Write a note on mercury cell $(Zn-Hg)$.

(N/A) This cell is an example of a primary cell. Its construction is as follows:
Construction: As shown in the diagram,
Anode: It consists of zinc-mercury amalgam.
Cathode: $A$ paste of $HgO$ and carbon.
Electrolyte: $A$ moist paste of $KOH$ and $ZnO$.
Reaction: The electrode reactions are given below:
Oxidation of $Zn$ in the zinc-mercury amalgam occurs at the anode:
$(i) \ Zn(Hg) + 2 OH^{-} \rightarrow ZnO_{(s)} + H_{2}O + 2 e^{-}$
Reduction of $HgO$ occurs at the cathode:
$(ii) \ HgO_{(s)} + H_{2}O + 2 e^{-} \rightarrow Hg_{(l)} + 2 OH^{-}$
The overall cell reaction is:
$(iii) \ Zn(Hg) + HgO_{(s)} \rightarrow ZnO_{(s)} + Hg_{(l)}$
In this cell,$Zn$ is oxidized and acts as a reducing agent,while $HgO$ is reduced and acts as an oxidizing agent.
Cell potential and characterization: The cell potential is approximately $1.35 \ V$ and remains constant throughout its life because the overall reaction does not involve any ions in the solution whose concentration changes during its operation.
Uses: Mercury cells are suitable for low-current devices like hearing aids,watches,etc.