1. Properties

2. Preparation and reactions
    Main reactions  Basic thermal properties  As an oxidizer   Hydrolysis  Formation from decomposition of nitric acid  Conversion to nitrates  Conversion to nitrites 

3. Ecology

4. Uses

5. Human-caused sources and exposure

6. Toxicity

7. Health effects of {{chem|NO|2}} exposure

8. Avoiding {{chem|NO|2}} toxicity
    Environmental limits 

9. See also

10. References

11. Cited sources

12. External links

Nitrogen dioxide is the chemical compound with the formula {{chem|NO|2}}. It is one of several nitrogen oxides. {{chem|NO|2}} is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year which is used primarily in the production of fertilizers. At higher temperatures it is a reddish-brown gas that has a characteristic sharp, biting odor and is a prominent air pollutant.^[8] Nitrogen dioxide is a paramagnetic, bent molecule with C_2v point group symmetry.

Properties

Nitrogen dioxide is a reddish-brown gas above {{convert|21.2|C|F K}} with a pungent, acrid odor, becomes a yellowish-brown liquid below {{convert|21.2|C|F K}}, and converts to the colorless dinitrogen tetroxide ({{chem|N|2|O|4}}) below {{convert|-11.2|C|F K}}.^[6]

The bond length between the nitrogen atom and the oxygen atom is 119.7 pm. This bond length is consistent with a bond order between one and two.

Unlike ozone, O₃, the ground electronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron,^[9] which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in {{chem|NO|2}} also means that this compound is a free radical, so the formula for nitrogen dioxide is often written as ^•{{chem|NO|2}}.

The reddish-brown color is a consequence of preferential absorption of light in the blue (400 – 500 nm), although the absorption extends throughout the visible (at shorter wavelengths) and into the infrared (at longer wavelengths). Absorption of light at wavelengths shorter than about 400 nm results in photolysis (to form NO + O, atomic oxygen); in the atmosphere the addition of O atom so formed to O₂ results in ozone formation.

Preparation and reactions

Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air:^[10]

2 NO + {{chem|O|2}} → 2 {{chem|NO|2}}

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

{{chem|O|2}} + {{chem|N|2}} → 2 {{chem|NO|}}

In the laboratory, {{chem|NO|2}} can be prepared in a two-step procedure where dehydration of nitric acid produces dinitrogen pentoxide, which subsequently undergoes thermal decomposition:

2 {{chem|HNO|3}} → {{chem|N|2|O|5}} + {{chem|H|2|O}}

2 {{chem|N|2|O|5}} → 4 {{chem|NO|2}} + {{chem|O|2}}

The thermal decomposition of some metal nitrates also affords {{chem|NO|2}}:

2 {{chem|Pb(NO|3|)|2}} → 2 PbO + 4 {{chem|NO|2}} + {{chem|O|2}}

Alternatively, reduction of concentrated nitric acid by metal (such as copper).

4 {{chem|HNO|3}} + Cu → {{chem|Cu(NO|3|)|2}} + 2 {{chem|NO|2}} + 2 {{chem|H|2|O}}

Or finally by adding concentrated nitric acid over tin; hydrated tin dioxide is produced as byproduct.

4 HNO₃ + Sn → H₂O + H₂SnO₃ + 4 NO₂

Main reactions

Basic thermal properties

2 {{chem|NO|2}} {{eqm}} {{chem|N|2|O|4}}

The equilibrium is characterized by {{nowrap|1=ΔH = −57.23 kJ/mol}}, which is exothermic. NO₂ is favored at higher temperatures, while at lower temperatures, dinitrogen tetroxide (N₂O₄) predominates. Dinitrogen tetroxide ({{chem|N|2|O|4}}) can be obtained as a white solid with melting point −11.2 °C.^[10] NO₂ is paramagnetic due to its unpaired electron, while N₂O₄ is diamagnetic.

The chemistry of nitrogen dioxide has been investigated extensively. At 150 °C, {{chem|NO|2}} decomposes with release of oxygen via an endothermic process ({{nowrap|1=ΔH = 14 kJ/mol}}):

2 {{chem|NO|2}} → 2 NO + {{chem|O|2}}

As an oxidizer

As suggested by the weakness of the N–O bond, {{chem|NO|2}} is a good oxidizer. Consequently, it will combust, sometimes explosively, with many compounds, such as hydrocarbons.

Hydrolysis

It hydrolyses to give nitric acid and nitrous acid:

2 {{chem|NO|2}} ({{chem|N|2|O|4}}) + {{chem|H|2|O}} → {{chem|HNO|2}} + {{chem|HNO|3}}

This reaction is one step in the Ostwald process for the industrial production of nitric acid from ammonia.^[11] This reaction is negligibly slow at low concentrations of NO₂ characteristic of the ambient atmosphere, although it does proceed upon NO₂ uptake to surfaces. Such surface reaction is thought to produce gaseous HNO₂ (often written as HONO) in outdoor and indoor environments.^[12]

Formation from decomposition of nitric acid

Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:

4 {{chem|HNO|3}} → 4 {{chem|NO|2}} + 2 {{chem|H|2|O}} + {{chem|O|2}}

The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid.

