Concentrated nitric acid is required for many chemical processes. It is produced by feeding the aqueous nitric acid resulting from the oxidation of ammonia (composition about 65% HNO3 by mass), into a concentration unit along with 60 - 67% by mass of concentrated sulphuric acid, H2SO4. This has two functions:

• The concentrated sulphuric acid is a dehydrating agent and will remove water from the nitric acid
• The concentrated sulphuric acid removes the azeotrope (the constant boiling mixture at 68.5% HNO3) from the nitric acid/ water system. Distillation of this mixture produces a vapour with 99% HNO3 (plus some nitrogen dioxide and oxygen from the dissociation of nitric acid during distillation