Electrodialysis procedure:

Considering the above transport processes, ion selective membranes for use in electrodialysis procedure should have a high selectivity for counter ions. The selectivity of ion selective membranes might be expressed in terms of transport number of counterions within the membrane phase ( ) transport number is a measure of the fraction of the total current carried through the counter ion by the membranes. These transport number reduce as the salt concentration of the solution in contact along with the membrane increases. The word perselectivity (P) is also described to characterize the counter ion selectivity of membranes.

P = - t / 1- t

where, t indicates the transport number of the ions in free solution.

The ion selective membranes also required to have a high electrical conductance while in equilibrium with the most dilute solution to be encountered within the procedure. The membranes should allow only a negligible rate of free electrolyte diffusion under the conditions of concentration difference expected within the process. As the concentration difference across the membrane rise, free diffusion of electrolyte also increases.