Sodium and its compounds

Sodium and its compounds

Sodium is a typical alkali metal. It occurs naturally in its ores chiefly as sodium chloride (common salt), sodium trioxonitrate(V) (Chile saltpetre) and sodium trioxocarbonate(IV) (soda ash). Sodium is extracted industrially by the Downs Process. Here, common salt is electrolysed in the molten condition. Calcium chloride is added to common salt to serve as a flux (to bring the melting point of common salt from 800°C to about 600°C). In the molten sodium chloride are Na+ and Cl- ions.

Sodium is used:

i. In the manufacture of lead(IV)tetraethyl, an anti-knock addictive used in petrol.

ii. In sodium vapour lamps (having intensity yellow illumination).

iii. In the production of titanium needed for the manufacture of heat-resistant alloys in rockets.

iv. In an alloy for making coolants for nuclear reactors.

Sodium has some important compounds (e.g NaCl, NaOH, Na2SO4, Na2CO3) that are useful industrially.

1. Sodium chloride (NaCl): When pure, it is not deliquescent (dampness is due to impurities (like MgCl2) which are deliquescent). It is a convenient starting material for manufacturing compounds like caustic soda, washing soda, baking soda and salt-cake. It is also used for the preparation of chlorine gas and hydrogen chloride needed for the manufacture of bleaching agents and fine chemicals. In addition, sodium chloride is used in salting out process in soap making or in solvent extraction of organic substance from water.

2. Sodium hydroxide (NaOH) is prepared industrially by the electrolysis of brine (a saturated solution of sodium chloride). The pellet is deliquescent. Its dissolution in water is exothermic. The resulting aqueous solution is alkaline (turns red litmus blue).

Sodium hydroxide is used in the manufacture of soaps (saponification). It is also used in the textile industries for treatment of cotton. Also, an aqueous sodium hydroxide solution is used in qualitative analysis to form the hydroxides of metal ions in solution as precipitates. This facilitates the identification of some actions in unknown samples.

3. Sodium tetraoxosulphate(VI) (Na2SO4):

It is prepared in the laboratory by neutralizing sodium hydroxide solution by dilute H2SO4. The hydrated form is the Glauber's salt, Na2SO4.10H2O.

Sodium tetraoxosulphate(VI) is used:

i. In the manufacture of glass (a mixture of two silicates e.g. Na2SiO3 and CaSiO3)/

ii. In the manufacture of sodium sulphide (by heating it with coke).

Sodium sulphide is used for stripping the hair from hides and for making shaving powder.

iii. In drying the ethereal layer during solvent extraction of organic compounds from water.

4. Sodium trioxocarbonate(IV) (Na2CO3):

It is obtained industrially by the Solvay process. The raw materials are concentrated brine (NaCl), ammonia gas (NH3) and limestone (CaCO3) which yields quicklime (CaO) and carbon(IV)oxide (CO2) when heated. The process involves essentially the following steps:

i. formation of ammoniacal brine when brine is saturated with ammonia gas.

ii. Reaction between the ammoniacal brine and carbon (IV) oxide (produced by heating limestone) to form sodium hydrogen trioxocarbonate(IV) and ammonium chloride.

iii. Sodium hydrogen trioxocarbonate (IV) (not very soluble in water) is filtered from the reaction mixture and the washed.

iv. Heating of sodium hydrogen trioxocarbonate (IV) to sodium trioxocarbonate (IV) and carbon (IV) oxide (which is recycled). Sodium trioxocarbonate (IV) is stable to heat i.e. not decomposed on heating.

v. If the anhydrous sodium trioxocarbonate (IV) (soda ash) is dissolved in hot water and allowed to cool, it gives the crystalline form (washing soda).

The important uses of sodium trioxocarbonate (IV) are:

i. Softening of water for domestic purposes.

ii. Manufacture of glass (formation of silicates). The mixture of Na2SiO3, CaSiO3 and unreacted SiO2 constituent the glass.

iii. Manufacture of caustic soda.