What is sodium hydroxide? Its Synthesis, Reactions and Uses
Sodium hydroxide is a very commonly used laboratory chemical. Its physical characteristics are it is white in color, odorless, has a crystalline texture and is solid. Sodium hydroxide is a highly caustic and corrosive compound. It melts at 591K and the crystals of sodium hydroxide are deliquescent i.e. the crystals absorb moisture from the air and hence when exposed for long they get dissolved in by moisture.
Chemically, sodium hydroxide is an ionic compound and dissolves in water to give an alkaline solution. It is highly water-soluble and. NaOH absorbs CO2 from the air to give Na2CO3.
2 NaOH + CO2→ Na2CO3 + H2O
Sodium hydroxide is a monoacidic base (each NaOH releases a single ion of OH–) and readily separates into ions of Na+ and OH– when dissolved in water. As it releases hydroxyl ions easily even under low concentration, it is 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–
Anode: 2Cl– → Cl2 + 2e–
H2O→ H+ + OH–
Na+ + OH– → NaOH
NaOH reacts with acids to give salt and water
NaOH + НСl (diluted) → NaCl + H2O
2 NaOH + H2SO4 (diluted) → Na2SO4 + H2O
NaOH + H2SO4 (conc.) → NaHSO4 + H2O
NaOH + HNO3 (diluted) → NaNO3 + H2O
NaOH + HF(diluted) → NaF + H2O
NaOH (conc.) + HCN → NaCN + H2O
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 is a process of removing 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) It is also used to refine raw materials used for wood products and in 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 bleaching processes
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 to control the 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 their 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 mercerization of cotton. 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 mercerization of cotton.
13) NaOH also finds application in the oven and drain cleaning where it 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.