Membrane Response Model for Ion-Selective Electrodes Operated by Controlled-Potential Thin-Layer Coulometry

Abstract

The electrochemical response behavior of controlled-potential thin-layer coulometric sensors based on solvent polymeric membranes doped with ionophores is elucidated by numerical simulation. This treatment forms the theoretical basis for the design of potentially recalibration-free ion-selective chemical sensors that operate by exhaustive coulometry. Mass transport is assumed to occur primarily by diffusion in each bulk phase, and interfacial ion exchange with interfering ions is described with modern ion-selective electrode theory. The ion-selective membrane is assumed to contain an ion exchanger that can form concentration gradients as a result of transmembrane ion fluxes. It is shown that the diffusion of ions in the membrane phase will become rate limiting for membrane components with diffusion coefficients of 10(-8) cm(2) s(-1) that are typical for traditional ion-selective electrode formulations. This characteristic may be advantageous for sample thicknesses of 20 μm or less, where otherwise exhaustive depletion occurs too quickly to be distinguishable from nonfaradic processes. In most other cases, however, it will be necessary to formulate membrane materials that permit much faster diffusion characteristics. Indeed, the simulations give guidance on sensor design for sample concentrations that approach millimolar levels. The treatment also considers interferences from ions of the same charge sign as the analyte ion, and it is shown that the required selectivity for a given analysis must be about 1 order of magnitude higher than in direct potentiometry. This is because the coulometric exhaustive depletion process occurs only for the analyte ion, not for the interfering one, and to avoid interference, the required selectivity must be maintained even if the sample contains a fraction of the original analyte levels.