Calomel Electrode
A reference electrode based on the equilibrium between mercury, mercury(I) chloride and chloride ions.
Formula / equation
Hg₂Cl₂ + 2e⁻ ⇌ 2Hg + 2Cl⁻
Unit
V
In depth
Its potential is fixed by the chloride concentration, so a saturated potassium chloride solution gives a highly reproducible +0.242 V at 25 °C. Robust and inexpensive, it was the standard laboratory reference for decades, though mercury toxicity has since limited its use.
Examples in the real world
Pairing a calomel electrode with a glass electrode in a traditional pH meter.