Indicator Electrodes:

An indicator electrode of a cell is one whose potential is dependent on the activity of a particular species whose concentration is to the determined. Several metals like silver, copper, mercury, lead and cadmium that show reversible half reactions are used for the construction of indicator electrodes. The potentials established are reversible and reflect the activities of these ions in solution.

Between the metals; some metals like nickel, cobalt, tungsten do not give reproducible potentials because of some chemical or physical reactions and hence are not used as indicator electrodes.

Usually these metal electrodes are constructed from coils of wire, flat metal, metal plates or button set in plastic or glass. Large surface area is preferable because of rapid attainment of equilibrium. The Metal surface is to be thoroughly cleaned. Usually it is achieved by dipping in concentrated nitric acid followed by repeated rinsing with distilled water.

Figure: Typical indicator electrode designs

In addition to the above where the metal electrode is used as an indicator electrode for its own cations, that could also be used indirectly; where it becomes responsive to anions that form slightly soluble precipitates with cations of the metal. For this application it is necessary to saturate the solution under study with the sparingly soluble salt, e.g. the potential of a silver electrode will accurately reflect the concentration of iodide ion in a solution that is saturated with silver iodide.