Conversion to nitrates

MO + 3 {{chem|NO|2}} → {{chem|M(NO|3|)|2}} + NO

Conversion to nitrites

Alkyl and metal iodides give the corresponding nitrites:

2 {{chem|CH|3|I}} + 2 {{chem|NO|2}} → 2 {{chem|CH|3|NO|2}} + {{chem|I|2}}

{{chem|TiI|4}} + 4 {{chem|NO|2}} → {{chem|Ti(NO|2|)|4}} + 2 {{chem|I|2}}

Ecology

Uses

Human-caused sources and exposure

For the general public, the most prominent sources of {{chem|NO|2}} are internal combustion engines burning fossil fuels.^[8] Outdoors, {{chem|NO|2}} can be a result of traffic from motor vehicles.^[17]

Indoors, exposure arises from cigarette smoke,^[18] and butane and kerosene heaters and stoves.^[19]

Workers in industries where {{chem|NO|2}} is used are also exposed and are at risk for occupational lung diseases, and NIOSH has set exposure limits and safety standards.^[6] Astronauts in the Apollo–Soyuz Test Project were almost killed when {{chem|NO|2}} was accidentally vented into the cabin.^[16] Agricultural workers can be exposed to {{chem|NO|2}} arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called "Silo-filler's disease".^[20][21]

Historically, nitrogen dioxide was also produced by atmospheric nuclear tests, and was responsible for the reddish colour of mushroom clouds.^[22]

Toxicity

Gaseous {{chem|NO|2}} diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and dissolves, and chemically reacts with antioxidant and lipid molecules in the ELF; {{chem|NO|2}}'s health effects are caused by the reaction products or their metabolites, which are reactive nitrogen species and reactive oxygen species that can drive bronchoconstriction, inflammation, reduced immune response, and may have effects on the heart.^[23]

Acute harm due to {{chem|NO|2}} exposure is only likely to arise in occupational settings. Direct exposure to the skin can cause irritations and burns. Only very high concentrations of the gaseous form cause immediate distress: 10–20 ppm can cause mild irritation of the nose and throat, 25–50 ppm can cause edema leading to bronchitis or pneumonia, and levels above 100 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema.^[24][25]

For skin or eye exposure, the affected area is flushed with saline. For inhalation, oxygen is administered, bronchodilators may be administered, and if there are signs of methemoglobinemia, a condition that arises when nitrogen-based compounds affect the hemoglobin in red blood cells, methylene blue may be administered.^[26][27]

It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and it is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.^[28]

Health effects of {{chem|NO|2}} exposure

For the public, chronic exposure to {{chem|NO|2}} can cause respiratory effects including airway inflammation in healthy people and increased respiratory symptoms in people with asthma. {{chem|NO|2}} creates ozone which causes eye irritation and exacerbates respiratory conditions, leading to increased visits to emergency departments and hospital admissions for respiratory issues, especially asthma.^[43]

The effects of toxicity on health have been examined using questionnaires and inperson interviews in an effort to understand the relationship between ({{chem|NO|2}}) and asthma. The influence of indoor air pollutants on health is important because the majority of people in the world spend more than 80% of their time indoors.^[29] The amount of time spent indoors depends upon on several factors including geographical region, job activities, and gender among other variables. Additionally, because home insulation is improving, this can result in greater retention of indoor air pollutants, such as ({{chem|NO|2}}) .^[29] With respect to geographic region, the prevalence of asthma has ranged from 2 to 20% with no clear indication as to what's driving the difference.^[29] This may be a result of the “hygiene hypothesis” or "western lifestyle” that captures the notions of homes that are well insulated and with fewer inhabitants.^[29] Another study examined the relationship between nitrogen exposure in the home and respiratory symptoms and found a statistically significant odds ratio of 2.23 (95% CI: 1.06, 4.72) among those with a medical diagnosis of asthma and gas stove exposure.^[30]

A major source of indoor exposure to ({{chem|NO|2}}) is from the use of gas stoves for cooking or heating in homes. According to the 2000 census, over half of US households use gas stoves^[31] and indoor exposure levels of ({{chem|NO|2}}) are, on average, at least three times higher in homes with gas stoves compared to electric stoves with the highest levels being in multifamily homes. Exposure to ({{chem|NO|2}}) is especially harmful for children with asthma. Research has shown that children with asthma who live in homes with gas stoves have greater risk of respiratory symptoms such as wheezing, cough and chest tightness.^[30][51] Additionally, gas stove use was associated with reduced lung function in girls with asthma, although this association was not found in boys.^[32] Using ventilation when operating gas stoves may reduce the risk of respiratory symptoms in children with asthma.

