Potential drifts of solid-contacted ion-selective electrodes due to zero-current ion fluxes through the sensor membrane

A thin aqueous layer is formed between the polymeric ion-selective membrane and an inner gold electrode. Its composition can be altered upon sample ch anges, which leads to drifting potentials. It is shown by theory and experi ment that interfering ions enter the inner layer much faster than primary o nes. Measurement protocols with equal contact times for the sample and the reconditioning solution can therefore not eliminate the effect of changes i n the composition of the inner solution. The formation of an inner aqueous film and corresponding drifts for solid-contacted membrane electrodes can b e avoided by creating a lipophilic self-assembled monolayer on the surface of the inner electrode.