Determination of ion-selective electrode characteristics by non-linear curve fitting

A simple practical method of determining potentiometric selectivity coefficients of ion-selective electrodes (ISEs) is described in which electrode characteristics (slope, potentiometric selectivity coefficients and cell constant) can be determined by fitting the experimental data obtained using the fixed interference (FI) method to an appropriate model by non-linear least-squares regression. The proposed method is simple to implement practically, and data processing can be easily achieved through use of the optimisation add-on, Solver, bundled with Microsoft Excel. The flexibility of the method is demonstrated by modelling the response of a valinomycin potassium-selective electrode with the Nikolskii-Eisenman equation and a recently proposed alternative to the Nikolskii-Eisenman equation for cases where the ionic charges on the primary and interfering ions are unequal.