7.1 INTRODUCTION

Potentiometric ion-selective electrodes (ISEs) are one of the most important groupsof chemical sensors. The application of ISEs has evolved to a well-established routine analytical technique in many fields, including clinical and environmental analysis, physiology, and process control. The essential part of ISEs is the ion-selective membrane that is commonly placed between two aqueous phases, i.e., the sample and inner solutions that contain an analyte ion. The membrane may be a glass, a crystalline solid, or a liquid (1). The potential difference across the membrane is measured with two reference electrodes positioned in the respective aqueous phases

reference electrode 2 sample solution membrane inner solution reference electrode 1(cell 1)

Under equilibrium conditions, the measured potential (or emf of the cell), E, can be expressed as

where z _I is the charge of the analyte ion, I, a _I ^W is its activity in the sample solution, and the constant term, E _I ⁰, is unique for the analyte and also includes the sum of the potential differences at all the interfaces other than the membrane/sample solution interface. This well-known "Nernst" equation for ISEs represents their unique response properties, i.e., Nernstian responses, where the sensor signal, E, is proportional to logarithm of the analyte activity rather than the activity itself. The slope in an E versus lna _I ^w(or more commonly loga _I ^w) plot is...