Saturated calomel electrode « Education Resources

April 21, 2006

Saturated calomel electrode

Filed under: Chemistry Terms — @ 8:55 am

Saturated calomel electrode

The Saturated calomel electrode (SCE) is a reference electrode which uses the reaction between mercury metal and mercury(I) chloride, to fix its potential. 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.

The Nernst equation is

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

As the mercury concentration can be worked out from the solubility product

K = [C l - ]2[H g 2 + + ]

Then as [Cl-] is fixed by the solubility of the potassium chloride then

[H g 2 + + ] = K / [C l - ]2

As the activity of a solid or liquid which is imissible is 1, also two electrons are transferred per Hg₂ dication the Nernst equation becomes

E = E_Hg2++/Hg - (RT/2F) ln (K / [Cl-]2)

This electrode and the silver/silver chloride reference electrode work in the same way, for both electrodes the redox active metal chloride has a very low solubility and is fixed by the chloride solution.

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