Ionenselektiv

Ionenselektiv, or ion-selective, refers to a property or devices that respond preferentially to a specific ion within a mixture. In electrochemistry and analytical chemistry, this term is most often applied to ion-selective electrodes (ISEs), which convert the activity of a target ion into an electrical potential.

The operating principle rests on a selective barrier or membrane that discriminates the target ion from others. The measured potential across the membrane, in contact with a reference electrode, is related to the ion activity by the Nernst equation. At room temperature, monovalent ions typically show a near-Nernstian slope of about 59 mV per decade change in activity; divalent ions yield smaller slopes proportional to their charge.

Construction and varieties: An ISE generally comprises a measuring electrode with a selective membrane, an internal reference solution, and a separate reference electrode. Membranes can be liquid-filled or solid-state. Ionophores or charged exchangers embedded in polymeric matrices confer selectivity for specific ions, such as potassium with valinomycin or calcium with calcium ionophores. Glass membranes are common for hydrogen ion (pH) selectivity, while other membranes target Na+, K+, Ca2+, Cl−, and many other ions. Solid-state ISEs using inorganic or carbon-based materials are also widely used.

Selectivity and performance: The selectivity toward the target ion is quantified by selectivity coefficients that describe interference from other ions. Calibration with standards, control of ionic strength, and temperature compensation are essential for reliable measurements. ISEs must be maintained, as membranes can foul or drift over time, affecting response time and lifetime.

Applications and impact: Ion-selective electrodes are central to clinical electrolyte analysis, environmental monitoring, food and beverage testing, and industrial process control, enabling rapid, relatively low-cost, and in-situ ion measurements.