The saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental mercury and mercury(I) chloride.
The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg2Cl2, "calomel") is a saturated solution of potassium chloride in water.
The only variable in this equation is the activity (or concentration) of the chloride anion.
This gives the SCE a potential of +0.248 V vs. SHE at 20 °C and +0.244 V vs. SHE at 25 °C,[1] but slightly higher when the chloride solution is less than saturated.
The SCE is used in pH measurement, cyclic voltammetry and general aqueous electrochemistry.