Ostwald process

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The Ostwald process is a chemical process used for making nitric acid (HNO3). Wilhelm Ostwald developed the process, and he patented it in 1902.[1][2] The Ostwald process is a mainstay of the modern chemical industry, and it provides the main raw material for the most common type of fertilizer production.[3] Historically and practically, the Ostwald process is closely associated with the Haber process, which provides the requisite raw material, ammonia (NH3).


Stage 1[edit]

Ammonia is converted to nitric acid in 2 stages. It is oxidized by heating with oxygen in the presence of a catalyst such as platinum with 10% rhodium, platinum metal on fused silica wool, copper or nickel,[4] to form nitric oxide (nitrogen(II) oxide) and water (as steam). This reaction is strongly exothermic, making it a useful heat source once initiated:[5]

H = −905.2 kJ/mol)

Stage 2[edit]

Stage two encompasses two reactions and is carried out in an absorption apparatus containing water. Initially nitric oxide is oxidized again to yield nitrogen dioxide (nitrogen(IV) oxide).[5] This gas is then readily absorbed by the water, yielding the desired product (nitric acid, albeit in a dilute form), while reducing a portion of it back to nitric oxide:[5]

H = −114 kJ/mol)
H = −117 kJ/mol)

The NO is recycled, and the acid is concentrated to the required strength by distillation.

And, if the last step is carried out in air:

H = −348 kJ/mol).[In Absorption Tower].

Typical conditions for the first stage, which contribute to an overall yield of about 98%, are:

  • pressure is between 4–10 standard atmospheres (410–1,000 kPa; 59–150 psi) and
  • temperature is about 870–1,173 K (600–900 °C; 1,100–1,700 °F).

A complication that needs to be taken into consideration is a side-reaction in the first step that reverts the nitric oxide back to nitrogen:

This is a secondary reaction that is minimised by reducing the time the gas mixtures are in contact with the catalyst.[6]

Overall reaction[edit]

The overall reaction is the sum of the first equation, 3 times the second equation, and 2 times the last equation; all divided by 2:

H = −740.6 kJ/mol)

Alternatively, if the last step is carried out in air, the overall reaction is the sum of equation 1, 2 times the equation 2, and equation 4; all divided by 2.

Without considering the state of water,

H = −370.3 kJ/mol)


  1. ^ GB 190200698, Ostwald, Wilhelm, "Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides", published January 9, 1902, issued March 20, 1902 
  2. ^ GB 190208300, Ostwald, Wilhelm, "Improvements in and relating to the Manufacture of Nitric Acid and Oxides of Nitrogen", published December 18, 1902, issued February 26, 1903 
  3. ^ Kroneck, Peter M. H.; Torres, Martha E. Sosa (2014). The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Dordrecht: Springer. p. 215. ISBN 978-94-017-9268-4.
  4. ^ Foist, Laura. "The Ostwald Process & Catalytic Oxidation of Ammonia". Study.com. Retrieved 5 January 2019.
  5. ^ a b c Alan V. Jones; M. Clemmet; A. Higton; E. Golding (1999). Alan V. Jones (ed.). Access to chemistry. Royal Society of Chemistry. p. 250. ISBN 0-85404-564-3.
  6. ^ Harry Boyer Weiser (2007). Inorganic Colloid Chemistry -: The Colloidal Elements. Read Books. p. 254. ISBN 978-1-4067-1303-9.

External links[edit]