Caustic Soda - Preparation and Uses

Sodium hydroxide, also known as caustic soda and lye, is an inorganic compound with NaOH as caustic soda formula. It is a white-colored solid ionic compound consisting of hydroxide anions OH^- and sodium cations Na⁺.

It's a highly caustic base and the alkali that decomposes proteins at ordinary ambient temperatures and can severely burn chemical burns. It readily absorbs carbon dioxide and moisture from the air and is highly soluble in water. It creates a series of hydrates NaOH·nH₂O. 

 Note: The monohydrate NaOH·H₂O crystallises from the water solutions between the temperatures of 12.3 and 61.8 °C. "Sodium hydroxide", which is commercially available, is often this monohydrate, and published data can refer to it instead of an anhydrous compound. It is frequently used as one of the simplest hydroxides alongside acidic hydrochloric acid and neutral water to demonstrate the chemistry students' pH scale.

Preparation of Caustic Soda

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 ionisation of brine solution takes place according to the following reaction:

2NaCl → 2Na⁺ + 2Cl^-

When the brine solution gets in contact with an electric current, ionisation 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 crystallised 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.

Caustic Soda Uses and Benefits

Sodium hydroxide or caustic soda is used to manufacture various everyday products, including aluminium, paper, commercial drain, oven cleaners, detergents and soaps.

Let us look at it in brief.

Sodium hydroxide or caustic soda is used to manufacture soaps and various detergents we use in homes and commercial applications. The chlorine bleach is formed by combining sodium hydroxide and chlorine. Drain cleaners containing caustic soda convert the fats, grease that may clog pipes into soap that dissolves in water. 

Caustic soda is used in various industries, such as in the manufacturing of paper and pulp, drinking water, textiles, detergents and soaps. The worldwide production of caustic soda as per 2004 was approximately 60 million tonnes, while the demand was about 51 million tonnes. Caustic soda’s chemical name is sodium hydroxide and the chemical formula of caustic soda is NaOH.