What do you know about Castner-Kellner process? Write the principle involved in it.

Explanation

This process is also called Mercury Cathode process.
Principle : Brine solution is electrolysed using mercury as cathode. Chlorine gas evolved at anode escapes out through the outlet. At the same time sodium amalgam is formed at cathode. This sodium amalgam is formed at cathode. This sodium amalgam reacts with water and gives NaOH along with the evolution of `H_(2)` gas.
`2NaCl to 2Na^(+) + 2Cl^(-)` ( ionization)
At Mercury Cathode :
`2Na^(+) + 2e^(-) + Hg to Na_(2)Hg`
At Graphite Anodes:
`2Cl^(-) to Cl_(2) uarr + 2e^(-)`
In the Central Compartment :
At anode : `Na_(3)Hg to 2Na^(+) + 2e^(-) + Hg`
At cathode : `2H_(2)O + 2e^(-) to 2OH^(-)+H_(2) uarr`
`2Na^(+) + 2OH^(-) to 2NaOH`
Process: The Castner's cell consists of a rectangular iron tank, divided into 3 compartments by tow slate partitions as shown in the diagram. The partitions will be upto the bottom but they do not touch the bottom. The bottom of the tank is covered with mercury. In the middle compartment mercury acts as anode and in the outer compartments it acts as cathode. Brine solution is taken in the outer compartments. Graphite rods, projected into the brine solution act as anode. In the middle compartment dil. NaOH solution is taken . A bunch of iron rods suspended in the middle compartment acts as cathode.
The cathode and anode in the outer and middle compartments are connected by means of wires to the terminals of a battery. Then the above reactions occur and a concentrated solution of NaOH is formed in the middle compartment. It is evaporated in iron pans, cooled and cast into moulds.