Electric mass transport of sodium chloride through cation-exchange membrane MK-40 in dilute sodium chloride solutions: A rotating membrane disk study

The polarization properties of an electromembrane system consisting of an MK-40 membrane and a dilute sodium chloride solution are investigated with an experimental apparatus, which includes a rotating membrane disk with a horizontally positioned membrane. For the electrochemical systems of MK-40/0.01 M NaCl and MK-40/0.001 M NaCl, effective ion transport numbers and partial current-voltage curves are determined for sodium and hydrogen ions, and limiting-current densities and the diffusion-layer thickness are calculated as functions of the rotation rate of the membrane disk. The space-charge distribution in the diffusion layer and in the membrane is calculated for various current densities and rotation rates of the membrane. It is shown that when electric-current densities are greater than the limiting value, ion fluxes of the salt increase as a result of a decrease in the effective thickness of the diffusion layer. This decrease is caused by the development of space charge, electroconvection, water dissociation, and the exaltation effect in the region near the membrane. It has been established that in dilute solutions the limiting current is not purely electrodiffusive in nature.