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Radioactive tracers can be used in what may be regarded as micro-chemical determinations such as the evaluation of very low solubilities, or very low vapour pressures. Suppose we wish to determine the solubility of barium sulphate, then 139Ba may be selected as a tracer. The activity (A), associated with a given mass of barium sulphate (W) must first of all be determined. A small quantity of the active sulphate is then agitated with water at a known temperature, until equilibrium is reached when the activity (A*) of a given volume (V cm3) of the solution is determined.

An activity A is produced by W g of BaSO4

Therefore, an A* activity is produced by :

Therefore, solubility:

Certain errors may arise in this type of determination. For example, it may be necessary to correct for the self-absorption of the active solid, or to arrange the experimental procedure so that this may be avoided. Care must also be taken to avoid errors due to absorption on apparatus.

In this experiment the solubility of strontium sulphate is determined and at the same time errors likely to arise are investigated.35S (t1/2 = 87 days) will be used. Alternatively 89Sr (t1/2 = 54 days) would also be suitable.