Mention the reaction occurring at (i) anode, (ii) cathode, during working of a mercury cell. Why does the voltage of a mercury cell remain constant during the operation?

Explanation

Mercury Cell : It is a miniature cell which finds a frequent use these days to supply energy for watches, video cameras, hearing aids and other compact devices. In mercury cell the anode is zinc-mercury amalgam, and the cathode is a paste of mercury (II) oxide and carbon, electrolyte is a moist paste of KOH-ZnO.
The cell reaction are as follows :
Anode :` Zn(Hg)+ 2OH^(-)rightarrow ZnO_((s))+ H_((2))O_((1)) + 2e^(-)`
Cathod : `HgO_((s)) + H_((2))O_((1)) + 2e^(-) rightarrow Hg_((1)) + 2OH^(-)`
Net reaction : `An_((Hg))+ HgO _((s)) rightarrow ZnO_((s)) + Hg_((1))`
The cell potential remains constant during its life as the overall reaction does not involve any ion in solution whose concentration can change during its use.