Interpretation of the selectivity and detection limit of liquid ion-exchanger electrodes

An equation has been derived which describes the e.m.f. of a liquid ion-exchanger membrane electrode in conditions of low concentration levels of the primary and interfering ions. The equation is based on the assumption that if the external solution contains no excess of ions which may exchange with the organic phase, then the concentration of the exchanger at the interface decreases, and this is responsible for formation of a diffusion layer inside the membrane. Therefore the potential response depends on the initial concentration of the ion-exchanger in the membrane phase, on the thicknesses of the diffusion layer on both sides of the interface, and on the diffusion coefficients of the species in both phases. This equation explains the non-Nernstian behaviour of the electrode in the presence of interferents, as well as the variation of the conditional selectivity coefficients. The parameters mentioned also influence the detection limit of an electrode. The electrode behaviour has been tested in unstirred solutions and in solutions stirred at different rates. Through its influence on the diffusion layer thickness, the stirring also influences the electrode potential and the characteristics of the electrode.