The critical component of an ion selective electrode is a membrane that acts to pass the selected ions into the interior of the electrode assembly. These are of two principal types. Crystal membranes consist of a polycrystalline or single crystal
plate. For example, fluoride (F) ion sensors are made from single crystal lanthanum trifluoride (LaF3), doped with europium difluoride (EuF2). The Eu2 ions substitute for La3 in the LaF3 matrix, and each substituted ion is accompanied by a vacancy on the F substructure, to maintain charge neutrality. The large number of vacancies thus generated increases the diffusion coefficient of F in LaF3 enormously. The membrane has a similar permeability to F as the surrounding liquid, and is found to be highly selective for the passage of F ions