ION AND PORE FLUID TRANSPORT-PROPERTIES OF A NAFION(R) MEMBRANE SEPARATING 2 ELECTROLYTE-SOLUTIONS .1. KINETICS OF THE PROTON AND ALKALI-METAL CATION-TRANSPORT

A theoretical model is developed to describe the impedance of two elec trolyte solutions separated by a fixed-site ion exchange membrane. The total impedance is the sum of the solution and membrane impedances, e ach of which splits into a resistance and a Warburg impedance owing to the diffusion-migration ion transport in the solution. Kinetics of th e ion transport across the membrane solution interface is taken into a ccount by using Schlogl's theory of membrane permeation. It is shown t hat the pore fluid flow can be responsible for the coupling between th e kinetic and transport impedances. As a result, the Warburg coefficie nt of the membrane impedance is predicted to decrease when the ratio o f the rate of the pore fluid flow (membrane permeability) and the ion transport in the membrane bulk and/or across the membrane\electrolyte interface increases. Impedance analysis can thus provide an insight in to the pore fluid transport properties of an ion exchange membrane. Eq uilibrium impedance measurements are used to analyse the transport pro perties of a Nafion(R) 117 membrane in the presence of alkali metal ca tions and protons. The Warburg coefficient of the membrane impedance d ecreases in the sequence Li+ > Na+ > K+ > Rb (+) approximate to Cs+ > H+ and even becomes negative for last four ions, which indicates an in creasing relative contribution of the pore fluid flow. An evaluation o f the permeability coefficient of the Nafion(R) membrane from the impe dance measurements confirms that in the presence of Na+ or Cs+ ions th e pore diameter is about 5 or 3 times smaller, respectively than in th e presence of protons.