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

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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.

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