# Nitrogen dioxide

Structural formula
General
Surname Nitrogen dioxide
other names
• Nitrogen dioxide
• Nitrogen (IV) oxide
• Nitric peroxide
Molecular formula NO 2
Brief description

red-brown, pungent smelling gas

External identifiers / databases
 CAS number 10102-44-0 EC number 233-272-6 ECHA InfoCard 100.030.234 PubChem 3032552 ChemSpider 2297499 Wikidata Q207895
properties
Molar mass 46.01 g mol −1
Physical state

gaseous

density
• 3.663 g l −1 (gas density at 0 ° C)
• 1.439 g cm −3 (liquid at boiling point)
Melting point

−11.2 ° C

boiling point

21.2 ° C

Vapor pressure

963 hPa (20 ° C)

solubility

Hydrolysis in water

safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary

danger

H and P phrases H: 270-280-330-314
EUH: 071
P: 220-244-260-280-303 + 361 + 353 + 315-304 + 340 + 315-305 + 351 + 338 + 315-370 + 376-403-405
MAK
• DFG : 0.95 mg m −3 (recommendation)
• Switzerland: 1.5 ml m −3 or 3 mg m −1
Thermodynamic properties
ΔH f 0

33.2 kJ / mol

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Nitrogen dioxide , NO 2 , is a red-brown, poisonous, pungent gas that smells similar to chlorine. The gas belongs to the group of nitrogen oxides and can easily be liquefied by increasing the pressure or cooling with dimerization to form N 2 O 4 ( dinitrogen tetraoxide ). Nitrogen dioxide has been produced on a large scale since 1908 and used for the production of nitric acid . Nitrogen dioxide is produced in traces from oxygen and nitrogen as the two main components of the atmosphere during natural processes such as B. by lightning as well as in technical combustion processes, z. B. in internal combustion engines .

## Manufacturing

In the laboratory, NO 2 can be produced by heating heavy metal nitrates ( e.g. lead nitrate ).

${\ displaystyle {\ ce {2 Pb (NO_3) _2 -> 2 PbO + 4 NO_2 + O_2}}}$

Technically, nitrogen dioxide is produced as an intermediate product in the synthesis of nitric acid by the air oxidation of nitrogen monoxide (NO). In addition, nitrogen dioxide is formed when nitric acid reacts with copper , since this reaction with the semi-precious metal does not take place with the formation of hydrogen by reducing the acid protons, but with the formation of nitrogen dioxide by reducing the nitrate anions of the nitric acid.

${\ displaystyle {\ ce {Cu + 2 NO_3 ^ - + 4 H ^ + -> Cu ^ 2 + + 2 NO_2 + 2 H_2O}}}$
Formed through natural and industrial processes

Since 1908, nitrogen dioxide has been an intermediate product in the technical production of nitric acid using the large-scale Ostwald process developed by Wilhelm Ostwald . In this process, ammonia is catalytically oxidized with atmospheric oxygen at 600–700 ° C to form nitrogen monoxide , which is further oxidized to nitrogen dioxide in a further step at a significantly lower temperature. The nitrogen dioxide formed is converted into nitric acid with air oxidation with water. During the process, residues of nitrogen dioxide NO 2 and nitrogen monoxide NO get into the air as so-called nitrous gases via the factory chimneys. Because of the reddish color of the gas, these exhaust gases were formerly known as the "red flags on the chimneys in the Ruhr area". Set in the wild, nitrogen dioxide with the highest values ​​collects near the ground.

NO 2 is a by-product of every combustion of fossil fuels such as gas, coal and oil, and is therefore also part of the exhaust gases from motor vehicles and aircraft, from oil and gas boilers and from gas and coal-fired power plants. It is formed from nitric oxide, whereby the concentration of nitrogen dioxide decreases with increasing temperature. At temperatures of 700 to 1700 ° C, only 0.15 to 1.8 ppm are present as nitrogen dioxide when nitrogen monoxide reacts with oxygen. Nitrogen dioxide is only formed to a significant extent below 600 ° C. In internal combustion engines, the maximum formation of nitrogen dioxide is in the temperature range from 200 to 300 ° C.

