Sodium Hydroxide, NaOH

It is also called caustic soda

Manufacture

Sodium hydroxide is manufactured by the following method

Lime soda or causticising process: When a 10% solution of sodium carbonate is allowed to react with lime, calcium carbonate and sodium hydroxide are formed.

Na₂CO₃ + Ca(OH)₂ CaCO₃ (ppt.) + 2NaOH

The reaction is carried out in iron tanks. Lumps of lime are placed in an iron cage suspended in the tank which is filled with sodium carbonate solution and heated by steam to a temperature of 80 − 85°C with continuous stirring. The process is continued until a filtered test sample gives no effervescence with dilute hydrochloric acid. The insoluble calcium carbonate is removed by filtration and the filtrate containing sodium hydroxide is concentrated in multiple effect vacuum evaporators till a concentration of 50% is attained. Further concentration is done by evaporation. This process gives caustic soda of about 98% purity, the impurities being the carbonate, chloride and sulphate of sodium.

Preparation of pure caustic soda

Pure caustic soda is obtained by dissolving the commercial sample in alcohol in which the impurities viz. NaCl, Na₂SO₄, Na₂CO₃, etc. do not dissolve. The filterate is heated to distil off the alcohol when pure caustic soda is left.

Physical Properties

(i) Caustic soda is a white, opaque crystalline solid of density 2.13. It melts at 318°C.

(ii) It is highly hygroscopic and becomes liquid when placed in the atmosphere for some time due to the absorption of atmospheric moisture. It also absorbs CO₂ from the atmosphere.

(iii) It is highly soluble in water and alcohols. Its solubility in water at 0°C is 42 g in 100 g of water.

Chemical Properties

(i) Caustic soda is a strong base: It furnishes hydroxyl ions in aqueous solution.

NaOH Na+ + OH−

(ii) Action on non−metals: Most of the non−metals are attacked by sodium hydroxide. For example,

(a)Halogens: Hypohalites and halates are formed

Cl₂ + 2NaOH(cold) NaCl + NaClO + H₂O

Sodium

hypochlorite

3Cl₂ + 6NaOH(hot) 5NaCl + NaClO3 + 3H₂O

Sodium

chlorate

Br₂ + 2NaOH (cold)   NaBr + NaBrO + H₂O

3Br₂ + 6NaOH (hot)   5NaBr + NaBrO₃ + 3H₂O

I2 + 2NaOH (cold)   NaI + NaIO + H₂O

3I₂ + 6NaOH (hot)   5NaI + NaIO₃ + 3H₂O

(b)Sulphur: Sulphur reacts with caustic soda to form sodium sulphide and sodium thiosulphate.

6NaOH + 4S   2Na₂S + Na₂S₂O₃ + 3H₂O

(c)Phosphorus: Phosphorus gives phosphine gas with caustic soda solution.

4P + 3NaOH + 3H₂O   3NaH₂PO₂ + PH₃↑

(d)Silicon: Silicon gives hydrogen gas with caustic soda

Si + 2NaOH + H₂O   Na₂SiO₃ + 2H₂↑

(iii) Action on metals: Metals like Al, Zn, Sn react with sodium hydroxide solution to give hydrogen.

Zn + 2NaOH   Na₂ZnO₂ + H₂↑

Sodium zincate

2Al + 2NaOH + 2H₂O   2NaAlO₂ + 3H₂↑

Sodium

meta aluminate

Sn + 2NaOH + H2O   Na₂SnO₃ + 2H₂↑

Sodium stannate

(iv) Action on acids: It reacts with acids forming salts.

NaOH + HCl   NaCl + H₂O

2NaOH + H₂SO₄  Na₂SO4 + 2H₂O

NaOH + HNO₃  NaNO3 + H₂O

3NaOH + H₃PO₄  Na₃PO₄ + 3H₂O

(v) Action on ammonium salts: Ammonium salts are decomposed on heating with caustic soda solution with the liberation of ammonia.

NH4Cl + NaOH   NaCl + H₂O + NH₃↑

2NaOH + (NH₄)₂SO₄ Na₂SO₄ + 2NH₃↑ + 2H₂O

(vi) Action on salts: It reacts with various salts to form stable insoluble hydroxides. In some cases the hydroxides decompose. In case of salts of aluminum, tin and zinc, the hydroxides react with sodium hydroxide to form a soluble salt.

(a) MnSO₄ + 2NaOH   Mn(OH)₂ + Na₂SO₄

(b) Ni(NO₃)₂ + 2NaOH   Ni(OH)₂ + 2NaNO₂

(c) FeSO₄ + 2NaOH   Fe(OH)₂ + Na₂SO₄

(d) CuSO₄ + 2NaOH Cu(OH)₂ + Na₂SO₄

(e) AgNO₃ + 2NaOH   2AgOH + 2NaNO₃

2AgOH   Ag₂O + H₂O

(f) ZnCl₂ + 2NaOH Zn(OH)₂ + 2NaCl

Zn(OH)₂ + 2NaOH Na₂ZnO₂ + 2H₂O

(g) AlCl₃ + 3NaOH Al(OH)₃ + 3NaCl

Al(OH)₃ + NaOH   NaAlO₂ + 2H₂O

Uses

(i) In the manufacture of soap paper and viscose rayon (artificial silk).

(ii) In the manufacture of organic dyestuffs.

(iii) For the manufacture of sodium hypochlorites, chlorates and nitrites.

(iv) In petroleum industry for refining.

(v) As a reagent in laboratory.

(vi) For mercerizing cotton