Question

Describe the working of the calomel electrode?

Answer 1

The mercury-mercurous chloride electrode is known as the calomel electrode. It is made up of a glass vase with a bent side tube. The bottom of the tube is filled with pure mercury. Although the silver chloride electrode has largely replaced it, the calomel electrode has a reputation for being more robust.

Complete answer:
The electrode is coated with a paste of mercury-mercurous chloride $(Hg + H{g_2}C{l_2})$ also known as calomel. The remainder of the cell is filled with a normal (1 N) or decinormal (0.1 N) or saturate $KCl$
solution. To provide an external electrical contact, a platinum wire enclosed within a glass tube and immersed in a layer of mercury is employed. The side tube makes electrical contact with a salt bridge.
The electrode can be expressed as follows:
$Hg(I)/H{g_2}C{l_2}(s)/C{l^ - }$
Depending on the nature of the other electrodes in the cell, the calomel electrode can function as an anode or a cathode.
When it acts as an anode, the electrode reaction occurs.
2Hg(I)$$$$ \to H{g_2}^{2 + } + 2{e^ - }
$H{g_2}^{2 + } + 2C{l^ - } \to H{g_2}C{l_2}$
$2Hg\, + \,2C{l^ - } \to H{g_2}C{l_2} + 2{e^ - }$ (Oxidation Reaction)
When it acts as cathode, the electrode reaction is,

$$H{g_2}^{2 + } + 2{e^ - } \to 2Hg \\\ H{g_2}C{l_2} \to H{g_2}^{2 + } + 2C{l^ - } \\\ Overall{\text{ }}reaction:H{g_2}C{l_2} + 2{e^ - } \to 2Hg(I) + 2C{l^ - } \\\$$

Note:
In electrochemistry, the saturated calomel electrode has numerous applications. pH measurement, cyclic voltammetry, and general aqueous electrochemistry all use saturated calomel electrodes. Cyclic voltammetry, abbreviated CV, is a type of potentiodynamic electrochemical measurement in which the working electrode potential is increased linearly versus time.