Concentration polarization on ion-exchange membranes in electrodialysis with natural convection: : Estimating interfacial concentrations by measurement of membrane potential decay caused by concentration relaxation after current cut-off

Abstract

Membrane potentials measured (after interruption of the electric current at zero time) with currentless probe electrodes are extrapolated back to zero time according to a t 1 2 relation, consistent with the equation for concentration relaxation in a Nernst diffusion layer (DICET method). Applying the laws of natural convection, the limiting current density value can be estimated by extrapolation of interfacial concentration/current density plots to zero interfacial concentration. When applying the method to a system with suitably selected unequal bulk concentrations on either side of the membrane (EQUI method), it allows determination of the current density at which the interfacial electrolyte concentrations are identical on both sides of the membrane. Under these conditions osmosis and electrolyte diffusion in the membrane are eliminated and Donnan potentials cancel out. This could facilitate a separate study of ionic and electro-osmotic transport in the membrane.