Preparation, Properties And Uses- Caustic Soda

What is Sodium Hydroxide?

Caustic soda is an alkali salt which is also called Lye. It is the common name of sodium hydroxide. This name is given due to the corrosive nature of this salt on animal and plant tissues. It has a wide range of applications. The chemical formula of sodium hydroxide is NaOH.

Preparation of Sodium Hydroxide:

Sodium hydroxide can be prepared by various methods like:

  • Castner-Kellner process
  • Nelson Diaphragm cell
  • Loewig’s process

caustic soda

Castner-Kellner process:

Principle: In Castner-Kellner method, electrolysis of brine solution is performed in order to obtain sodium hydroxide.

Castner-Kellner cell: It is a steel tank which is rectangular in shape. Ebonite is lined inside the tank. Titanium acts as an anode and a layer of mercury at the bottom of the tank acts as the cathode.

Ionization of brine solution occurs according to the following reaction:

\( 2NaCl \rightarrow 2Na^{+} + 2Cl^{-}\)

When the brine solution comes in contact with electric current, ionization takes place. As a result positive and the negative ions move towards the electrodes. Sodium ions get deposited at the mercury cathode forming a sodium amalgam. Chlorine ions move towards the anode and exit the cell from the top.

Reaction at the anode:

\( 2Cl^- \rightarrow Cl_2 + 2e^- \)

Reaction at the cathode:

\( 2Na^+ + 2e^- \rightarrow  2 Na\)

Formation of NaOH:

The amalgam formed is then transferred to another chamber called denuder. In the denuder, it is treated with water to obtain sodium hydroxide solution. On evaporation of the solution, solid sodium hydroxide is formed. This is a very efficient process in order to obtain pure caustic soda.

Properties of Sodium Hydroxide:

  • It is a white solid which has a melting point of 591K
  • It is a stable compound.
  • NaOH is bitter in taste and has a soapy feel to it.
  • It is highly soluble in water and moderately soluble in alcohol.
  • Sodium hydroxide is strongly alkaline in nature.

Uses of Sodium Hydroxide:

  • It is used as a cleansing agent and in the manufacturing of washing soda.
  • Sometimes, sodium hydroxide is also used as a reagent in the laboratories.
  • It is used in the preparation of soda lime.
  • It is used in the extraction of aluminium by purifying bauxite.

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Practise This Question

Cl2 + 2NaOH  NaCl + A + H2O to form. A is

                (Cold & dilute)

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