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

Working of mercury cell:
Anode `Zn(Hg)+2OH^(-) to ZnO(S)+H_(2)O(l)+2e^(-)` (Amalgam)
Cathode `HgO(s)+H_(2)O(l)+2e^(-) to Hg(l)+2OH^(-)`
Net reaction `Zn(Hg)+HgO(s) to ZnO+Hg(l)` Amalgam
The operating voltage for mercury cell is 1.35 V, and show consistency throughout its life as the overall reaction does not involve any ion in solution whose concentration can change during its lifetime.