Nitric oxide

Nitric oxide also known as nitrogen monoxide is a chemical compound made of one nitrogen and one oxygen atom. It has the chemical formula ·NO, where the · is an unpaired electron. Because of this electron, ·NO is a free radical. Nitric oxide is an important compound in both the chemical industry and in mammal biology.

Properties

Nitric oxide is a colorless gas with a sharp, sweet odor.[1]

Nitric oxide is a rare example of a stable free radical. Most simple radicals with a single electron form a dimer, similar to the halogens. Nitric oxide usually stays as a monomer in the gas phase, only combining to dinitrogen dioxide when it is a liquid.[2]

Reactions

It is easily oxidized to nitrogen dioxide by air. It can react with a mixture of air and water to make nitrous acid. It glows when reacted with ozone.

Preparation

In the chemical industry, nitric oxide is made from ammonia and oxygen in the first step of the Ostwald process. This process uses an alloy of platinum and rhodium as a catalyst. The chemical equation for this reaction is:[3]

4 NH3 + 5 O2 → 4 NO + 6 H2O

Nitric oxide is one of the nitrogen oxides that are made from nitrogen and oxygen in the atmosphere by a lightning strike.[4]

It can be made in small amounts by reacting copper with nitric acid. It can also be made by reducing nitrous acid with compounds such as iron(II) sulfate.

Uses

The main use of nitric oxide in the chemical industry is making nitric acid in the Ostwald process. As of 2016, people made around 60 million tonnes of nitric acid a year, mostly by using the Ostwald process.[5]

Nitric oxide is an important molecule in the biology of mammals. It is used by the circulatory system to control vasodilation, the growing and shrinking of blood vessels.[6]

Safety

Nitric oxide is toxic when breathed in. It also easily is oxidized to nitrogen dioxide, a toxic brown choking substance.

Sources

  1. ↑ "Nitrogen Oxides". Agency for Toxic Substances and Disease Registry. United States Centre for Disease Control and Prevention. Retrieved 2026-05-08.
  2. ↑ Saji, Tijin H. G.; Vlugt, Thijs J. H.; Calero, Sofia; Bagheri, Behnaz (2025). "Modeling nitric oxide and its dimer: Force field development and thermodynamics of dimerization". Physical Chemistry Chemical Physics. 27 (25): 13662–13674. Bibcode:2025PCCP...2713662S. doi:10.1039/D5CP00784D. PMID 40521930.
  3. ↑ Gopakumar, Jithin; Myrstad, Rune; Bo̷Rresen Anda, Rebecka; Øien, Halvor; Enger, Bjo̷rn Christian; Waller, David; Ro̷Nning, Magnus (2025). "Ostwald Process Intensification by Catalytic Oxidation of Nitric Oxide". ACS Omega. 10 (2): 2197–2211. doi:10.1021/acsomega.4c09111. PMC 11755144. PMID 39866597.
  4. ↑ "How Does Lightning Impact Air Pollution?". United States Environmental Protection Agency. 2018-09-05. Retrieved 2026-05-08.
  5. ↑ Thengane, Sonal K.; Bandyopadhyay, Santanu; Mitra, Sagar; Bhattacharya, Sankar; Hoadley, Andrew (2016). "An alternative process for nitric oxide and hydrogen production using metal oxides". Chemical Engineering Research and Design. 112: 36–45. Bibcode:2016CERD..112...36T. doi:10.1016/j.cherd.2016.06.015.
  6. ↑ Lancaster, Jack R. (2015). "Nitric Oxide: A Brief Overview of Chemical and Physical Properties Relevant to Therapeutic Applications". Future Science Oa. 1 (1) FSO59. doi:10.4155/fso.15.59. PMC 5137977. PMID 28031866.
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