Anomalous potentiometric response of solid-contact ion-selective electrodes with thin-layer membranes

Abstract

In this work, the potentiometric response of solid-contact ion-selective electrodes (SCISEs) with thin-layer ion-selective membranes (ISMs) was characterized. Thin-layer potassium ISMs were deposited on the solid contact by spin-coating. When calibrating the solid-contact potassium ion-selective electrodes (K-SCISEs) at a constant background concentration of 0.1 M NaCl, an anomalous increase in the potential was observed at low concentrations of the primary potassium ion. The origin of the unusual response was elucidated in this work by varying the solid-contact material based on conducting polymers with different lipophilicity and by varying the composition and thickness of the ISM as well as the timescale of the potentiometric measurements. The composition of the ISM was found to have only a minor effect, while the thickness of the ISM, the lipophilicity of the solid-contact material and the timescale of the potentiometric measurement were found to strongly influence the potentiometric behavior. Furthermore, the potentiometric response of classical K-ISEs with inner filling solution did not show any unexpected behavior. It was concluded that the anomalous potentiometric response is a characteristic feature of SCISEs with thin-layer ISMs and originates from a relatively rapid change of the composition of the water layer between the ISM and the solid contact. Thus, the reported anomalous behavior of SCISEs with thin-layer ISMs, together with the established water layer test, can be used as a diagnostic tool indicating the presence of water between the ISM and the solid contact.