Unlike dry cell, the mercury cell has a constant cell potential throughout its useful life. Why?

Question

Philoid · Accepted Answer

The following diagram represents a mercury cell.

The reactions take place inside a mercury cell are as follows:

Anode: Zn(Hg) + 2OH^–→ ZnO(s) + H₂O + 2e^–

Cathode: HgO + H₂O + 2e^–→ Hg(l) + 2OH^–

Galvanic cell equation: Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)

The above overall reaction contains no ions whose concentration can change over time. So, the cell potential remains constant throughout the mercury cell life.