Counterion transport numbers of poly(acrylic acid)-grafted porous ion-exchange membranes as determined from current step measurements

Abstract

The effect of an electric current on the concentration polarization of the external bathing solutions and the permselectivity of poly(acrylic acid)-grafted porous ion-exchange membranes has been studied. The experimental approach is based on the transient behavior of the total electric potential drop through the membrane cell when a current step is imposed from external nonpolarizable electrodes. When this voltage drop is recorded as a function of time, a transition time characteristic of each membrane system is obtained. From this time, the counterion transport number for the salt solution (KClH 2O) in the membrane can be obtained. The theoretical modeling is based on the time-dependent Nernst-Planck equations. It is shown that the transport number, and then the membrane permselectivity, decreases with the electric current. The higher the membrane grafting ratio and the lower the external salt concentration the larger the permselectivity changes.