An electrode half-cell, with a membrane that is permeable only to a single ion species. The electrical potential measured between this and a reference half-cell such as a calomel electrode is the Nernst potential for the ion concerned and the activity (rather than concentration) of the ion in the unknown solution can be measured if the ionic content of the solution used to fill the electrode is known. Ion-selective electrodes often have a hydrophobic membrane incorporating an ionophore such as valinomycin or monensin. Proton-permeable glass is used in pH electrodes.
Subjects: Chemistry — Medicine and Health.