1. An electrode that develops a standard electric potential.

The calomel electrode is used as a standard in determining the pH (potential of hydrogen) of fluids which refers to a measure of acidity or alkalinity in which the pH of pure water is 7, with lower numbers indicating acidity and higher numbers indicating alkalinity.

2. An electrode consisting of mercury surrounded by a suspension of calomel (a colorless, white or brown tasteless compound, used as a fungicide or insecticide; formerly, as a purgative for purging the bowels; especially, as a laxative in a solution of potassium chloride of a specified strength.

The calomel electrode gives a highly reproducible potential, and is used as a standard, often as a half cell with a glass electrode for determining pH, or with a platinum electrode for establishing redox (oxidation-reduction) potentials or any chemical reactions in which electrons are transferred.

One of two practical reference electrodes, used with a mercurous chloride (calomel) paste in pH (measure of the acidity or alkalinity of a solution) and other potentiometric (voltage-measuring) instruments. The other is the silver/silver chloride electrode.

The calomel (mercury compound) electrode has been the standard secondary reference electrode used in the laboratory since the introduction of the pH electrode.