Mercury cell used in hearing aid devices contains anode comprising of

The correct answer is Option (2) → Zinc-mercury amalgam

The mercury cell is a primary cell used in low current devices such as hearing aids and watches. Its anode is zinc–mercury amalgam, its cathode is a paste of HgO and carbon, and the electrolyte is a paste of KOH and ZnO. Zinc, the reducing agent, is oxidised at the anode, and mercury(II) oxide is reduced at the cathode:

    \( \displaystyle \text{Anode: } \ce{Zn(Hg) + 2OH- -> ZnO(s) + H2O + 2e-} \)

    \( \displaystyle \text{Cathode: } \ce{HgO(s) + H2O + 2e- -> Hg(l) + 2OH-} \)

    \( \displaystyle \text{Cell: } \ce{Zn(Hg) + HgO(s) -> ZnO(s) + Hg(l)} \)

No ion appears in the cell reaction, so its concentration cannot change during use, and the cell gives a constant potential of about 1.35 V throughout its life.

NCERT (2026-27 edition):
Primary Batteries – “… Mercury cell, (Fig. 2.9) suitable for low current devices like hearing aids, watches, etc. consists of zinc – mercury amalgam as anode and a paste of HgO and carbon as the cathode. The electrolyte is a paste of KOH and ZnO.”