Kalomelelektrode

KalomelElektrode, commonly known as the calomel electrode, is a reference electrode used in electrochemistry. It consists of mercurous chloride (Hg2Cl2) in contact with a saturated potassium chloride (KCl) electrolyte and is housed in a body that allows ionic contact with the test solution through a porous junction. The design provides a stable and well-defined potential that serves as a reference for measuring the electrode potential of other half-cells.

Construction and operation involve an inner chamber containing a paste or solid pellet of Hg2Cl2 in saturated KCl, connected by a conductor to an external circuit. The outer portion, often a glass or ceramic housing, ends in a porous plug or frit that permits the escape and entry of chloride-containing solution while limiting mixing of the calomel electrolyte with the test solution. The electrode potential arises from the redox couple Hg2Cl2/ Hg and Cl-, described by the reaction Hg2Cl2 + 2 e- ⇌ 2 Hg + 2 Cl-.

In standard practice, the standard calomel electrode (SCE) has a defined potential of about +0.241 V versus the standard hydrogen electrode at 25°C when using saturated KCl. This potential is relatively stable and temperature-dependent, providing reliable reference behavior across many electrochemical measurements. However, the exact value can shift with temperature and chloride activity.

Applications primarily include serving as a reference electrode in potentiometric measurements, electrochemical cells, and pH instrumentation. Advantages include a stable, low-drift potential and compatibility with many electrolytes. Limitations involve the use of mercury, requiring careful handling and disposal, and potential drift with temperature or chloride changes. In modern laboratories, alternatives such as silver/silver chloride reference electrodes are sometimes preferred for environmental and safety considerations.