Question

Describe the working of the calomel electrode?

Answer 1

The calomel electrode:

• The calomel electrode is the mercury-mercurous chloride electrode.
• It consists of a glass vessel having a bent side tube.
• Pure mercury is placed at the bottom of the tube, which is covered with a paste of mercury- mercurous chloride (Hg⁺Hg₂Cl₂) that is calomel.
• The remaining portion of the cell is filled with a solution of normal (1 N) or saturated KCl (Potassium chloride).
• A platinum wire sealed into a glass tube is dipped into a mercury layer and is used to provide external electrical contact.
• The side tube is used for making electrical contact with a salt bridge.

The calomel electrode can act as an anode or cathode depending on the nature of another electrode of the cell.

Reaction at the anode:

$$\begin{gathered} 2\mathrm{Hg}\left(\mathrm{l}\right)\to {{\mathrm{Hg}}_2}^{2+}+2{\mathrm{e}}^{-} \\ {{\mathrm{Hg}}_2}^{2+}+2{\mathrm{Cl}}^{-}\to {\mathrm{Hg}}_2{\mathrm{Cl}}_2 \end{gathered}$$

Overall reaction:

$2\mathrm{Hg}+2{\mathrm{Cl}}^{-}\to {\mathrm{Hg}}_2{\mathrm{Cl}}_2+2{\mathrm{e}}^{-}(\mathrm{Oxidation}\mathrm{reaction})$

Reaction at the cathode:

$$\begin{gathered} {{\mathrm{Hg}}_2}^{2+}+2{\mathrm{e}}^{-}\to 2\mathrm{Hg} \\ {\mathrm{Hg}}_2{\mathrm{Cl}}_2\to {{\mathrm{Hg}}_2}^{2+}+2{\mathrm{Cl}}^{-} \end{gathered}$$

Overall reaction:

${\mathrm{Hg}}_2{\mathrm{Cl}}_2+2{\mathrm{e}}^{-}\to 2\mathrm{Hg}\left(\mathrm{l}\right)+2{\mathrm{Cl}}^{-}(\mathrm{Reduction}\mathrm{reaction})$