Ionenselektrode

Ionenselektrode, commonly known as an ion-selective electrode (ISE), is a chemical sensor that converts the activity of a specific ion in solution into an electrical potential. It usually comprises a sensing element featuring a membrane selective for the target ion and a reference electrode; the potential difference is measured with a high-impedance voltmeter or potentiometer.

The electrode operates potentiometrically: the membrane develops a potential due to selective interaction or transport of the target ion. According to the Nernst equation, E = E0 + (RT/zF) log a, where a is the activity of the ion. At 25°C, the ideal slope is about 59 mV per decade of activity for monovalent ions (z = 1); for divalent ions the slope is about 29.6 mV per decade, reflecting the ion’s charge.

Common membrane types include glass for hydrogen ions (pH measurement) and polymeric or crystalline membranes containing ionophores tailored to specific ions such as K+, Na+, Ca2+, or Cl−. Configurations include coated-wire, solid-state, and liquid-contact electrodes, with a separate reference electrode completing the electrochemical cell. The sensing element is typically paired with a stable reference, such as Ag/AgCl, to provide a reproducible baseline.

Performance characteristics depend on selectivity against interfering ions, temperature, and membrane integrity. Calibration with standard solutions is necessary, and selectivity coefficients quantify potential interferences. Response times range from seconds to minutes, and practical lifetimes span from months to years with proper maintenance.

Applications span clinical and biomedical analysis, environmental monitoring, water quality assessment, food and beverage analysis, and research settings where rapid, in situ ion measurements are required.