Calomel electrode

A calomel electrode ( popularly called Saturated calomel electrode or SCE) is a based on the reaction between elemental mercury and mercury(I) chloride (calomel).

It is used as a reference electrode in many electrochemical experiments since its electric potential is known. It is also sometimes used in pH calculations and cyclic voltammetry.

(Photo credit: dc181.4shared.com)

The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg₂Cl₂, "calomel") is a saturated solution of potassium chloride in water.

In this electrode, the activity of the metal ion is fixed by the solubility of the metal salt.

Thus, the half reaction that occurs inside this electrode can be denoted as follows:

Hg₂ ²⁺ + 2e ↔ 2Hg

(Note that in the above reaction the left-right arrow represents that this reaction is under equilibrium conditions or that both the forward and the reverse reactions are in equilibrium)

Therefore, the final reaction inside a calomel electrode can be denoted as follows:

Hg₂Cl₂ + 2e ↔ 2Hg + 2Cl^-

The cell notation for the electrode is written as:

Cl^- (4M) | Hg₂Cl₂(s) | Hg(l) | Pt
The saturated calomel electrode may get clogged if the solution dries out.

The redox potential of the SCE is about +0.2444V as measured from the standard hydrogen electrode. This reduction potential depends upon the saturation percentage of the chloride solution ( potential increases when the saturation percentage is lowered).

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