Calomel electrode
This consists of a platinum wire dipping into mercury which is itself in contact with calomel (a salt with the formula Hg2Cl2, practically insoluble in mercury) and with an aqueous saturated solution of potassium chloride.
This electrode, denoted as Hg2Cl2 / Hg , is used as secondary reference electrode .
When the solution of KCl is saturated, its potential at 25°C is 0.2444 V compared to the standard hydrogen electrode.