The phase-boundary potential model

The response of ion-selective electrodes (ISEs) can be described on the basis of two different theoretical approaches. On one hand, the phase-boundary model is based on the assumption of local equilibria at the aqueous/organic interface. The phase-boundary model allows the description of all practically relevant cases of steady state and even transient responses with sufficient accuracy. Moreover, it has the advantage of relating simple thermodynamic parameters to the response function of the electrodes and hence allowing an intuitive interpretation of many observed facts. On the other hand, the comprehensive but quite involved dynamic model requires knowledge of mobilities and ion transfer rate constants. It has never been applied to ionophore-based electrodes in its full complexity. Both models were first suggested decades ago but have been recently extended to explain so far poorly understood aspects of ionophore-based ISEs. Due to space restrictions, only the most important original references are given in this paper, which summarizes the major assumptions of the phase-boundary potential model and discusses the usefulness and limits of this approach. Recent applications are discussed towards understanding sensor selectivity, upper and lower detection limits (even when concentration polarizations are relevant), the so-called sandwich membrane method to determine thermodynamic parameters, apparently non-Nernstian responses, potential drifts with solid contact electrodes, polyion sensors, and galvanostatically controlled ion sensors.