The Ostwald process is the dominant industrial route for making nitric acid from ammonia. Developed and patented by Wilhelm Ostwald in the early 20th century, the process is closely coupled with the Haber process, which supplies the ammonia feedstock. Nitric acid produced by this route is a vital precursor for nitrogenous fertilizers, explosives and many industrial chemicals.

Chemical steps and principal reactions

The Ostwald process converts ammonia into nitric acid through a series of oxidation steps carried out at elevated temperature over a platinum-based catalyst. The commonly cited first-stage reaction is the catalytic oxidation of ammonia to nitric oxide (NO):

  • Ammonia oxidation: 4 NH3 + 5 O2 → 4 NO + 6 H2O (over a platinum catalyst at roughly 900 °C).
  • Oxidation of NO: 2 NO + O2 → 2 NO → 2 NO2 (nitric oxide is readily oxidized to nitrogen dioxide in the gas phase).
  • Absorption to form acid: NO2 reacts with water in an absorption tower to yield dilute nitric acid, with some NO reformed and recycled back to the oxidation stage.

When written as an overall stoichiometric balance, the net conversion for each mole of acid formed is often represented as NH3 + 2 O2 → HNO3 + H2O, with several intermediate exchange and recycle steps required by industrial practice.

Equipment, catalysts and process design

Industrial units use platinum or platinum–rhodium gauzes as the oxidation catalyst. Ammonia and air are passed over these gauzes in a converter at high temperature; the reaction is strongly exothermic so heat-management and temperature control are essential to avoid catalyst damage and unwanted side reactions. Downstream, gas coolers and absorbers capture NO2 and dissolve it into water to produce nitric acid, while unconverted gases and NO/NO2 are recycled to improve yield.

Applications, significance and integration

Nitric acid from the Ostwald process is the principal feedstock for manufacturing ammonium nitrate and other nitrogen fertilizers, and it is used to make nitrates for explosives, pickling agents, and chemical intermediates. Modern plants are often integrated with ammonia production (Haber units), creating an efficient flow of intermediate chemicals and heat between processes.

Environmental, safety and operational considerations

The process generates nitrogen oxides (NOx), which are regulated pollutants because of their role in acid rain and smog formation; rigorous gas treatment and recycling reduce emissions. Catalysts are sensitive to poisons (for example sulfur or arsenic compounds) and mechanical degradation, so gauzes are periodically replaced. Because the reactions occur at high temperature and produce corrosive acid, plants emphasize corrosion-resistant materials, leak prevention and careful control of reaction conditions for safety.

Historical and technical notes

The Ostwald process, patented in 1902, replaced earlier laboratory methods with an industrially scalable route and helped enable the large-scale production of fertilizers that support modern agriculture. Over time, incremental improvements—better catalyst alloys, optimized converter geometry, improved heat recovery and tighter gas recycling—have raised conversion efficiency, reduced emissions and extended catalyst life.

For further technical references and industrial descriptions, see introductions to nitric acid manufacture and materials on process design, and historical treatments of Wilhelm Ostwald and the coevolution of the Haber process. Technical summaries and operating guidance are available from chemical engineering handbooks and standards organizations (fertilizer industry sources), while environmental controls and NOx abatement methods are described in regulatory and engineering literature (process conditions, nitric oxide, nitrogen dioxide).