↪ 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