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

摘要

The electrochemical response behavior of controlled-npotential thin-layer coulometric sensors based on solventnpolymeric membranes doped with ionophores is eluci-ndated by numerical simulation. This treatment forms thentheoretical basis for the design of potentially recalibration-nfree ion-selective chemical sensors that operate by ex-nhaustive coulometry. Mass transport is assumed to occurnprimarily by diffusion in each bulk phase, and interfacialnion exchange with interfering ions is described withnmodern ion-selective electrode theory. The ion-selectivenmembrane is assumed to contain an ion exchanger thatncan form concentration gradients as a result of transmem-nbrane ion fluxes. It is shown that the diffusion of ions innthe membrane phase will become rate limiting for mem-nbrane components with diffusion coefficients of 10 8 cm2ns 1 that are typical for traditional ion-selective electrodenformulations. This characteristic may be advantageousnfor sample thicknesses of 20 µm or less, wherenotherwise exhaustive depletion occurs too quickly tonbe distinguishable from nonfaradic processes. In mostnother cases, however, it will be necessary to formulatenmembrane materials that permit much faster diffusionncharacteristics. Indeed, the simulations give guidancenon sensor design for sample concentrations that ap-nproach millimolar levels. The treatment also considersninterferences from ions of the same charge sign as thenanalyte ion, and it is shown that the required selectivitynfor a given analysis must be about 1 order of magnitudenhigher than in direct potentiometry. This is becausenthe coulometric exhaustive depletion process occursnonly for the analyte ion, not for the interfering one, andnto avoid interference, the required selectivity must benmaintained even if the sample contains a fraction ofnthe original analyte levels.