Water dissociation in ion-exchange membrane electrodialysis

When an electrical current larger than the limiting current density passes across an ion-exchange membrane, H+ ions and OH− ions are generated and transported in a water dissociation layer formed between the ion-exchange membrane and the boundary layer. Water dissociation reactions consist of a forward reaction and a reverse reaction. The forward reaction rate increases along with the increase of electrical potential difference in the water dissociation layer. From this, we introduced equations expressing pH changes in the concentrating and desalting side of the membrane; electrical current efficiency and the forward reaction rate constant of the water dissociation reaction; thickness of the water dissociation layer; and concentration distribution of H+ and OH− ions, electrical resistance, electrical potential difference and electrical potential gradient in the water dissociation layer. The reasonability of values calculated using these equations was confirmed by electrodialysis of several electrolyte solutions. The intensity of water dissociation on an anion-exchange membrane was generally greater than on a cation-exchange membrane. This phenomenon is explained by the fact that the hydrophilicity of the anion-exchange membrane is less than that of the cation-exchange membrane. When a cation-exchange membrane was electrodialyzed in a MgCl2 or a NiCl2 solution, violent water dissociation occurred. This phenomenon is due to the hydrophilic effect of Mg(OH)2 or Ni(OH)2 crystals formed in the water dissociation layer.