}} Nitrogen dioxide is the chemical compound with the formula NO₂. It is one of the several nitrogen oxides. This reddish-brown toxic gas has a characteristic sharp, biting odor. NO₂ is one of the most prominent air pollutants.

Preparation and reactions

Nitrogen dioxide (NO₂) arises from the aerobic oxidation of nitric oxide:

2 NO + O₂ → 2 NO₂

In the laboratory, NO₂ can be prepared by treating of nitric acid (HNO₃) with copper metal. The reaction is the following:

4 HNO₃ + Cu → Cu(NO₃)₂ + 2 NO₂ + 2 H₂O

NO₂ exists in equilibrium with its dimer, N₂O₄, which is colourless and diamagnetic.

Safety and pollution considerations

Nitrogen dioxide is toxic by inhalation but the material is so acrid that accidental poisoning is easily avoided. For example, fuming nitric acid is often contaminated with NO₂. 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 anesthetize the nose, thus creating a potential for overexposure.

Long-term exposure to NO₂ at concentrations above 40–100 µg/m³ causes adverse health effects.

Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:

2O₂ + N₂ → 2 NO₂

The most important sources of NO₂ are internal combustion engines , thermal power stations and, to a lesser extent, pulp mills. Atmospheric nuclear tests are also a source of nitrogen dioxide, which is responsible for the reddish colour of mushroom clouds The excess air required for complete combustion of fuels in these processes introduces nitrogen into the combustion reactions at high temperatures and produces nitrous oxides (NOx). Limiting NOx production demands the precise control of the amount of air used in combustion.

The map shown below, depicting results of satellite measurements over Europe, 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.

See also

• Nitryl
• 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 that is short lived because it converts to NO2 in the presence of free oxygen.
• NO_x = all of the above in unspecified proportions but tending toward NO₂.

More esoteric nitrogen oxides include N₂O₅ and the blue species N₂O₃.

Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite (NO₂⁻), nitrate (NO₃⁻), nitronium or NO₂⁺, and nitrosonium or NO⁺. NO₂ is intermediate between nitrite and nitronium:

NO₂⁺ + e⁻ → NO₂

NO₂ + e⁻ → NO₂⁻

• Birkeland-Eyde process

References

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
• Nitrogen dioxide pollution in the world (image)
• Computational Chemistry Wiki