ABSTRACT
A survey is given of the kind of information which can be drawn from electromotive force studies of electrolyte concentration cells with membranes as separators. The theoretical analysis spans from the most general formalism of irreversible thermodynamics (in practice only useful in the case of two kinds of ions only) to a generalized Nernst-PlanckDonnan treatment able to yield approximate information about ratios of ion diffusion coefficients and Nernst distribution coefficients. The methodologies are exemplified with studies of weak ion exchange or nonionic membranes. Dense and asymmetric cellulose acetate membranes are examples of the first kind of membranes, since these membranes contain a small "fixed charge" of dissociated glucuronic acid groups. It is important to realize that such membranes inserted between two aqueous solutions of, e.g., NaCl cannot, in general, be treated as a two ion + membrane system, since either H+ or OH~ or both ions participate in the diffusion process and in the Donnan equilibrium process. Also, the glucuronic acid groups may be neutralized by protons or bind divalent ions, so that the "fixed charge" changes sign. The redissociation of such bound, divalent ions is often an extended kinetic process with relaxation times of the order of weeks, depending on temperature and ionic strength. In the case where the influence of the "third ion" can be neglected, general irreversible thermodynamics leads to a method for the characterization not only of the "mean transport numbers" of the membrane but also of the "surface transport numbers" which are generally different for the two sides of an asymmetric membrane. The difficulties of measuring correctly such surface transport numbers are discussed by a critical evaluation of recent experimental studies of this kind. Completely new measurements on a strongly asymmetric, supported polyether sulfone membrane are presented. In one series of experiments, three different kinds of measuring cells have been used with NaCl as the electrolyte in order to check any influence of "unstirred layers" on surface and mean transport numbers. In another set of experiments, the surface and
mean transport numbers of Ba2+ in BaCl2 have been measured. The different concentration dependence of the two surface transport numbers reflects the asymmetry of the membrane.
