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Chapter 18 – Preparation of Sodium Hydroxide

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:


Preparation of Sodium Hydroxide is an essential industry practice. Sodium Hydroxide, or NaOH, is a strong alkali manufactured by electrolysis of brine (NaCl – common salt) solution. You must have seen the solid white crystals of sodium hydroxide in the labs. You must have felt the crystal being slippery to the touch (hence saponification – manufacture of soap). NaOH reacts with the skin’s natural oils.

It is also known as lye or caustic soda. It possesses a corrosive nature that can harm animals and plant tissues. It is used in paper and pulp, water treatment, textiles, and soap and detergent manufacture.

There are three primary methods to prepare caustic soda: the Castner-kellner process, the Nelson Diaphragm process, and Loewig’s process.

NaOH is a byproduct of the chlorine extraction process. It is generally made in an industrial process known as the Chloralkali process. This process is easy electrolysis of brine water (aqueous NaCl). We shall study this process in more detail.

The diaphragm cell contains two separate compartments where the brine water is poured down. The two containers have a titanium anode and steel cathode, respectively. Yes, it may be a little surprising, but titanium cathode is being used in this device because it prevents corrosion by chlorine.

As we all know that reduction occurs at the cathode, and oxidation occurs at the anode in electrochemistry. Hence, NaCl dissociates into ions in the given solution.


The H2O(l) can also dissociate into:


When the current is passed through the solution, the anions are attracted to the anode. The cations are attracted to the cathode. In the anode, there is a competition between the OH−(aq) and the Cl−(aq) ions. Cl−(aq) wins because it is easier to discharge.

Similarly, there is a competition between the Na+(aq) and the H+(aq) ions at the cathode. H+(aq) wins because it is easier to discharge.

So, we can now write the equation at the cathode as:


And the equation at the anode as:


These ions get discharged at the electrodes, and remaining ions are present in the solution that makes the sodium hydroxide.

Some of the properties of sodium hydroxide are given below.

  • NaOH is a white colored solid that has a high melting point of 591K.
  • It feels soapy to touch and is bitter. Although it is a stable compound, NaOH possesses corrosive nature.
  • It is highly alkaline and is water-soluble.
  • It is used as a cleansing agent due to its corrosive cleaning properly.

It’s a strong base and a salt at the same time!

A salt is an ionic compound consisting of a metal and a nonmetal. The metal is sodium, and the acid is water (aka hydroxylic acid). Water’s corresponding base is OH(-).

Na(+)OH(-) is just a salt like Na(+)Cl(-) or Na2SO4 or NaHSO4.

·NaOH is a very alkaline salt. This salt exists in countless modifications, including several hydrates, like Orthorhombic NaOH Space group: Cmcm, No. 63.

Sodium hydroxide has some distinguishable physical properties. They are given below:

  • They appear as pellets of white color.
  • Their boiling point is 140 degrees C for a 40% aqueous solution.
  • Molecular weight is 39.997 g/mol.
  • Specific gravity is 1.5.

Let’s check some uses of sodium hydroxide.

  • It is used as an essential chemical reagent in laboratories.
  • Soda-lime is prepared using sodium hydroxide.
  • Aluminum is extracted from bauxite with the help of sodium hydroxide.

Caustic soda / sodium hydroxide is a very alkaline compound that can have corrosive effects on animals or plant tissues. They are extensively used for cleaning purposes as a washing powder or to remove stains from drains or floors. Preparation of sodium hydroxide can be done in different ways. The widely used method is the electrolysis of the brine solution.

  1. How is sodium hydroxide prepared?

The synthesis starts from the electrolysis of ultra-pure Sodium Chloride (a highly purified table salt). Sodium and chlorine are separated immediately; while chlorine can be employed for several different reactions (mainly depending on market demands and prices), sodium reacts with water to form NaOHor Sodium Hydroxide, which comes out from the reactor in an aqueous solution of 50%. The solid form (pellets, prills, or flake) requires heat treatment to dry the solution.

The most extensive part of the production is the energy needed for the electrolysis. The means to separate chlorine and sodium used to be a mercurial cell reaction; nowadays, a particular membrane is used in modern reactors.

  1. How is NaOH prepared from NaCl?

The process is called the Castner- Kellner process, where we do the electrolysis of the brine solution (NaCl) to get our desired result of sodium hydroxide.

  1. Where is sodium hydroxide found?

You can buy sodium hydroxide from shops, including hardware shops or grocery markets. It is also called caustic soda in general terms.

  1. What is sodium hydroxide lye used for?

You can use it to strip paint, unclog drains, and make soap. Also, use it with hydrogen peroxide (30% hydrogen peroxide) to make homemade wood bleach. Another possible home use is the preparation of foods such as hominy, some pretzels, and cleaning.

  1. What are the sources of sodium hydroxide?

Sea Water (Brine) electrolysis, and you can also get it using advanced membrane technology. Or simply buy it from your nearby hardware shop.

You can learn about the manufacturing process of sodium hydroxide through explanatory videos available on MSVgo.

High School Physics

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