Question

Descibe the reaction involved in the preparation of caustic soda

Answer 1

SINGAPORE: Sodium hydroxide, also known as caustic soda or lye, is an inorganic compound with the chemical formula NaOH. It is a white solid, and is a highly caustic metallic base and alkali salt. It is available in pellets, flakes, granules, and as prepared solutions at a number of different concentrations.

Sodium hydroxide is industrially produced as a 50 per cent solution by variations of the electrolytic chloralkali process. Chlorine gas is also produced in this process. Solid sodium hydroxide is obtained from this solution by the evaporation of water.

Various methods of preparation of sodium hydroxide are -

Castner — Kellener Process

Principle — In castner-kellner method NaOH is prepared by the electrolysis of aqueous solution of NaCl (Brine).

Castner-kellner cell — It is a rectangular tank of steel. Inside of tank is lined with ‘ebonite.’ Anode is made of titanium. Flowing layer of mercury (Hg) at the bottom of tank serves as cathode.

Ionization of NaCl — 2NaCl e- 2Na+ + 2Cl-

When electric current is passed through brine, +ve and -ve ions migrate towards their respective electrodes. Na+ ions are discharged at mercury cathode. The sodium deposited at mercury forms Sodium Amalgam. Chlorine produced at the anode is removed from the top of the cell.

Reaction at cathode

2Na+ +2 e- à 2Na

(Na forms amalgam)

Na + Hg à Na/Hg

Na+ ions are discharged in preference to H+ ions due to high over voltage.

Reaction at anode

2Cl- à Cl2 + 2e-

Formation of NaOH

Amalgam moves to another chamber called ‘denuder,’ where it is treated with water to produce NaOH which is in liquid state. Solid NaOH is obtained by the evaporation of this solution.

2Na/Hg + 2H2O à 2NaOH + H2 + 2Hg

NaOH obtained is highly pure and the process is very effeicient.

Chemical properties of sodium hydroxide

Nelson Diaphragm Cell

Principle: Electrolyte used in this process is aqueous NaCl (Brine).

Procedure: Porous diaphragm of asbestos or metal oxide with polymer separates anode and cathode compartments. Diaphragm prevents hydroxide ions entering anode compartment and prevents chloride ions entering cathode compartment. Saturated brine enters anode compartment where chlorine gas is produced.