOn)2.2H~Ois listed as 7.9 g/100 , at 160C (5). Any soluble urrtnyl would be suitable; the wetate is relatively inexpensive and found in most general chemistry stockrooms.
Volume 47, Number 6, June 1970 / 459
as constant in form as possible; stirring the resin bed or allowing the liquid level to drop below the top of the bed will lead to erroneous activity counts and (3) the geometry of the counting set-up, as mentioned above, should be held as constant as possible. Statistical analysis of the data provides an interesting supplementary exercise in judging indeterminate errors and is a recommended addition to the experiment. Acknowledgment
The author is extremely grateful to Dr. George Lowry for his assistance and many helpful discussions during the adaptation of this experiment' to classroom use. Thanks are also due the General Chemistry class, Fall Semester, 1909, for their services rendered as checkers. Literature Cited
0
7
14 21 28 TIME DAYS)
35
Dway of thorium-234.
observed: (1) elution of the uranium must be quantibe observed Or no decrease in activity (2) the physical state of the resin column must remain
460
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Journal o f Chemical Education
(1) KAPLAN. I.. ' ' N ~ d e a r Phyeio~." Addiaon-Wesley Publishing Co.. Reading. Masa.. 1958, pp. 197-210. (2) SCHWEITEER. G. K., A N D WHITNEI. I. n., "~adioactive racer T~CILninuea." . . ~ Van. Nostrand and Co.. New Yark. 1949. ~ o 118-23. (3) Smm. V. R. J.. P E I ~ A C X , M., A& SST.E&.F. W:. "Seoond International Conference on t h e Peaceful Uses of Atomic Enerav," Paper 1119. 1958. (4) C ~ ~ a w e b lD. . , J., A N D LAWRANCB. J. J.. J. CHEM. EDUC.. 36, 499 (1959). ( 5 ) wE.,. R. c.. ~ d i l ~ . ., . - ~ ~ ~ ofd chemistry h ~ ~ ksnd PI,~.IOS;~ (48th ed.),Chemical Rubber Co.. Cleveland. Ohio. 1967. o. R-237. (6, TREADWELL. F. P., A N D HALL,W. T.,"Analytical Chemistry," John wiley dr sons. ~oc.,N ~ Wyork, 1956, VOI. I. p. 515.
