Fuming Nitric Acid

Fuming Nitric Acid:- Fuming HNO3 contains dissolved NO2 in concentrated nitric acid. Its also obtained by distilling concentrated HNO3 with a little starch. Nitric acid is reduced by starch into NO2 which dissolves in the remaining acid to form fuming nitric acid.

Concentration of nitric acid:- Dilute nitric acid (50-60%) obtained by any to the method is concentrated by distillation till it forms a constant boiling mixture. This is the ordinary azcotropic HNO₃ and 68% in strength. More concentrated HNO3 is produced by distilling the ordinary concentrated acid with concentrated sulphuric acid. The distillate is 98% HNO₃. 100% HNO₃ is obtained by cooling 98% HNO3 in a freezing mixture. The colourless crystals are taken out and melted to get 100% HNO₃. Some times HNO3 obtained is brown yellow. To have colourless HNO₃, either dry air is passed through it or some PbO₂ is added. This helps in removal of dissolved oxides of N from HNO₃ and colourless solution is obtained.