# Preparation of NaOH by Castner Kellner process

Preparation of NaOH by Castner Kellner process : Castner Kellner process is also called as Mercury cathode process. The chemical formula of Sodium hydroxide is NaOH.  Sodium hydroxide is also known as caustic soda or lye.  The ph value of sodium hydroxide is 12.

Principle of Preparation of NaOH : Brine solution is subjected to electrolysis using mercury as cathode.  $\text&space;{cl}_{2}$ gas evolves at anode. Sodium Amalgum formed at cathode. When sodium amalgum reacts with water. NaOH is formed by evolving $\text&space;H_{2}$ gas.

Electrode reactions in the cell :

1. In the outer compartments :

$\text&space;{2Nacl}&space;\longrightarrow&space;\text&space;{2Na}^{+}&space;+&space;2\text&space;{cl}^{-}&space;\text{(ionization)}$

At graphite anode :

$2\text&space;{cl}^{-}&space;\longrightarrow&space;\text{cl}_{2}&space;\text&space;{(g)}&space;+&space;2&space;\text&space;e^{-}&space;\text{(oxidation)}$

At mercury cathode :

$\text&space;{Hg}&space;+&space;2&space;\text&space;{Na}^{+}&space;+&space;2\text&space;e^{-}\longrightarrow&space;\text&space;{Na}_{2}&space;\text&space;{Hg}$

• In the central compartment :

At anode : $\text&space;{Na}_{2}&space;\text&space;{Hg}&space;-&space;2&space;\text&space;e^{-}&space;\longrightarrow&space;2&space;\text&space;{Na}^{+}&space;+&space;\text&space;{Hg}$

At cathode : $\text&space;{2H}_{2}&space;\text{}&space;O&space;+&space;2&space;\text{}&space;e^{-}&space;\longrightarrow&space;\text{}&space;H_{2}&space;+&space;2\text&space;{OH}^{-}&space;\text&space;{(reduction)}$

$2&space;\text&space;{Na}^{+}&space;+&space;2&space;\text&space;{OH}^{-}&space;\longrightarrow&space;2&space;\text&space;{NaOH}$

Process :

A rectangular iron tank is divided into three compartments by the suspended slate partitions. The partitions do not reach the bottom of the tank. The bottom of the tank is covered with mercury.

In the two side compartments, brine solution is taken. The two graphite rods projected into brine solution acts as an anode.

In the central compartment, Sodium Hydroxide (NaOH) solution is taken. A bunch of iron rods are suspended in the central compartment which functions as a cathode.

When electricity is passed, the mercury layer at the bottom functions as a cathode in the outer compartments and as anode in the central compartment. Hence, the mercury layer at the bottom is called an intermediate Electrode by induction. $\text&space;{cl}^{-}$ ions are discharged at anode and $\text{cl}_{2}$ gas evolves from outer compartments. $\text{Na}^{+}$ ions lose their charge at the mercury and form Na metal. The sodium metal readily forms sodium amalgam with mercury. The amalgam is then brought into the central compartment by the use of the eccentric wheel. Sodium amalgam forms Sodium Hydroxide with water and $\text{H}_{2}$ gas is released.

When the NaOH solution becomes 20% strong, is removed and concentrated. It is evaporated at $500^&space;\circ&space;-600^&space;\circ&space;\text&space;C$ in iron pans to obtain fused NaOH. This is cooled and cast into different types of moulds.