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The S-Block Elements

Question
CBSEENCH11007106

Discuss Down's process for the isolation of sodium. 

Solution

In this method, sodium is prepared by the electrolysis of fused anhydrous sodium chloride. Some calcium chloride and potassium fluoride are added to it. It lowers the melting point of sodium chloride from 1085 to 873K. The electrolysis is carried out in an iron box lined with fire bricks known as Down’s cell.
It consists of a graphite anode projected up through the bottom of the cell which is surrounded by a cylindrical iron cathode. The anode and cathode are separated by a wire gauze shell through which molten sodium chloride can easily pass but melted sodium cannot. The cell is also produced with a storage drum for receiving molten metal, an outlet for the removal of chlorine gas at the top.


On electrolysis, chlorine is liberated at the anode which escapes through the iron hood at the top. Sodium is liberated at the cathode and remains in the wire gauze shell. The level of molten sodium rises and it overflows into the receiver. The following reactions take place:
NaCl space space space space rightwards arrow space space space space Na to the power of plus space plus space Cl to the power of minus
space space space At space cathode colon space space space Na to the power of plus space plus space straight e to the power of minus space space space rightwards arrow space space space space Na
space space space At space anode colon space space space space space space space space space space 2 Cl to the power of minus space rightwards arrow space space space Cl subscript 2 space plus space 2 straight e to the power of minus
Sodium metal obtained by this method is about 99.5% pure.