Other Solid-State Ion-Selective Electrodes

Other types of solid-state membranes include single crystals of sparsely soluble salts and are often called heterogeneous membranes, in which the insoluble salt is embedded in some inert polymer matrix. Obviously, in order for these membranes to be at equilibrium they should be in a saturated solution. In practice, these membranes are used in solutions that are below saturation. In that case, the insoluble salt slowly dissolves. 

The key common property of such membranes is their ionic conductivity. An example is the Ag/AgCl electrode discussed previously. Although the primary charge-transfer reaction at this interface is that of the silver ion (6.27), the electrode will respond also to chloride ion (6.33), because of the low solubility product of AgCl. Let us now consider what happens if other ions that also form an insoluble silver salt are present in the solution. If the solubility product of the other salt (AgX) is lower than that of silver chloride and/or the activity ax is sufficiently high 

The membrane undergoes metathesis and its surface becomes gradually partially covered with the solid phase of AgX. Clearly, this is a nonequilibrium situation that leads to formation of mixed potential. It also underscores the nonequilibrium 

Primary ion Orion electrode3 Homogeneous membranes 3 Heterogeneous membranes0 

Let us consider mixed PbS-Ag2S membrane, for which the respective solubility 

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