Concentration polarization and water dissociation in ion-exchange membrane electrodialysis. Mechanism of water dissociation

Abstract

A relationship between the current density i and the current efficiency η of H⁺ and OH^– ions generated by dissociation of water above the limiting current density in ion-exchange membrane electrodialysis has been proposed. Generation rates of H⁺ and OH^– ions in the dissociation of water, H₂O [graphic omitted] H⁺+ OH^– are given as the difference between the forward and the reverse reaction rate. The transport of H⁺ and OH^– ions generated is described according to the Nernst–Planck equation. The relationship between i and η is derived by using the conservation law for the generation and the transport of H⁺ and OH^– ions. Comparing the calculated values with experimental ones, it is clear that the water dissociation reaction rate constant increases with increasing i. Such an increase is usually larger for an anion-exchange membrane than for a cation-exchange membrane. An especially remarkable increase is seen for a cation-exchange membrane placed in aqueous MgCl₂, NiCl₂ or CoCl₂ solution. This is due to violent water dissociation generation by cascade (snow slide) splitting of water molecules in the water dissociation layer.