In the mercury cathode (Castner-Kellner) cell for the electrolysis of brine, the substance formed at the mercury cathode is

$\mathrm{Na}{}^{+}$ is reduced at the mercury cathode to sodium, which dissolves in mercury as sodium amalgam. Chlorine is formed at the graphite anode. The amalgam is later treated with water to give $\mathrm{NaOH}$ and $\mathrm{H}{}_2$.