To a lesser extent, nitrogen dioxide is produced in paper production. In addition, arcing occurs in air, e.g. B. Jacob's ladders .

Thunderstorms are a natural source of atmospheric nitrogen dioxide. It also arises from microbiological reactions in the soil. In indoor spaces, nitrogen oxides are mainly caused by open fireplaces (e.g. gas stoves, water heaters, kerosene lamps or candles) and tobacco smoking (100 to 600 µg NO x per cigarette).

## properties

Bond angle
Nitrogen dioxide at different temperatures

Nitrogen dioxide is a brown-red, characteristic-smelling, extremely corrosive and highly toxic gas that can be easily liquefied. The liquid is red-brown just below the boiling point (21.15 ° C), turns lighter to pale yellow as it cools and solidifies to colorless crystals at −11.20 ° C.

At 150 ° C the decomposition of NO 2 into O 2 and NO begins , at 650 ° C the decomposition is complete.

Nitrogen dioxide has a pungent, throat-irritating odor. It is relatively soluble in water, with nitric acid and nitric oxide being formed with disproportionation .

${\ displaystyle {\ ce {3 NO_2 + H_2O -> 2 HNO_3 + NO}}}$

The paramagnetic NO 2 is in equilibrium with the diamagnetic colorless dinitrogen tetroxide N 2 O 4 , this equilibrium shifting to the left with increasing temperature and almost completely dimerized below 0 ° C to colorless dinitrogen tetroxide. There are (in each case at 1 bar total pressure) at the boiling point about 20%, at 50 ° C 40%, at 100 ° C 90% and at 140 ° C almost 100% of the gaseous dinitrogen tetroxide split into nitrogen dioxide. Liquid dinitrogen tetroxide is still 99.9% undissociated at its boiling point, while solid dinitrogen tetroxide is 99.99% undissociated at its melting point.

${\ displaystyle {\ ce {2 NO2 <=> N2O4 (g)}} \ quad \ Delta H = -57 ~ {\ rm {kJ / mol}}}$
${\ displaystyle {\ ce {2 NO2 <=> N2O4 (fl)}} \ quad \ Delta H = -86 ~ {\ rm {kJ / mol}}}$

This equilibrium also changes the density. At 0 ° C this is 2.05 g / l for the ideal pure gas NO 2 and 4.1 g / l for the ideal gas N 2 O 4 . The real value of 3.6 g / l therefore applies to an equilibrium mixture of the two gases. The critical point is 157.8 ° C, 101.32 bar and 0.557 kg / l; the triple point at −11.20 ° C and 0.1864 bar.

Nitrogen dioxide is a powerful oxidizing agent that keeps combustion (e.g. of potassium, phosphorus, coal, sulfur, hydrogen) lively and can react explosively with organic compounds. Its oxidizing power is roughly the same as that of bromine . It can also act as a reducing agent for strong oxidizing agents such as ozone.

N 2 O 4 and NO 2 behave like the mixed anhydride of nitric acid and nitric acid . With alkali hydroxide solutions they form nitrates and nitrites , e.g. B:

${\ displaystyle {\ ce {2 NO_2 + 2 NaOH -> NaNO_2 + NaNO_3 + H_2O}}}$

When NO 2 is introduced into water, disproportionation to nitric acid and nitrous acid takes place , the latter breaking down in the acidic solution to form NO 2 , NO and water.

