Sodium hydroxide or caustic soda can be prepared by three successive methods as given below:
- Castner-Kellner process
- Nelson Diaphragm cell
- Loewig’s process
Let us now discuss the Castner-Kellner process in brief.
Castner-Kellner Process
Principle
In the process of Castner-Kellner, electrolysis of brine solution is performed to obtain the sodium hydroxide.
Castner-Kellner cell
It is a rectangular steel tank, where the ebonite is lined inside the tank. The titanium acts as an anode, and a mercury layer, residing at the bottom of the tank, acts as the cathode.
The ionization of brine solution takes place according to the following reaction:
2NaCl → 2Na+ + 2Cl–
When the brine solution gets in contact with an electric current, ionization takes place. As a result, both the positive and negative ions move towards the electrodes. And, the sodium ions get deposited at the mercury cathode by forming a sodium amalgam. In contrast, chlorine ions move towards the anode position and exit the cell from the top.
Reaction at the Anode:
2Cl– → Cl2 + 2e–
Reaction at the Cathode:
2Na+ + 2e– → 2Na
Nelson Diaphragm Cell
Principle
The electrolyte used in this process is an aqueous NaCl solution (also called Brine).
Procedure
The porous diaphragm of metal oxide or asbestos with the polymer separates cathode and anode compartments. The diaphragm prevents the hydroxide ions from entering the anode compartment and the chloride ions from entering the cathode compartment. The saturated brine enters into the anode compartment, and resultantly, chlorine gas is produced.
Anode (otherwise called a positive electrode): carbon (otherwise called graphite) or titanium is coated with the Ru-Ti oxide.
Cathode (or called a negative electrode): Steel mesh
Reaction at the anode (or oxidation):
2Cl– (aq) → Cl2(g) + 2e
Cathode reaction (or reduction):
2H2O (l) + 2e → H2(g) + 2OH– (aq)
Na+ migrates across the diaphragm to the cathode compartment by combining with OH- to form NaOH.
Overall cell reaction (showing the Na+ Spectator Ions):
2H2O (l) + 2Cl– (aq) + 2Na+ (aq) → 2Na+ (aq) + 2OH– (aq) + H2(g) + Cl2(g)
This product contains sodium hydroxide and sodium chloride, and the NaOH(s) can be crystalized out.
Loewig’s Process
The process of Loewig for the preparation of caustic soda depends on the formation of sodium ferrate (Na2FeO4) and then decomposed with water. Thereafter, the soda liquors mix with ferric oxide and mass evaporated to dryness and calcined at a bright red heat, usually in a revolving furnace. By the process of calcination, a reaction between the iron oxide and sodium carbonate is brought about, escaping the carbon dioxide and remaining sodium ferrate in the furnace.
The mass is washed with the cold water until the total soluble matter is removed; then, water at 900 °C is run over the sodium ferrate. It is decomposed, forms caustic soda, and the iron oxide is regenerated. The last is returned to the process of calcining. Ferric oxide, which is used, is natural iron ore, free and very clean from silica or other impurities; made by calcining a precipitated ferric hydroxide is not adapted well to the process, because it forms a product, which is difficult to lixiviate.