The Theory of Radioactive Ion Exchange

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Abstract
Experiments reported by other workers have shown that when a precipitate of silver bromide is shaken with a solution containing either silver or bromide ions, some of which are radioactive, the radioactive strength of the solution decreases. The final value reached corresponds to a uniform distribution of the active material throughout both solid and solution. No agreement on the interpretation of these results has up until now been reached. Two possible mechanisms for this exchange are described, and their consequences are examined. A new theoretical treatment is given which includes a consideration of the effect of the spread in crystal sizes, which was neglected in previous discussions of the problem. From those results, a new method of comparison between theory and experiment is suggested. This shows unambiguously that the exchange of silver ions is in agreement with predictions based on the hypothesis that the ions diffuse through the solid. It is shown that the exchange of bromide ions cannot be explained in this way. Values obtained for the diffusion constant of silver are in fair agreement with those from other sources. The relation of these results to the various suggestions put forward by other workers is discussed.