In the presence of air, NO is oxidized to NO 2 , so that ultimately all of the NO 2 introduced is converted into nitric acid:

${\ displaystyle {\ ce {4 NO_2 + O_2 + 2 H_2O -> 4 HNO_3}}}$

Nitrogen dioxide promotes increased ozone levels near the ground. It breaks down by UV-A radiation near the ground (320-380 nm) into nitrogen monoxide and atomic oxygen, which react with the oxygen in the air to form ozone. Since ozone in turn reacts with nitrogen monoxide to form nitrogen dioxide, an equilibrium is formed.

Nitrogen dioxide influences the atmospheric chemistry and the ozone content in the troposphere .

Acid rain results from the formation of nitric acid (HNO 3 ) in the earth's atmosphere through the reaction of (2 NO 2 + H 2 O → HNO 3 + HNO 2 ) or through the absorption of N 2 O 5 in aerosol particles and the subsequent formation of NO 3 - in the liquid phase.

## use

Nitrogen dioxide is used to produce nitric acid , for which purpose it is introduced into water and reacts with it. Its dimer, nitrous tetroxide , is used in rocket science as an oxidizing agent. It also serves as a non-aqueous solvent and is used to produce addition compounds with metals (e.g. copper, nickel) and ammonium nitrate (mixed with NO).

## safety instructions

Nitrogen dioxide is very toxic . Inhaled nitrogen dioxide causes headaches and dizziness in higher concentrations . It has a negative effect on respiratory diseases and can lead to shortness of breath and pulmonary edema . Basically, the smell perception of humans towards nitrogen dioxide is regarded as a safe warning sign. Nitrous gases have a characteristic pungent odor and can lead to pulmonary edema with a delay of more than 24 hours (latency period) after inhalation. There are indications of a reduction in fertility as a long-term consequence.

In higher concentrations (> 10 ppm = 20,000 µg / m³) nitrogen dioxide has  an irritating effect on the mucous membranes of the eyes. The extent of the clinical effects depends less on the duration of exposure than on the concentration. The following dose-response relationships were shown for a 60-minute exposure of humans: 100 ppm (200,000 µg / m³) - pulmonary edema with fatal consequences, 50 ppm (100,000 µg / m³) - pulmonary edema with possible consequences of subacute or chronic lung damage, 25 ppm (50,000 µg / m³) - respiratory tract irritation and chest pain. A new study on volunteers has shown that even short-term exposure (3 hours) to 1.5 ppm nitrogen dioxide can increase respiratory reactivity in healthy people. After long-term exposure to 1 ppm nitrogen dioxide, disorders of the lung function (increased airway resistance, reduced lung extensibility and reduced vital capacity) were observed. Asthma patients and people with chronic bronchitis do not seem to be more sensitive than healthy people. Of particular interest are the effects of nitrogen dioxide on the systemic immune status (changes in antibody production, spleen histology and the composition of the lymphocyte subpopulations). A possibly increased disposition of people with an existing immunosuppression to nitrogen dioxide was derived from this.

### Limit values

#### Workplace and interiors

The limit value for nitrogen dioxide of 950 µg / m³ applies to certain industrial and craft workplaces and relates to the mean value of a shift, which is usually eight hours long; it may be exceeded briefly and up to four times per shift by twice (as an average value over 15 minutes, exceedance factor 2). The occupational exposure limit applies within the meaning of the Ordinance on Hazardous Substances for employees who are expected to be exposed to an increased level of nitrogen dioxide as a result of their work. This value is also listed as a guideline occupational exposure limit value in Directive (EU) 2017/164. The value applies to healthy workers eight hours a day and for a maximum of 40 hours a week. Workers who are exposed to harmful substances for work-related reasons also receive occupational health care and are therefore under closer observation than the general population. False reports that the limit value of 950 µg / m³ applies to offices led to criticism of the limit value of 40 µg / m³ for the annual mean value outdoors.

