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Chemistry · Ch 5 — Electrochemistry

Mercury battery

5.10.4

Mercury battery

The mercury battery consists of a zinc anode amalgamated with mercury. The cathode is a paste of Hg and carbon. The electrolyte is strongly alkaline, made of a paste of KOH and ZnO. The electrode reactions and the net cell reaction are:

Zn(Hg)+2 OH− (aq)⟶ZnO (s)+H2O (l)+2 e−(anode oxidation)\mathrm{Zn(Hg) + 2\,OH^-\,(aq) \longrightarrow ZnO\,(s) + H_2O\,(l) + 2\,e^-} \quad \text{(anode oxidation)}

HgO (s)+H2O (l)+2 e−⟶Hg (l)+2 OH− (aq)(cathode reduction)\mathrm{HgO\,(s) + H_2O\,(l) + 2\,e^- \longrightarrow Hg\,(l) + 2\,OH^-\,(aq)} \quad \text{(cathode reduction)}

Zn(Hg)+HgO (s)⟶ZnO (s)+Hg (l)(overall cell reaction)\mathrm{Zn(Hg) + HgO\,(s) \longrightarrow ZnO\,(s) + Hg\,(l)} \quad \text{(overall cell reaction)}

The overall reaction involves only solid substances: there is no change in the electrolyte composition during operation. The mercury battery therefore provides a more constant voltage (1.35 V) than the Leclanche' dry cell; it also has considerably higher capacity and longer life than the dry cell.

The mercury dry cell finds use in hearing aids, electric watches, pacemakers, etc. …

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