explain the working of mercury cell

The half-reaction at the cathode is:
HgO + H2O + 2e- → Hg + 2OH -

The anode is made of zinc (Zn) and separated from the cathode with a layer of paper or other porous material soaked with electrolyte; this is known as a salt bridge. Two half-reactions occur at the anode. The first consists of an electrochemical reaction step:

Zn + 4OH- → Zn(OH)4-2 + 2e-

followed by the chemical reaction step:

Zn(OH)4-2 → ZnO + 2OH- + H2O

yielding an overall anode half-reaction of:

Zn + 2OH- → ZnO + H2O + 2e-

The overall reaction for the battery is:

Zn + HgO → ZnO + Hg
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