kalomelektrodi

Kalomelektrodi, or calomel electrode, is a reference electrode used in electrochemical cells. It consists of mercurous chloride (Hg2Cl2) in contact with elemental mercury, immersed in an aqueous saturated potassium chloride (KCl) solution, and housed in a glass body with a porous junction to the sample solution. An external conductor connects the electrode to the rest of the circuit.

The electrode potential is set by the calomel/mercury redox couple. The half-reaction is Hg2Cl2(s) + 2 e− ⇌ 2 Hg(l) + 2 Cl−. The potential is primarily determined by the chloride ion activity in the internal filling; at 25 °C a saturated calomel electrode has a standard potential of about +0.244 V relative to the standard hydrogen electrode. The exact value varies with temperature and the chloride concentration of the filling solution.

Construction and operation: The device contains a mercury pool in contact with a solid or paste of Hg2Cl2, connected to a porous plug wetted by saturated KCl. The plug provides ionic contact with the test solution, while an external wire transmits the potential. Modern variants may use different materials for the body or filling, but the essential principle remains the same: a stable, well-defined potential determined by the Hg2Cl2/Cl− couple in saturated KCl.

Applications: Kalomelektrodi has been widely used as a reference electrode in potentiometry, voltammetry, and pH measurements in combination with other working electrodes. It offers low drift and good stability, but contains mercury and requires proper handling and disposal. In many current instruments, it is increasingly replaced by Ag/AgCl reference electrodes due to toxicity concerns and comparable performance, though it remains in use where historical data comparability or specific calibration is important.