Saturated calomel electrode

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The Saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental mercury and mercury(I) chloride. The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg2Cl2, "calomel") is a saturated solution of potassium chloride in water. The electrode is normally linked via a porous frit to the solution in which the other electrode is immersed. This porous frit is a salt bridge.

In cell notation the electrode is written as: Cl- (1M)|Hg2Cl2(s)|Hg(l)|Pt


[edit] Theory of operation

The Nernst equation is

E = E^0 - \frac{RT}{nF} \ln\frac{a_{\mbox{red}}}{a_{\mbox{ox}}}

The mercury concentration can be calculated from the solubility product

K_{sp} = [Cl-]^2[Hg_2^{++}]

The concentration of Cl- is fixed by the solubility of potassium chloride,

[Hg_2^{++}] = K_{sp}/{[Cl-]^2}

The activity of a solid is 1. Two electrons are transferred per Hg2 dication. The Nernst equation thus becomes

E = E_{Hg_2^{++}/Hg} - \frac{RT}{2F} \ln{[Cl^-]_2}

At standard conditions, the potential of the SCE is +0.244 V versus the SHE.

[edit] Application

The SCE is used in pH measurement, cyclic voltammetry and general aqueous electrochemistry.

This electrode and the silver/silver chloride reference electrode work in the same way. In both electrodes, the activity of the metal ion is fixed by the solubility of the metal salt.

The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in the Ag/AgCl electrode.

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