Concentration polarization with monopolar ion exchange membranes: current-voltage curves and water dissociation

Concentration polarization is studied using a commercial anion and cation e xchange membrane. Current-voltage curves show the occurrence of an overlimi ting current. The nature of this overlimiting current is investigated in mo re detail, especially with respect to the contribution of water dissociatio n, pH measurements reveal that water dissociation is more pronounced in cas e of the anion exchange membrane than with the cation exchange membrane whe n the limiting current is exceeded. However, even with the anion exchange m embrane it is found that the contribution of water dissociation is very low and more than 97% of the current is carried by the salt ions. Furthermore measurements are described showing that the membrane permselectivity (co-io n transport) remains constant in the overlimiting region. This means that t he overlimiting current is virtually all carried by the salt counter ions f or the two membranes investigated. (C) 1999 Elsevier Science B.V. All right s reserved.