Sodium Hydroxide Synthesis, Reactions and Uses

Sodium hydroxide is a regular laboratory chemical that is white, odorless, and a crystalline solid.

It is a highly caustic and corrosive compound and its crystals absorb moisture from the air when exposed for long. Hence, they get dissolved in the air by moisture. Sodium hydroxide has a melting point at 591K.

sodium hydroxide pellets
sodium hydroxide pellets in the glass plate

Chemically, sodium hydroxide is an ionic compound and is highly water-soluble.

When dissolved in water, it gives an alkaline solution.

It also absorbs CO2 from the air to give Na2CO3.

  2 NaOH + CO2→ Na2CO3 + H2O

Sodium hydroxide is a monoacidic base (i.e., each NaOH molecule releases a single OHion). In water, it readily separates into ions of Na+ and OH.

As it releases hydroxyl ions easily, even under low concentrations, known as a strong base.

NaOH→ Na+ + OH

Synthesis of sodium hydroxide

NaOH is generally prepared by Castner-Kellner cell, a process developed in the mid-1800s.

The cell consists of a tank and an anode and a cathode (electrodes).

A sodium chloride (brine) solution is put in the tank.

Na+ and Cl ions split and the Cl ions are attracted to the anode (positive electrode) where it combines loses its electron and combines to become Chlorine gas.

The free sodium ions are present in the tank. The water splits into ions with the cathode (negative electrode), attracting the hydrogen ions (which form into hydrogen gas) and hydroxide (OH) ions.

The Na+ and OH ions combine to create sodium hydroxide.

NaCl→Na+ + Cl

At Anode:  2Cl → Cl2 + 2e

H2O→ H+ + OH

Na+ + OH → NaOH

Chemical Reactions of NaOH

NaOH reacts with acids to give salt and water.

With hydrochloric acid, it reacts to produce sodium chloride salt and water.

NaOH + НСl (diluted) → NaCl (sodium chloride)+ H2O

On reacting with dilute hydrogen sulfate it produces sodium sulfate.

2 NaOH + H2SO4 (diluted) → Na2SO4 + H2O

On the contrary, reacting with concentrated hydrogen sulfate produced Sodium biSulfate.

NaOH + H2SO4 (conc.) → NaHSO4 + H2O

With nitric acid it produces sodium nitrate salt and water.

NaOH + HNO3 (diluted) → NaNO3 + H2O

With dilute fluoric acid, it produces sodium fluoride and water.

NaOH + HF(diluted) → NaF + H2O

On reaction with hydrogen cyanide, it produces sodium cyanide and water.

NaOH (conc.) + HCN → NaCN + H2O

Disproportionation reactions

3S + 6NaOH → Na₂SO3 + 2Na₂S + 3H₂O

3Cl₂ + 6NaOH → NaClO3 + 5Na­Cl + 3H₂O (on heating)

Cl₂ + 2NaOH → NaClO + NaCl + H₂O

NaOH reacts with glycols to give soaps (saponification reaction)

HOCH₂CH₂OH + 2NаOH → NaOCH₂CH₂ONa + 2H₂O

Sodium hydroxide uses

NaOH is a standard commercial and industrial chemical and has many uses.

1) It is used in bauxite refining to give aluminum

Bauxite ore is a hydrated form of Al2O3. Concentration and extraction of Al2O3 are carried out.

    Al2O3(s) + 2NaOH(aq) + 3H2O → 2Na[Al(OH)4] (aq)

    2Na[Al(OH)4] (aq) + CO2(g)→ Al2O3.X H2O(s) + 2NaHCO3

    Al2O3.X H2O(s) → Al2O3(s) + X H2O(g)

    This Al2O3(s) is reduced to give Aluminum.

2) In petroleum refining.

NaOH is used for caustic wash. Caustic wash removes mercaptans and other sulfur compounds from petroleum products, thus making them purer.

3) It is used in the production of soap, lye and detergents.

(CH2OCOC17H35)3 + 3NaOH →3C17H35COONa + (CH2OH)3

Glyceryl ester of stearic acid                   Sodium Stearate/ Soap       Glycerol

4) It is used to produce inorganic salts in the laboratory

For salts like NaCl, NaCN, NaNO3, Na2SO4, etc.

5) It is used as an ingredient in food preservatives.

Changing the alkalinity of the food helps prevent the growth of bacteria and mold.

6) Sodium hydroxide is used to produce sodium hypochlorite (NaClO), a water disinfectant.

 2 NaOH + Cl2 = NaClO + NaCl + H2O

7) To refine raw materials

It is used for wood products and wood bleaching and cleaning.

8) For papermaking

Sodium hydroxide solution is used in both papermaking and paper recycling processes.

Wood chips are pulped to produce fibers by the chemical pulping process. Sodium hydroxide is used to regenerate those chemicals used in the pulping process.  Thus many chemicals are reused, thereby reducing the cost of the process. It is also used in the bleaching process.

It is also used is in the recycling of paper. When the fibers are placed in a solution of sodium hydroxide, they undergo swelling. This swelling helps to remove the ink that is fixed to the fibers. Thus the paper fibers devoid of ink can be reused.

9) Wastewater treatment

NaOH is used in water treatment to remove heavy metal from water and control water acidity levels.

Wastewater is passed into a reactor tank and the pH is adjusted to 9.0 by addition of NaOH or lime (a mixture of calcium oxide or calcium hydroxide. The higher pH will generate the Hydroxide Ion. The Hydroxide ion will react with the heavy metal ion and form a metal hydroxide, leading to precipitation. Thus the wastewater devoid of heavy metals.

10) For fuel cell

It is also used in fuel cell production, which works like batteries to produce electricity in an environmentally safe but efficient manner.

11) For the preparation of fats and oils.

It is used in the preparation of pure fats and oils.

12) In the textile industry

Sodium hydroxide is also used in the textile industry for cotton mercerization. This process adds luster and brightness to the cotton fibers.

For this, the cotton is held under specified tension with an application of caustic soda and wetting agents at room temperature.  Then the fabric is neutralized in an acid bath. The fibers swell up and become rounder. The light reflects off this round surface giving it more luster and a brighter shine. This process is called the mercerization of cotton.

13) NaOH also finds application in the oven and drain cleaning, where it is used to break up fats and clean dirt. It emulsifies the fats and makes the grease easier to clean up.

14) NaOH is used in many pharmaceutical products and medicines like aspirin, anti-coagulants of blood clots, etc.

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