Principle

↪ A mixture of ammonia gas with air (or oxygen) in the ratio of 1:8 by volume is passed through a catalyst chamber packed with platinum gauze or a rhodium-platinum alloy maintained at a temperature of 800°C.
↪ The ammonia is oxidized under these conditions to nitric oxide.

↪ The nitric oxide so formed is cooled rapidly down to 50°C and led to an oxidizing tower where it combines with oxygen of the air to form nitrogen dioxide at low temperature (50°C) under pressure because the reaction is reversible, exothermic and involves the decrease in volume.

↪ The resulting NO₂ gas mixed with oxygen of the air passes up a tower packed with acid proof stone balls down which a spray of water trickles.
↪ Nitrogen dioxide reacts with water in the presence of oxygen forming nitric acid.

4NO₂ + 2H₂O + O₂ → 4HNO₃

↪ The reaction is exothermic and once the reaction starts it proceeds automatically.

⁕ Flowsheet diagram of Manufacture of HNO₃ by Ostwald's process