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.