The Castner-Kellner cell consists of large rectangular trough divided into three compartments with partition short of reaching the bottom of the tank Thus mercury in one compartment can flow into another but solution cannot mix. Graphite anodes are used in outer compartments filled with NaCl solution. The middle compartment contains very dilute solution of caustic soda and filled with iron rods as cathode.
On passing electric current Cl2 is liberated in outer compartments and sodium liberated at cathode. Mercury forms amalgam which is passed into middle compartment in which mercury acts as anode (having induced +ve potential).
At anode Na+ + e- → Na
At cathode Na + Hg → Na – Hg
2 (Na – Hg) + 2H2O → 2NaOH + Hg + H2
The concentration of NaOH goes on increasing in the middle compartment.
When the concentration of NaOH reaches 20% the solution is replaced by dilute solution.
