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Caustic Soda - Preparation and Uses

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Last updated date: 17th Mar 2024
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What is Caustic Soda?

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.

FAQs on Caustic Soda - Preparation and Uses

1. Explain the properties of caustic soda.

The properties of caustic soda include the following:

  1. It is a stable compound.

  2. NaOH is a white solid with a melting point of 591K.

  3. It is extremely soluble in water and mild soluble in alcohol.

  4. NaOH is bitter and has a soapy sensation.

  5. Caustic soda is also strongly alkaline.

  6. Purified caustic soda is a colourless crystalline solid. It melts at 318° C without decomposing and with a boiling point of 1,388° C.

  7. It is very soluble in water, with less solubility in polar solvents such as ethanol and methanol.

2. Why is caustic soda called sodium hydroxide?

Sodium hydroxide is also called soda or lye, or caustic soda. Here, caustic means "to burn," and caustic soda gets its name from how it can burn the skin and has a chemical formula of NaOH.


At room temperature, caustic soda (NaOH) is an odourless white crystalline solid that absorbs moisture from the air. It is a synthetically produced substance. We can say that it is a polyvalent alkali. It is mainly used in the production of paper, soap and detergents, alumina, petroleum and chemicals. Some other applications include textiles, food for water treatment, mining glassmaking, metalworking.

3. List the practical uses of caustic soda and mention its use in energy.

Caustic soda has various uses such as cleaning, pesticide, detergents, soaps, inorganic benefits in chemistry such as producing electricity, which is utilised in cells. It is used to produce soaps, radii, paper, explosives, dyes and petroleum products. 


Caustic Soda in Energy

In the energy industry, caustic soda is used in the production of fuel cells. These fuel cells function like batteries to generate electricity cleanly and efficiently for various applications, such as transportation, material handling, portable emergency power, stationary and emergency applications. In addition, epoxy resins are made from caustic soda, and they are used in wind turbines.

4. What occurs if caustic soda is mixed with bleach?

Bleach can be made by mixing caustic soda with chlorine gas and has less remaining caustic soda. Adding more bleach will simply raise the pH level of the solution from 10.5+ to 14. 


Explore our website to learn more about caustic soda. 

5. Does caustic soda absorb CO2?

There are two orthodox chemical processes for absorbing CO2. One is to use NaOH solution (caustic soda). Sodium hydroxide is an alkali, carbon dioxide is an acid gas, and the two react to form sodium carbonate (sodium carbonate). A second method is to use calcium oxide.