A reversible electrochemical cell or a galvanic cell consists of two reversible half cells or electrodes. There are four types of reversible electrodes according to their compositions.
(1) Metal-metal ion electrode :
This electrode is set up by dipping a metal in a solution containing its own ions, e.g. Zn rod dipped into ZnSO4 solution containing Zn++ ions of concentration C.
It is represented as,
\(Zn^{2+}{_{(aq)}}|Zn_{(s)}\)
The reduction reaction at the electrode is,
(2) Metal-sparingly soluble salt electrode :
This electrode consists of a metal coated with one of its sparingly soluble salts and immersed in a solution containing an electrolyte having a common anion as that of the salt.
For example,
Silver electrode coated with sparingly soluble AgCl dipped in KCl solution with common anion Cl-. This electrode is represented as,
(3) Gas electrode :
This is developed by bubbling pure and dry gas around a platinised platinum foil dipped in the solution containing ions (of the gas) reversible with respect to the gas bubbled.
The gas is adsorbed on the surface of platinum foil and establishes an equilibrium with its ions in the solution. Pt electrode provides electrical contact and also acts as a catalyst.
Some of the gas electrodes are represented as follows :
(4) Redox electrode (Oxidation reduction electrode) :
This electrode consists of a platinum wire dipped in a solution containing the ions of the same metal (or a substance) in two different oxidation states, like \(Fe^{2+} - Fe^{3+}, Sn^{2+} - Sn^{4+},\) \(Mn^{++} - MnO^-{_4},\) etc.
A platinum electrode which provides an electrical contact and acts as catalyst aquires an equilibrium between two ions in the solution, due to their tendency to undergo a change from one oxidation state to another. The electrodes are represented as,
