The Calomel Electrode

A diagram of the calomel electrode is shown in figure 9. It consists or an outer glass tube fitted with a frit at the bottom to permit electrical contact with the outside solution. In side there is another tube, the bottom of which is packed with glass wool to allow further electrical connection between the contents of the inner tube and the contents of the outer tube.

The inner tube is packed with a paste of mercury and mercurous chloride dispersed in a saturated solution of potassium chloride represented by,

                               Hg|Hg₂Cl₂KCl (xM)  saturated

And the reaction for the electrode will be,

Hg₂Cl₂  +  2e Hg₂Cl₂    2Hg + 2Cl^–

Figure 9.   The Calomel Electrode

The electrode potential will depend on the concentration of the potassium chloride and, thus, the electrode potential must be reported together with the potassium chloride concentration. Thus, for the saturated calomel electrode the common reference voltage is +0.244 V.

The calomel electrode operating temperature is restricted to below 80^oC due to the mercurous chloride disproportioning into mercury and mercuric chloride at higher temperatures. Unfortunately due to its design and the thermal characteristics of the materials from which it is made, it’s rate of progress towards equilibrium after a temperature change is rather slow.