Standard Potential of Ion-Sensors

Abstract

The potentiometric measurement of ions belongs to the most frequently applied electroanalytical methods. It is owing to the need of routine determinations of the main blood electrolytes (Na+, K+, Cl-, and HCO3 -), pH and PCO2. Growing demand for improved reliability of the sensors response and manageable response time is associated with their fundamental properties characterized by sensitivity, selectivity, detection limit and standard potential [1]. The latter should be preferably stable over certain time, and in this way not to be a source of analytical error.The interpretation of the standard (resp. formal) potential is often ignored owing to the application of internal electrode solution, i.e. symmetric ion-sensor systems. However, the present wave in ion-sensor technology typically utilizes all-solid-state sensor architecture, in which the internal liquid contact is substituted by a solid contact, with resulting assymetricion-sensors. This type of contact was first used for ion-selective electrodes with a solid-state (crystalline) membrane, and made by platinum, silver, carbon. More recently, conducting polymer as a solid-contact material for the ion-selective electrodes with plastic membranes was proposed [2]. Very recently the application of nanostructured materials was offered [3]. Beyond any doubt, the understanding of the reason(s) of standard potential stability of the asymmetric solid-contact ion-sensors is of a primary importance, but rarely undertaken. Thermodynamic interpretation the standard potential of the asymmetric all-solid-state ion-selective sensors will be presented. This interpretation is related to the fundamental concepts metallic electrodes and all-solid-state electrodes presented by Trasatti [4] and Buck & Koebel [5], and extended by Lewenstam [6] for the solid contacts made of conducting polymers. The applicability of the interpretation for the whole family of all-solid-state ion-selective sensors will be discussed.