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Preparation of Sodium Hydroxide

Sodium Hydroxide is an inorganic compound made of sodium cations and hydroxide anions. It is a strong base that exists as a white solid at room temperature. Its chemical formula is NaOH and is popularly known as caustic soda or lye. Sodium hydroxide (NaOH) is a monoacidic base, as it has only one OH^– ion that can replace or neutralize only one H⁺ ion of an acid.

Sodium hydroxide preparation

It is a strong base and widely used for laboratory and industrial purposes. Generally, it is prepared by electrolysis of brine solution in diaphragm or mercury cell.

Some of the other processes used to produce sodium hydroxide are described below;

1) Leblanc process

It is an industrial process that was used to produce sodium carbonate from sodium chloride, sulphuric acid, coal and calcium carbonate. This process was invented by Nicolas Leblanc in 1791.

It comprises two stages; production of sodium sulphate from sodium chloride and production of sodium carbonate from the reaction of sodium sulphate with coal and calcium carbonate.

In this process, NaOH is produced as a side product along with chlorine gas which is produced on a large scale. See the process;

Leblanc process goes like this

4HCl + MnO₂ → 2Cl₂ + Mn²⁺ + 2H₂O

In the above reaction, MnO₂ serves as the oxidising agent. It oxidises HCl to Cl₂. Earlier, the preparation of HCl was not easy. It can be prepared from NaCl by the action of a strong acid as shown in the following chemical reaction;

i) Formation Sodium sulphate (salt cake):

NaCl + conc. H₂SO₄ → NaHSO₄ + HCl

NaHSO₄ + NaCl →Na₂SO₄ + HCl

The HCl produced in the above reaction is then oxidised as shown below;

HCl + MnO₂ →Cl₂ + Mn^e+

ii) Formation of Sodium Carbonate

The Na₂SO₄ produced in the above reaction is used in the formation of other chemicals such as glass, Na₂CO₃ or NaOH as follows;

Na₂SO₄ + C + CaCO3 → Na₂CO3 + CaSO₄

Na₂CO₃ + Ca (OH) ₂ →2NaOH + CaCO₃

Sodium carbonate reacts with calcium hydroxide to form Sodium hydroxide (NaOH) and Calcium Carbonate.

The reactants included in this reaction are H₂SO₄, NaCl, CaCO₃ and C. NaOH and Cl₂ are the main products produced in this reaction. The CaCO₃ gets converted to Ca(OH)₂ as shown below;

CaCO₃ →CaO → Ca(OH)₂

2) Lowig’s Process

In this sodium carbonate (Na2CO3) is used as a starting material which leads to the formation of sodium ferrate (Na₂Fe₂O₄). The chemical reaction that occurs is given below;

Na₂CO₃ + Fe₂O₃ → 2NaFeO₂ + CO₂

The above reaction is carried out by calcination. Carbon dioxide formed in the above reaction escapes out, whereas, sodium ferrate remains in the furnace. The mass is cleaned with cold water to remove the soluble particles. Thereafter, at 90 degrees centigrade, water is passed over the sodium ferrate to carry out its decomposition and to form sodium hydroxide and iron oxide. The chemical reaction is shown below;

2NaFeO₂ + H₂O → 2NaOH + Fe₂O₃

So, the sodium ferrate formed in the first reaction is decomposed with water and the solution obtained is filtered and evaporated to obtain sodium hydroxide flakes.

Electrolytic Process (Chlor-alkali process)

It is the commonly used method that involves the electrolysis of brine solution. This process is called the chlor-alkali process. It involves the electrolysis of Brine (NaCl) solution to produce NaOH. The titanium and steel or nickel rods are used as anode and cathode respectively, whereas, the NaCl is used as an electrolyte as shown in the below image;

When current is passed, sodium ions and chlorine ions are produced. Hydrogen ions being positively charged move towards the cathode, which has a negative charge. Whereas, the negatively charged chlorine ions move towards the positively charged anode.

The electrolysis of aqueous sodium chloride solution gives chlorine gas and sodium hydroxide. It results in the formation of an aqueous solution of NaOH 50%, which is dried to get sodium hydroxide as flakes or pellets.

