Nitric acid is an important oxyacid of nitrogen. It was called as `aqua tortis' by alchemists. It means strong water. It was first prepared by Glauber (1650). Later Cavendish (1784) stated that nitric acid may be formed by passing electric sparks through the mixture of nitrogen and moist oxygen. Traces of nitric acid occur in air where it is formed by electric sparks through the mixture of nitrogen and moist oxygen. Traces of nitric acid occur in air where it is formed by electric discharges and is washed down by rain.
1. Laboratory preparation
Nitric acid is prepared in the laboratory by heating a nitrate salt with concentrated sulphuric acid.
NaNO3 + H2SO4 -- > NaHSO4 + HNO3
Vapours of nitric acid are condensed to a brown liquid in a receiver cooled under cold water. Dissolved oxides of nitrogen are removed by redistillation or blowing a current of carbondioxide or dry air through the warm acid.
2. Manufacture of nitric acid
Nitric acid is manufactured by blowing air into an electric arc struck between two water cooled copper electrodes and spread into a disc with the help of a magnetic field at right angle. The serious disadvantage of the method is now obsolete.
3. Ostwald's process
Large quantities of ammonia manufactured by Haber's process are converted into nitric acid by Ostwald's process.
4 NH3 - (Platinum gauze) -- > 4NO + 6H2O
2NO + O2 -- > 2 NO2
4NO2 + 2H2O + O2 -- > 4 HNO3
Dilute nitric acid may be concentrated by distillation until a constant boiling point mixture is obtained (98%). Fuming nitric acid is obtained by distilling this acid with concentrated sulphuric acid. Crystals of pure nitric acid may be obtained by cooling 98% acid in a freezing mixture.
1. It is a colourless fuming liquid when pure, but may be coloured yellow by its dissociation products mainly nitrogen dioxide.
2. It has extremely corrosive action on the skin and causes painful sores.
3. Pure acid has a specific gravity of 1.54. It boils at 359K and freezes to a white solid (m.p. 231K).