In a cohort study with inner-city minority African American Baltimore children to determine if there was a relationship between ({{chem|NO|2}}) and asthma for children aged 2 to 6 years old, with an existing medical diagnosis of asthma, and one asthma related visit. Families of lower socioeconomic status were more likely to have gas stoves in their homes. The study concluded that higher levels of ({{chem|NO|2}}) within a home were linked to a greater level of respiratory symptoms among the study population. This further exemplifies that ({{chem|NO|2}}) toxicity is dangerous for children.^[33]

Avoiding {{chem|NO|2}} toxicity

While using a gas stove, it is advised to also use ventilation. Studies show that in homes with gas stoves, if ventilation is used while using gas stoves, then children have lower odds of asthma, wheezing and bronchitis as compared to children in homes that never used ventilation.^[34] If venting isn't possible, then replacing gas stoves with electric stove could be another option. Replacing gas stoves with electric ranges could greatly reduce the exposure to indoor NO₂ and improve the respiratory function of children with asthma. It is important to keep gas stoves and heaters in good repair so they are not polluting extra NO₂. 2015 International Residential Code that requires that vent hoods are used for all stoves and set standards for residential buildings. This requires that all range hoods have a vent that discharges outside. You can also prevent NO₂ exposure by avoiding cigarette smoking and not idling your car whenever possible.^[35]

Environmental limits

The U.S. EPA has set safety levels for environmental exposure to {{chem|NO|2}} at 100 ppb, averaged over one hour, and 53 ppb, averaged annually.^[8] As of February 2016, no area of the US was out of compliance with these limits and concentrations ranged between 10–20 ppb, and annual average ambient NO₂ concentrations, as measured at area-wide monitors, have decreased by more than 40% since 1980.^[36]

However, {{chem|NO|2}} concentrations in vehicles and near roadways are appreciably higher than those measured at monitors in the current network. In fact, in-vehicle concentrations can be 2–3 times higher than measured at nearby area-wide monitors. Near-roadway (within about {{convert|50|m|ft}}) concentrations of NO₂ have been measured to be approximately 30 to 100% higher than concentrations away from roadways. Individuals who spend time on or near major roadways can experience short-term NO₂ exposures considerably higher than measured by the current network. Approximately 16% of U.S. housing units are located within {{convert|300|ft|m}} of a major highway, railroad, or airport (approximately 48 million people). Studies show a connection between breathing elevated short-term NO₂ concentrations, and increased visits to emergency departments and hospital admissions for respiratory issues, especially asthma. NO₂ exposure concentrations near roadways are of particular concern for susceptible individuals, including asthmatics, children, and the elderly.^[37]

For limits in other countries see the table in the Ambient air quality criteria article.

See also

• Dinitrogen tetroxide
• Nitric oxide (NO) – pollutant that is short lived because it converts to {{chem|NO|2}} in the presence of ozone
• Nitrite
• Nitrous oxide ({{chem|N|2|O}}) – "laughing gas", a linear molecule, isoelectronic with {{chem|CO|2}} but with a nonsymmetric arrangement of atoms (NNO)
• Nitryl

References

Cited sources

• {{cite book | editor= Haynes, William M. | year = 2011 | title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = CRC Press | isbn = 1439855110}}

External links

• International Chemical Safety Card 0930
• National Pollutant Inventory – Oxides of nitrogen fact sheet
• [https://www.cdc.gov/niosh/npg/npgd0454.html NIOSH Pocket Guide to Chemical Hazards]
• WHO-Europe reports: Health Aspects of Air Pollution (2003) (PDF) and "[https://web.archive.org/web/20050909162153/http://www.euro.who.int/document/E82790.pdf Answer to follow-up questions from CAFE (2004)] (PDF)
• Nitrogen Dioxide Air Pollution
• Nitrogen dioxide pollution in the world (image)
• A review of the acute and long term impacts of exposure to nitrogen dioxide in the United Kingdom IOM Research Report TM/04/03

• Akhand Hindustan
• Akhand (surname)
• Akhand Surname
• Akhasheni, Gurjaani
• Akhat Achirov
• Akhat Akhirov
• Akhateli
• Akhaten
• Akhatwada
• Akhaura-Kulaura-Chhatak line
• Akhaura-Laksam-Chittagong line
• Akhaïa
• Akhbare Islamia
• Akhbār majmūʿa
• Akhdiyat Duta Modjo
• Akhenaten songs
• Akhenaten (verse novel)
• Akhepatar
• Akher Zapheer
• Akhetaa
• Akhethetep
• Akhethetep (disambiguation)
• Akhethetep (father of Ptahhotep)
• Akhethetep Hemi
• Akhet (hieroglyph and season)