2 NaCl + 2 H₂O → 2 NaOH + Cl₂ + H₂

Let us see what happens in the Chlor-alkali process?

When electricity is passed through brine solution, the NaCl breaks into sodium and chlorine ions (Na⁺ and Cl^–) and water also breaks into H⁺ and OH⁺ ions. So, in the brine solution, there will be four types of ions.

The negatively charged ions will move towards the positively charged rod, and the positively charged ions will move towards the negatively charged rod.

The negatively charged ions get oxidised on reaching the positive rod as they lose their electron. This positive rod is called anode as it causes oxidation of negative and oxidation also takes place at the anode.

Similarly, the gain of electrons by positively charged ions occurs at the rod with a negative charge. The positively charged ions that need electrons will move towards the negatively charged rod and gain electrons, which is known as reduction. So, reduction takes place at the negatively charged rod, which is called cathode as the rod at which reduction takes place is called the cathode. So, it is not the charge of a rod that decide whether it is a cathode or anode.

Reaction at anode

It refers to the reaction that takes place at anode in this process. The anode is positively charged so ions with a negative charge will be attracted toward it.

There are two negative ions (Cl^– and OH^–) in solution, out of which Cl^– ions are able to reach the anode where they are oxidised. Whereas, the OH^– ions remain in the solution. After oxidation, Cl^– ions gets converted into Cl atoms at the anode and one Cl atom combines with another Cl atom to form chlorine gas, which is released from the anode out of the solution.

Reaction at cathode

Reduction takes place at the cathode as it gives electrons to positively charged ions and thus reduce them. There are two types of positively charged ions in the solution Hydrogen ion (H⁺) and Sodium ion (Na⁺). Out of these two ions, hydrogen ions reach the cathode and undergo reduction by gaining electrons. The hydrogen ions (H⁺) change to hydrogen atoms and one hydrogen atom combines with another hydrogen atom to form hydrogen gas which is released from the cathode out of the solution.

So, Cl₂ is formed at the anode and H₂ is formed at the cathode.

Now, sodium and hydroxide left in the solution combine with each other to form Sodium hydroxide (NaOH), thus the formation of NaOH takes place in the Chlor-alkali process.

Reactions of Sodium hydroxide (NaOH)

i) Reaction with Acids

Sodium hydroxide when reacts with acids forms salts and pure water. See the below reaction;

NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (liquid)

ii) Reaction with metals and oxide

It reacts with metals at a high temperature to produce metal oxides as shown in the below chemical reaction;

4 Fe + 6 NaOH → 2 Fe₂O3 + 6 Na + 3 H₂

Some transition metals such as aluminium react strongly with Sodium hydroxide.

iii) Reaction with acidic oxides

It also reacts with acidic oxides. This reaction is used to purify harmful acidic gases as shown in the following chemical reaction

2NaOH + SO₂ → Na₂SO₃ + H₂O

Safety Hazards or Risks associated with NaOH

• It is a strong acid, which is highly corrosive and thus can decompose or burn living tissues such as skin.
• It can cause blindness when comes in contact with the eye.
• Its solvation is highly exothermic which can cause splashing.

Physical properties of Sodium Hydroxide

• In its purest form, it is found as a white crystalline solid.
• It is odourless, density is 2.13 g.ml and melting point is 318 degrees centigrade.
• It dissolves easily in water, ethanol, and glycerol.
• It leads to a highly exothermic reaction when dissolved in water.
• Its viscosity is higher than water; 78 mPas.
• It can be used to form hydrates.
• It has a high tendency to absorb carbon dioxide and water from the air.
• It can be found in liquid form.

Chemical properties of sodium hydroxide

• It contains an ionic bond between sodium and hydroxide ions.
• It is a strong base, so, reacts easily with acids to form water and salts.
• It has high acidity, indicated by its high pH value of 13.
• It is soluble in polar solvents like water, ethanol, and methanol.
• It is insoluble in non-polar solvents like benzene, carbon tetrachloride, etc.

Uses of Sodium Hydroxide

• It is widely used in the paper, petroleum and textile industries.
• It is required in the production of soaps and detergents.
• It is used in the Bayer process, in which aluminium is produced.
• It is also needed in industries for cleaning purposes and pH regulation.

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