Mercury Button Cell

The mercury button cell is a small, flat, long-lasting primary battery valued for its very stable voltage throughout its working life, which makes it well suited to precision devices such as watches, cameras and pacemakers. Its construction is:

• Anode: zinc metal amalgamated with mercury.
• Cathode: mercury(II) oxide (HgO) mixed with graphite.
• Electrolyte: a paste of KOH and ZnO.
• Emf: approximately 1.35 V.

At the anode, zinc is oxidised in the alkaline electrolyte: $\text{Zn(s)} + 2\text{OH}^{-}\text{(aq)} \rightarrow \text{ZnO(s)} + \text{H}_2\text{O(l)} + 2e^{-}$. At the cathode, mercury(II) oxide is reduced to liquid mercury: $\text{HgO(s)} + \text{H}_2\text{O(l)} + 2e^{-} \rightarrow \text{Hg(l)} + 2\text{OH}^{-}\text{(aq)}$. Adding the two half-reactions, the hydroxide ions and water molecules cancel exactly, leaving a remarkably simple overall reaction with no ions at all: $\text{Zn(s)} + \text{HgO(s)} \rightarrow \text{ZnO(s)} + \text{Hg(l)}$. …