John C. Sheppardl and Reilly C. Jensen
Son Diego State College Son Diego, California
II
A radiochemistry experiment
The Solubility of Thallium(l) Bromide
There are several reasons why the study of TlBr solubility is particularly adaptable to student experimentation. The 0.77 Mev beta emitted by thallium-204 is easily counted by standard end-window Geiger-Muller counters. The solubility of thallium(1) bromide in water is such that 5 microcuries of thallium204 and 15 mg of thallium(1) nitrate are sufficient to obtain convenient and accurate counting rates with 0.1 ml assays of the supernatant solution. Fifty microcuries of thallium-204, which can be obtained without exceeding the general AEC license, is sufficient tracer for a class of about ten students. The experiment is completed with little difficulty in a 3-hr laboratory period and demonstrates the concept of specific activity and its application to the determination of a slightly soluble salt. The students first determine the activity of the thallium-204 tracer solution by evaporating an aliquot of the tracer on a planchet and counting it with an end-window Geiger-Miiller counter. If t,he tracer is not thallium(I), a few milligrams of sulfite should be added to the solution to reduce thallium(II1) before the experiment is performed. While evaporating the tracer assay, the student carefully weighs out 15 mg of dry thallium(1) nitrate and dissolves it in a few milliliters of water. About 5 microcuries of the assayed thallium-204 tracer is then carefully pipetted into the thallium(1) nitrate solution, and a few milliliters of 0.1 M sodium bromide are added. Under these conditions the tracer is almost completely precipitated but the student should check this by assaying the supernatant solution. The thallium(1) bromide precipitate is washed with 0.01 M sodium bromide, centrifuged, and the supernatant solution discarded, T~~milliliters of wateris added to the 1 Present addreas; Hanford Laboratories, General Electric Go., Richland, Washington.
34
/
Journol of Chemical Education
centrifuge tube which is then heated to about 80°C for a few minutes to dissolve some of the precipitate. The centrifuge tube containing the precipitate is placed in a constant temperature bath at 2j°C and equilibrium is approached by the reprecipitation of the dissolved thallium(1) bromide. After 30 min the centrifuge tube is removed from the bath, centrifuged, and an aliquot of the supernatant solution removed and evaporated on a planchet under a heat lamp. This assay is then counted to determine the thallium204 content of the solution. The sampling is repeated a t 30-min intervals to determine whether or not equilibrium has been attained. Generally, equilibrium is reached in about an hour and a half. In order to determine the solubility of thallium(1) bromide, the student must first calculate the specific activity from a knowledge of the quantit~of tracer added to the weighed amount of thallium(1) nitrate. The solubility of this salt is then determined from the activity of the supernatant solution. The solubility of thallium(1) bromide found by our students ranged from 0.4 to 0.5 g/ml, and the solubility product ranged from 2.8 X lo-= to 3.8 X (Lit. value = 3.6 X 10-8.2) The solubility as a function of bromide ion concentration and ionic strength may also be determined. In this way the common ion effect and complex ion formation38 can be investigated. A demonstration experiment of the common ion effect on the solubility of thallium(1) chloride5 has been described. LATIMER, W. M., "Oxidation Potentials," 2nd ed., PrenticeHall,Inc.,NewYork, 952, p, 164, See for example: Hu, K. H., AND SCOTT, A. B., J . Am. Chem. SOC.,77,1380(1955). a SCOTT, A. B.,DAETAU,R., AND SAPSOONTHORN, S., Abstracts, Northwestern Regional Meeting, American Chemical Society, June 16,17 (1960). a ROIG,E., RIEKEHOFF, I. G., RUSSO,C. S., CURET,J. D., J. CAEM.EDUC.,38,350 (1961).
