Calomel Electrode: Structure, Composition, Properties

Calomel Electrode (Saturated)

The Calomel Electrode is a widely used secondary reference electrode. It is robust, easy to maintain, and provides a stable potential.

The electrode assembly consists of a glass tube (cell) designed to hold the necessary components. Key structural elements include:

A main compartment holding the internal elements.
Two side tubes: one is used for filling the saturated Potassium Chloride (KCl) solution, and the other forms a salt bridge to connect to another electrode system.
Pure Mercury (Hg) is placed at the very bottom of the main tube, ensuring reliable electrical connection.
Above the pure mercury is a layer of a paste made from Mercurous Chloride (Hg₂Cl₂, also known as Calomel) mixed with KCl solution.
A Platinum (Pt) wire is sealed into a glass tube and makes contact with the pure mercury, providing an external electrical circuit connection.

The electrode can act as either an anode or a cathode, depending on the relative potential of the coupled electrode.

Secondary Electrode: Its potential is determined relative to the Standard Hydrogen Electrode (SHE).
Concentration Dependent: Its potential is directly affected by the concentration of the KCl solution used.
Standard Potential: The standard electrode potential for the Saturated Calomel Electrode (SCE) is precisely 0.2415V at 25°C.

Depending on its role as anode or cathode, the electrode undergoes the following reactions:

2Hg + 2Cl⁻ → Hg₂Cl₂ + 2e⁻
(Oxidation at the Anode)

Hg₂Cl₂ + 2e⁻ → 2Hg + 2Cl⁻
(Reduction at the Cathode)