Calomel electrode is the mercury-mercurous chloride electrode. It consists of a glass vessel having a bent side tube. Pure mercury is placed at the bottom of the tube. Which is covered with a paste of mercury- mercurous chloride (Hg+Hg2Cl2) i.e., calomel. The remaining portion of the cell is filled with a solution of normal (1 N) or decinormal (0.1 N) or saturated KCl. A platinum wire sealed into a glass tube is dipped into mercury layer is used to provide the external electrical contact. The side tube is used for making electrical contact with a salt bridge.
The electrode can be represented as
Hg(l)/Hg2Cl2 (s) / Cl–
The calomel electrode can act as anode or cathode depending on the nature of other electrode of the cell.
When it acts anode, the electrode reaction is
2Hg(l) → Hg22+ + 2e-
Hg22+ + 2Cl- → Hg2Cl2
Overall reaction: 2 Hg + 2Cl- → Hg2Cl2 + 2e- (Oxidation reaction)
When it acts as cathode, the electrode reaction is,
Hg22+ + 2e- → 2 Hg
Hg2Cl2 → Hg22+ + 2Cl-
Overall reaction: Hg2Cl2 + 2e- → 2Hg (l) + 2 Cl- [Reduction reaction]