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 a deadly poison by inhalation.

1 Formation and occurrence
2 Reactions
3 Structure and bonding
4 Safety and pollution considerations
5 See Also
6 External links

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Formation and occurrence

Nitric oxide (NO), also a common pollutant, oxidizes in air to the dioxide:

2 NO + O₂ → 2 NO₂

NO₂ is generated by the action of nitric acid on a variety of metals, such as copper or silver.

2 HNO₃ + Ag → AgNO₃ + NO₂ + H₂O

"Red fuming nitric acid" owes its red color to the presence of NO₂.

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.

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Reactions

Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide.

2 NO₂ ↔ N₂O₄

The equilibrium favors NO₂ at higher temperatures. Solid NO₂ can be obtained as a solid from NO₂ by very rapid cooling (for example with liquid nitrogen), although it is commonly contains 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 up to a maximum at about 183 °C, at which point a 2 cm thick layer of the gas between glass is essentially black and opaque. The color of the gas is a good indicator of the concentration of NO₂, which below 183 °C increases with increasing temperature. Above 183 °C, NO₂ converts to NO and O₂.

NO₂ reacts with water, even at ambient temperatures and pressure, where it disproportionates to nitric and nitrous acid. That reaction is used in the Ostwald process, the industrial route to nitrate-based fertilizers and explosives.

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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.

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Safety and pollution considerations

Nitriogen 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 [1]. 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.

Image:NO2 pollution europe hires.jpg

A recent study by researchers at the University of California, San Diego, suggests a link between NO₂ levels and Sudden Infant Death Syndrome [2].

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