Concentration polarization of ion-exchange membranes in electrodialysis: An interferometric study

The three-frequency laser interferometry method is used for studying the concentration polarization of ion-exchange membranes in a wide range of current densities. The concentration acid temperature fields of solutions involved in electrodialysis of sodium chloride are measured simultaneously. The proposed method makes it possible to measure absolute values of local concentrations of acids and bases that form during irreversible dissociation of water molecules at the solution-membrane interfaces following an increase in the current density above the limiting value. According to an analysis of the concentration and temperature distributions in an electrodialyzer channel, maximum variations in the measured quantities occur near the interface. Dependences of the near-membrane concentrations and temperatures on the current density are compared with the voltammogram. The comparison shows that reaching the limiting current density sharply accelerates the nonequilibrium dissociation of water molecules and increases the temperature.