Other than the standard hydrogen electrode (SHE), two other standard or reference electrodes are widely used: the calomel electrode and the silver chloride electrode. Each of these electrodes, like the SHE, provides a constant potential that is insensitive to the electrolyte.
Silver chloride electrode
The silver chloride reference electrode consists of a AgCl coated silver wire that is dipped in a saturated KCl solution (also saturated with AgCl). The porous frit, which allows for the slow passage of ions, forms a liquid junction with the test solution. The electrode potential of 0.199 V versus SHE at rtp is given by the following half-cell reaction:
Just as the constant bubbling of H2 in a H+ (1 M) electrolyte of the SHE half-cell provides the electrode with a constant potential, the Ag wire and the equilibrium between AgCl coated on the wire and the saturated AgCl internal solution ensure that the AgCl electrode maintains a constant potential. Some literature quotes the potential vs SHE as 0.22 V at rtp. This value corresponds to a KCl concentration of 1.0 M, whereas 0.199 V is measured when KCl is saturated.
The calomel electrode
The calomel reference electrode consists of mercury in contact with a saturated solution of Hg2Cl2 (calomel), which is in turn in contact with saturated KCl. The electrode potential of 0.244 V versus SHE at rtp is given by the following half-cell reaction:
Both AgCl and calomel reference electrodes are used for a wide range of electrochemical measurements. However, the toxicity of mercury in the calomel electrode poses health and environmental issues.
