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Preparation of Sodium Hydroxide in Castner-kellner Cell

The Castner-Kellner cell consists of a tank with a mercury layer at the bottom. The tank is divided into compartments with a slate partition not touching the bottom.


The two outer compartments contain sodium chloride solution while the central compartment contains dilute solution of sodium hydroxide and a series of iron rods which act as the cathode. Two graphite anodes are in the outer compartments. Mercury acts as the cathode in the outer compartments and as the anode in the central compartment. On passing the electric current, sodium chloride solution is electrolysed in the outer compartments. Chlorine is liberated at the anode and sodium forms amalgam with mercury (Na/Hg) at the cathode. The amalgam is continually pumped into a separate chamber and is treated with water to obtain caustic soda and dihydrogen gas.

2Na/Hg + 2H2O 2NaOH + H2 + Hg

The liberated mercury is sent back to the cell.

Sodium hydroxide is a white deliquescent crystalline solid (melting point 591 K) which is readily soluble in water. Its aqueous solution is strongly alkaline, soapy to touch and is corrosive.

Sodium hydroxide is widely used in the manufacture of soap, paper, viscose rayon and many other chemical industries.

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