IMPEDANCE OF A POROUS ION-EXCHANGE MEMBRANE

A model for a porous membrane with ion selective properties is present ed, and the impedance based on the model is theoretically calculated o ver the whole practical frequency region. The semi-circle usually obta ined in the measurements at high frequencies is explained by a transmi ssion line model which includes both the ohmic and ion-exchange resist ance along with the capacitance of the membrane pore. In the case of h igh porosity, the mass transfer impedance consists of a parallel combi nation of a semi-infinite diffusion element, i.e., a Warburg impedance , and a limited diffusion element. In the case of low porosity, the me mbrane forms an array of microholes, and spherical transport into the pores has to be considered. This brings an additional resistive elemen t into the equivalent circuit, and thus, a clear distinction between t he two cases can be found.