A calomel electrode is a type of reference electrode based on reactions between mercury (I) chloride (calomel) and elemental mercury. These electrodes are commonly used in voltmeters and pH meters.
A good non-polarizable calomel electrode is very robust, which is why it is used by many two-electrode systems where the supporting electrolyte is a non-reactive chloride salt.
The structure of the calomel electrode consists of an outer glass tube with a frit on the bottom that enables electrical contact with a solution outside the electrode. An inner tube is placed in an outer tube. The bottom of the inner tube is made of glass. Wool on the bottom to create electrical contact between the contents of both tubes.
The mercury paste is packed in the innermost tube and the mercury chloride is dispersed in a saturated solution of potassium chloride.
Diagram of a calomel electrode.
The calomel electrode is a type of half-cell in which the electrode is mercury coated with calomel (Hg2Cl2) and the electrolyte is a solution of potassium chloride and saturated calomel.
For the calomel electrode to work, the Hg-Hg 2 Cl 2 pair must react, either gaining or losing electrons.
When the reduction or gain of electrons occurs inside the calomel electrode, we have the following reactions:
Hg 2 Cl 2 → Hg 2 2+ + 2Cl – (Ionization)
Hg 2 2+ + 2e – → 2Hg (Reduction)
Hg 2 Cl 2 + 2e – → 2Hg + 2Cl – (Net reaction)
Therefore, Hg 2 Cl 2 gains electrons reducing to metallic mercury.
The potential E of the electrode when the reduction occurs is given by the equation:
E = Eº – 0.0591 Log [Cl – ]
Where it is observed that E depends exclusively on the concentration of Cl – ions , E ° being the standard reduction potential for this electrode measured against the standard hydrogen electrode.
Inside the electrode an oxidation process can also occur:
2Hg → Hg 2 2+ + 2e – (Oxidation)
Hg 2 2+ + 2Cl – → Hg 2 Cl 2 (Precipitation)
2Hg + 2Cl – → Hg 2 Cl 2 + 2e – (Net reaction)
That is, the mercury is oxidized to generate more Hg 2 Cl 2 .
The potential E in this case is given by:
E = Eº + 0.0591 Log [Cl – ]
And again, E depends on [Cl – ].
The general reaction for the calomel electrode is:
Hg 2 Cl 2 (s) + 2e – ⇌ 2Hg (l) + 2Cl –
The sense of balance above will depend on the medium where the electrode is contacted. The Cl – ion determines the solubility of Hg 2 Cl 2 , which in turn affects the formation or oxidation of Hg.
And the potential determined for a specified concentration of Cl ions – will be equal to:
E calomel = E red – E ox
E calomel being the potential that is reported as a reference in certain potential tables
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