Ion selective electrode
An electrode half cell, with a semi permeable membrane that is permeable only to a single ion. The electrical potential measured between this and a reference half cell (e.g. A calomel electrode) is thus the Nernst potential for the ion. Given that the solution filling the ion selective electrode is known, the activity (rather than concentration) of the ion in the unknown solution can be measured. Commercial ion selective electrodes frequently use a hydrophobic membrane containing an ionophore, such as valinomycin (for potassium) or monensin (for sodium). A ph electrode is made with a thin membrane of ph sensitive (i.e. Proton permeable) glass.