Sodium hydroxide is a white, odorless and crystalline solid substance available as circular pellets in the chemical laboratory.
One should be careful while handling it due to its caustic and corrosive nature.
When exposed to air, the pellets start to melt as they absorb moisture from the air.
Its chemical formula is NaOH and has a melting point of 591K and a boiling point of 1611.15 K.
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 OH– ion). 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
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 extract 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. This caustic wash removes organo-sulfurous (mercaptans) and other sulfur compounds from petroleum products.
This makes the petroleum compounds cleaner and less toxic to the environment.
3) It is used in the manufacture of soap, lye and detergents.
Soaps are the sodium or potassium fatty acids salts. When fats or oils are exposed to NaOH, they undergo saponification (hydrolysis) to form soap.
(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
Salts like NaCl, NaCN, NaNO3, Na2SO4, etc. can be produced in the lab by reaction of NaOH with other chemicals.
For example, Sodium sulfate (Na2SO4) can be produced by the reaction of sulfuric acid and sodium hydroxide.
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) To produce disinfectants
Sodium hydroxide is used to produce sodium hypochlorite (NaClO). This NaClO is used as a water disinfectant.
2 NaOH + Cl2 = NaClO + NaCl + H2O
When added to water, it releases chlorine which acts as a disinfectant.
7) To refine raw materials
It is used for wood products and wood bleaching and cleaning.
8) For paper manufacture
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 the 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.