COUNTERION TRANSPORT NUMBERS OF POLY(ACRYLIC ACID) GRAFTED POROUS ION-EXCHANGE MEMBRANES AS DETERMINED FROM CURRENT STEP MEASUREMENTS

The effect of an electric current on the concentration polarization of the external bathing solutions and the permselectivity of poly(acryli c acid)-grafted porous ion-exchange membranes has been studied, The ex perimental approach is based on the transient behavior of the total el ectric potential drop through the membrane cell when a current step is imposed from external nonpolarizable electrodes. When this voltage dr op is recorded as a function of time, a transition time characteristic of each membrane system is obtained. From this time, the counterion t ransport number for; the salt solution (KCl-H2O) in the membrane can b e obtained. The theoretical modeling is based on the time-dependent Ne rnst-Planck equations. It is shown that the transport number, and then the membrane permselectivity, decreases with the electric current. Th e higher the membrane grafting ratio and the lower the external salt c oncentration the larger the permselectivity changes. (C) 1997 Elsevier Science Ltd.