Diffusion of Radiotracer Ions in a Cation-Exchange Membrane

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Abstract
The tracer diffusion coefficients of Na + , Cs+ , Sr2 + , and Br− in a homogeneous cation‐exchange membrane have been measured in the homoionic counterion states over the external solution concentration range 0.005–1.0 equiv/liter and in the and heteroionic counterion states over the whole equivalent fraction range at 0.02 and 0.10 equiv/liter total external concentration. Steady‐ and transient‐state permeation and diffusion techniques have been used. The results show that the resin material is essentially uniform apart from a small volume of material of low crosslinking which forms a labyrinthine network interpenetrating the main structure. This labyrinthine network is important mainly in the diffusion of Br− at low external concentrations. In concentrations above about 0.02 equiv/liter the diffusion coefficient D Br of Br− does not vary greatly either with concentration or with counterion type. D Br is greatest in the Cs+ state and least in the Na+ state. These observations and the way in which D Br varies with the equivalent fraction of the counterions in the heteroionic states can be explained by considering the expected frictional interactions between Br− coions and the counterions and by the influence of the local variations in electric potential on the movements of the ions.

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