Sodium hydroxide is manufactured by electrolysis of concentrated
aqueous sodium chloride; the other product of the electrolysis,
chlorine, is equally important and hence separation of anode and
cathode products is necessary. This is achieved either by a diaphragm
(for example in the Hooker electrolytic cell) or by using a mercury
cathode which takes up the sodium formed at the cathode as an
amalgam (the Kellner-Solvay cell). The amalgam, after removal from
the electrolyte cell, is treated with water to give sodium hydroxide
and mercury. The mercury cell is more costly to operate but gives a
purer product.