Polarization at ion-exchange membranes in electrodialysis

In an attempt to investigate polarization phenomena (including substantial participation of H⁺ and OH^- in conduction) which seriously limit the production rates of electrodialysis units, current-voltage curves were measured in different sections along the tortuous flow path of an electrodialysis apparatus containing a single anion-exchange membrane of size 20″ × 18″. The electrical potential drops across this membrane and the adjacent solution layers were measured with silver electrodes terminating in exposed chloridized tips, placed in the two flow compartments adjacent to the anion-exchange membrane. While the measured current- voltage curves exhibit true limiting-current plateaus only in the last sections, all sections yield plateau currents after correction for the measured variation of coulombic efficiency. Moreover, these corrected curves are in fair agreement with those computed from the solution of the Nernst-Planck equations of ion migration, as developed in this paper to take due account of the variation of salt concentration along the flow path.