Indicator glass electrode:

As described in Figure (a), these electrodes have thin glass membrane fused to the end of a glass or plastic body. The primary body of the electrode holds an internal reference electrode classically Ag/AgCl and is filled along with a solution which is commonly the aqueous HCl of concentration around 1.0 mol dm ^-3. The selectivity coefficient of glass-membrane is such that excellent dissemination against common cationic species is achieved.

(a)                                                    (b)

Figure: Glass electrode: (a) Typical glass electrode consisting of both an indicator glass electrode and a silver/silver chloride reference; (b) illustration showing an ion exchange

The pH electrode responds to hydrogen ions as a result of the thin ion-exchange sites on the surface of a hydrated glass membrane. An electrode consists of a thin layer of glass, typically about 50 µ m thick. Charge is transported across the membrane by sodium or lithium ions within the glass. The membranes are primarily made from Lithia (Lithium oxide) or sodium oxide, and SiO₂, and some amount of Al₂O₃ and B₂O₃ or multi-component glasses whose sensitivity pattern depends on the composition of the glass. A surface layer of the glass consist of silicate group associated with sodium ion (- Si O^- Na⁺) as shown in Figure (b). When this electrode is dipped in water and the sodium ions exchanged with the protons in water.

- Si O^- Na⁺ +   H⁺ → - SiO ^- H⁺ +Na⁺