IONselective

Ionselective refers to sensors or membranes designed to respond predominantly to a specific ion in a mixture. In electrochemistry, ion-selective electrodes (ISEs) are potentiometric sensors that produce a potential determined by the activity of a target ion in solution, ideally independent of other ions.

Most ISEs use a membrane that contains an ionophore—an ion-specific receptor—embedded in a polymer matrix such as poly(vinyl chloride) with a plasticizer. The membrane forms an interface with the sample and, together with a reference electrode, yields a measurable potential according to the Nernst equation. For monovalent ions the near-Nernstian slope at 25 C is about 59 mV per decade change in ion activity; for divalent ions the slope is about 29.6 mV per decade.

Selectivity for the target ion relative to interfering ions is described by selectivity coefficients, often derived from the Nicolsky-Eisenman equation. The electrode response is influenced by temperature, pH, ionic strength, and membrane composition. Types of ISEs include cation-selective (e.g., Na+, K+, Ca2+) and anion-selective electrodes, as well as solid-contact or coated-wire and liquid-contact configurations.

Applications span clinical diagnostics (electrolyte analysis), environmental monitoring (water quality), agriculture (soil nutrient assessment), and industrial process control. While highly selective, ISEs require careful calibration and maintenance to account for drift and interferences.