Ion Selective Electrodes / Glass Electrode

Glass Electrodes make use of the ion-selective properties of glass and are used today in pH measurements.

The indicator electrode consists of a thin glass membrane that is contained inside a thick-walled glass or plastic tube. Inside the tube, there exists a solution of HCl (0.1M) satured with AgCl together with a silver wire. This is an internal Ag/AgCl reference electrode. The indicator electrode is connect with another refenrece electrode (the external one) and thus the cell is complete.

Before using the glass electrode, one must assure that it is hydrated. During hydration, single charged cations that are loosely placed in the glass lattice are overwhelmingly exchanged by hydrogen cations.

The hydrated glass interface is conductive due to the presence of hydrogen cations.

In each interface of the membrane an equilibrium is established:

A potential difference is again caused due to the difference in ionization.

The equilibrium's position depends on the concentration of $H^{+}$ in each interface.