A brine solution is electrolyzed using a mercury cathode and a carbon anode .Sodium metal discharged at the cathode combines with mercury to form sodium amalgam.
2Na+ + 2e- →2Na (reduction)
Na + Hg → Na-Hg (amalgam)
Chlorine gas is evolved at the anode
2Cl- → Cl2 +2e- (oxidation)
The mercury containing dissolved in sodium is sent to another chamber called decomposer, where sodium reacts with water forming sodium hydroxide and hydrogen. Decomposer is packed with graphite blocks as hydrogen is easily liberated over graphite surface.
2Na-Hg +2H2O → 2NaOH +H2+2Hg
The solution which flows out from decomposer is NaOH solution which is evaporated to dryness.