Question

Describe the construction and working of calomel electrode.

Answer 1

Calomel electrode is a metal-sparingly soluble salt electrode. It is used as secondary reference electrode to determine the standard potentials of the electrode.
Construction : The electrode consists of a glass tube provided with a bent side tube and another side tube $B$. A little $Hg$ is placed at the bottom of the dry glass tube. A rubber bung carrying a thin glass tube with a platinum wire is then inserted, taking care that the platinum wire drips into mercury. The mercury is then covered with a layer of mercury and mercurous chloride (calomel) paste. The glass tube is then filled with $KCl$ solution of definite concentration as shown in fig.
Hence, three types of Calomel electrodes are available, viz., normal $\left(1N\right)$, decinormal $\left(0.1N\right)$ and saturated.
Working : The potential of the calomel electrode depends upon the concentration of the chloride ions in solution. If the electrode is saturated calomel electrode (SCE), some crystals of $KCl$ are placed over the $Hg-{Hg}_2{Cl}_2$ paste so as to keep the solution saturated.
The electrode is represented as :
$K{Cl}_{(satu.)}{Hg}_2{Cl}_{2\left(s\right)}{Hg}_{\left(l\right)}$
Oxidation : If the electrode serves as anode, then half reaction that occurs on it will be oxidation. Mercury is first oxidised to mercuric ions.
$2{Hg}_{\left(l\right)}⟶{Hg}_2^{2+}+2e^{-}$
The chloride ions supplied by $KCl$ solution combine with mercuric ions $\left[{Hg}_2^{2+}\right]$ to form insoluble mercurous chloride. Thus,
${Hg}_2^{2+}+2{Cl}^{-}⟶{Hg}_2{Cl}_{2\left(s\right)}$
Overall reaction is,
$2{Hg}_{\left(l\right)}+2{Cl}_{\left(aq\right)}^{-}⟶{Hg}_2{Cl}_{2\left(s\right)}+2e^{-}$
Reduction : If the electrode is cathode $(+)$ in the galvanic cell, the half reaction that occurs on it will be reduction :
${Hg}_2{Cl}_{2\left(s\right)}+2e^{-}⟶2{Hg}_{\left(l\right)}+2{Cl}_{\left(aq\right)}^{-}$
The potential of calomel electrode decreases with increase in the concentration of chloride ions at a given temperature. Thus, electrode is reversible with respect to concentration of chloride ions.