Sodium hydroxide is generally prepared commercially by the electrolysis of aqueous NaCl (brine) in Castner-Kellner cell by using mercury cathode and carbon anode.

⇒ Sodium metal which is discharged at the cathode combines with mercury to form sodium amalgam.

⇒ Cl₂ gas is evolved at the anode.

At cathode:

At anode: 2Cl^-→ Cl₂ + 2e^-

The sodium amalgam thus obtained is treated with water to form sodium hydroxide and hydrogen gas.

Note: Caster- Kellner cell is used in the manufacture of sodium hydroxide (NaOH). The working of Caster Kellner process is given below: