Answer: the calomel electrode is a secondary reference electrode made of mercury in contact with mercurous chloride (calomel, \(\mathrm{Hg_2Cl_2}\)) and a KCl solution. It is used in place of the SHE because it is easy to make and gives a stable, reproducible potential.
Representation: \(\mathrm{Hg\,|\,Hg_2Cl_2(s)\,|\,KCl\ (solution)}\)
Construction
- A glass tube contains pure mercury at the bottom.
- Above it is a paste of mercury and mercurous chloride (calomel).
- The tube is filled with KCl solution of known concentration (saturated, 1 N or 0.1 N).
- A platinum wire dipping into the mercury makes the electrical contact.
- A side tube with a porous plug or salt bridge connects it to the other half-cell.
Electrode reaction (as reduction):
\[\mathrm{Hg_2Cl_2(s) + 2e^- \rightarrow 2Hg(l) + 2Cl^-}\]
Its potential depends only on the chloride ion concentration:
\[E = E^\circ - 0.0591\log[\mathrm{Cl^-}]\ \ (25\ ^\circ\mathrm{C})\]
Approximate potentials at 25 °C: saturated KCl about 0.242 V, 1 N KCl about 0.280 V, 0.1 N KCl about 0.334 V.
Uses: as a reference electrode to measure the potential of other electrodes (for example in pH metres and potentiometric titrations). It can act as anode or cathode depending on the other electrode.