Membranes and Commercial Ion Selective Membrane Electrodes:

A huge range of different types of membranes are used in ion selective membrane electrodes, including glass, solid state, heterogenous and liquid ion exchanger based. Table gives a selection of commercial ion selective membrane electrodes in which all the four types are used.

Table:  Typical Membranes and Commercial Ion Selective Membrane Electrodes

Various types of glasses have been used as membrane electrodes in ion selective membrane electrodes. When these electrodes were developed originally, they were primarily used in measuring pH values. Therefore, the scope of application has been widened to involve several univalent cations such as sodium, potassium, lithium, ammonium, silver, rubidium, and caesium. Glass membranes are generally based on Na₂O-Al₂O₃-SiO₂ mixtures. Membranes selective to hydrogen ions are rich within SiO₂ whereas membranes selective to alkaline metal ions have a higher content of Al₂O₃.

A huge variety of various solid state membranes are used for different ion detection. Several of these use a combination of Ag₂S + AgX (where X can be chloride, bromide, or thiocyanide) in the form of a pressed disc and the electrode responds to X^-. For the determination of cations (Mⁿ⁺), the membrane material is a mixture of Ag₂S + M_n/2S.

Heterogenous membranes use same active components as solid state devices but on the other hand the active material is deposited within the pores of an inert support like silicone rubber or polyvinyl chloride plastic.

Ion selective electrodes based on liquid ion exchanger membranes immobilize an active species, (an organic molecule dissolved in a solvent), into the pores of an inert polymer. The classical active species are phosphate di esters for calcium ion detection, metel complexes for anion detection and neutral macrocyclic crown ethers for alkali metal detection.