Nitrogen dioxide
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The chemical compound nitrogen dioxide (NO2) is a reddish or orange/brown gas with a characteristic sharp, biting odor. It is one of the most prominent air pollutants and an insidious poison by inhalation.
Formation and occurrence
Nitric oxide (NO), also a common pollutant, oxidizes in air to the dioxide:- :2 NO + O2 → 2 NO2
- :2 HNO3 + Ag → AgNO3 + NO2 + H2O
NO2 is generated in biological settings from decomposition of peroxynitrite (ONOO-), a potent oxidizing and nitrating agent formed from the reaction of nitric oxide with superoxide.
Reactions
Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide.- :2 NO2 ↔ N2O4
At −50 °C the crystals of N2O4 are colorless, but honey-yellow at the melting point. The vapour at −10 °C is pale yellow and deepens as the temperature rises
Structure and bonding
NO2 is a radical having a lone - or unpaired - electron, which renders this molecule paramagnetic and is involved in the electronic transitions that give rise to its color. The dimer, N2O4, is diamagnetic.Safety and pollution considerations
Nitrogen dioxide is toxic by inhalation. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthesize the nose, thus creating a potential for overexposure.Long-term exposure to NO2 at concentrations above 40–100 µg/m³ causes adverse health effects [link]. The most important source of NO2 are internal combustion engines, which emit nitrogen oxides near people.
This map, depicting results of satellite measurements, illustrates nitrogen dioxide as large scale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthful levels. Nitrogen dioxide plays a role in atmospheric chemistry, including the formation of tropospheric ozone.
A recent study by researchers at the University of California, San Diego, suggests a link between NO2 levels and Sudden Infant Death Syndrome [link].
See also
- Nitrous oxide or N2O, "laughing gas", a linear molecule, isoelectronic with CO2 but with a nonsymmetric arrangement of atoms (NNO)
- Nitric oxide or NO, a problematic pollutant, related to CO but with one additional electron.
- Nitrogen dioxide, NO2, also a serious pollutant, with a bent structure .
- NOx = all of the above in unspecified proportions but tending toward NO2.
Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite or NO2-, nitrate or NO3-, nitronium or NO2+, and nitrosonium or NO+. NO2 is intermediate between nitrite and nitronium:
- :NO2+ + e → NO2
- :NO2 + e → NO2-
External links
- [International Chemical Safety Card 0930]
- [National Pollutant Inventory - Oxides of nitrogen fact sheet]
- [NIOSH Pocket Guide to Chemical Hazards]
- WHO-Europe reports: [Health Aspects of Air Pollution (2003)] (PDF) and "[Answer to follow-up questions from CAFE (2004)] (PDF)
- [Nitrogen Dioxide Air Pollution] — A summary for non specialists by GreenFacts of the above WHO reports.
- [Nitrogen dioxide pollution in the world (image)]
- [Computational Chemistry Wiki]
- For a full list of external links to MSDSs, spectroscopic data, commercial chemicals suppliers etc. for this compound, see [Chemical sources].
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