The group of ion-selective electrodes with fixed ion-exchange sites includes systems with various membrane structures. The membranes are either homogeneous (single crystals, pressed pellets, sintered materials) or heterogeneous, set in an inactive skeleton of various polymeric materials. Important electrode materials include silver halides, silver and divalent metal chalcogenides, lanthanum trifluoride and various glassy materials. Here, the latter will be surveyed only briefly, for the sake of completeness.

Ion-selective electrodes for halide ions

Silver halides have the character of solid electrolytes, where the silver ion acts as the charge carrier (see [125, 204, 266] for AgCl) which moves according to the Frenkel mechanism in the crystal. This type of transport is depicted schematically in fig. 6.1. As the halide ions are located in fixed sites, no diffusion potential is formed within the membrane and (3.4.9) to (3.4.13) are valid for the membrane potential. As mentioned in chapter 3, they can be used for determining either halide ions or silver.

Work with silver halide electrodes is complicated by two specific problems; (a) the effect of nonstoichiometric defects on the membrane behaviour and (b) the effect of the character of the internal contact in the all solid-state electrodes. The effect of excess silver [264] appears both in the dependence of the ISE potential on the determinand activity and in the detection limit.