1. Castner-Kellner method
2. The Castner-Kellner cell consists of a large tank. The bottom of the tank is filled with mercury. The tank is made into 3 compartments by 2 partitions. The outer compartments are filled with NaCl solution and the middle compartment is filled with a very dilute solution of NaOH. The mercury layer of the bottom of cell in the outer compartments acts as cathode and the mercury in the middle compartments acts as anode. As a result of electrolysis Na and Cl2 are produced in the outer compartments. Na gets coated with mercury, and sodium amalgam is produced in the outer compartments. Due to the action of eccentric wheels at the bottom of the tank, the tank can be tilted. As a result of this Na is carried into the middle compartment and is then allowed to react with water in the middle compartment. As a result of this reaction NaOH is produced. When the concentration of NaOH in the middle compartment is raised to 40%, it is taken out from the tank. On cooling, crystals of NaOH separate out.
3. NaCl → Na+ + Cl-
At cathode: Na+ + e- → Na
Na + Hg → Na/Hg (Sodium amalgam)
At anode: Cl- → Cl + e-
Cl + Cl → Cl2 (g)