The precautionary limit value of 80 µg / m³ as a 60-minute average and the hazard limit of 250 µg / m³ as a 60-minute average apply to all other indoor spaces in which no corresponding activities are carried out, e.g. B. offices and schools, as well as for living spaces. These values ​​were set by the Committee for Indoor Standard Values ​​(AIR, formerly the ad hoc working group of the Indoor Air Hygiene Commission) of the Federal Environment Agency in 2018. The hazard guideline value, also called short-term guideline value II, represents an effect-related value, which must be acted upon immediately when it is reached or exceeded, since a permanent excess of this guideline value can pose a health risk, especially for sensitive people.

The AIR and the Workplace Ordinance also provide for the limit value of 40 µg / m³ for the annual mean value that applies to the outside air for the assessment of long-term indoor exposure.

#### Outside air

Across Europe, a 1-hour limit value of 200 µg / m³ has been set for nitrogen dioxide in the outside air, which must not be exceeded more than 18 times in a calendar year. The EU Directive 2008/50 / EC - (implemented in German law with the 39th  BImSchV : the regulation on air quality standards and maximum emissions ) sets an annual average of 40 µg / m³. When the limit value came into force (implemented in German law with the ordinance on immission values for pollutants in the air ) in 2002, a tolerance margin of 16 µg / m³ still applied to this annual limit value . It decreased gradually from January 1, 2003 to January 1, 2010 by 2 µg / m³ annually. Since 2010 the tolerance margin no longer applies and the annual limit value of 40 µg / m³ is binding.

In the USA, according to the requirements of the EPA responsible for the national environmental standards, a 1-hour limit value (100 ppb or 191 µg / m³) that is approximately comparable to the EU criteria has been in effect since 2010, but a limit value that is more than twice as high Annual mean as in the EU (53 ppb or 100 µg / m³). In California and sixteen other US states, however, the limit value for the annual mean is only 30 ppb or 57 µg / m³.

An alarm threshold of 400 μg / m³ has been set for nitrogen dioxide in the EU. If this value is measured in three consecutive hours at locations that are representative of the air quality in areas of at least 100 km² or in the entire area / metropolitan area, the member state concerned must immediately take appropriate measures.

### Air pollution

Between 1990 and 2015, nitrogen oxide emissions in Germany decreased by 59%. Despite this significant decrease in emissions, i.e. what power plants, heating systems or cars emit, this did not lead to a comparable reduction in nitrogen dioxide concentration, i.e. what affects people and the environment. In 2016, the EU limit value of a maximum of 40 µg / m³ on an annual average was exceeded at 57% of the traffic-related measuring stations. Road traffic is the main source of nitrogen oxides in urban areas, as the highest concentrations are only measured on busy roads. The annual mean nitrogen dioxide values ​​at measuring stations close to traffic have been between 30 and 60 µg / m³, with a slightly decreasing trend, since 1995, and in some cases even around 90 µg / m³. Around 60% of the proportion of nitrogen oxide emissions caused by motor vehicle traffic is accounted for by diesel vehicles. The reason is, on the one hand, that in extreme cases diesel cars emit up to ten times as much nitrogen dioxide as comparable gasoline-powered vehicles, on the other hand, the very consumption-intensive vehicle classes of buses and trucks mainly use diesel. In urban or suburban areas, the annual mean values ​​for nitrogen dioxide are in the range of around 20 to 30 µg / m³, in rural areas around 10 µg / m³.

### EU infringement proceedings

On May 17, 2018, the European Commission sued Germany, France and the United Kingdom before the Court of Justice of the European Union (ECJ) because the limit values ​​for nitrogen dioxide were not being complied with in these countries. Nor did these Member States take appropriate measures to keep the periods in which the limit values ​​were exceeded as short as possible. The member states had "not proposed any convincing, effective and timely measures to reduce pollution as quickly as possible - as prescribed by EU law - below the agreed limit values." In Germany, the limit values ​​were not complied with in 26 areas. The major cities of Berlin, Munich, Hamburg, Cologne, Stuttgart and Düsseldorf were particularly hard hit. The annual concentrations reported in 2016 were e.g. B. in Stuttgart up to 82 µg / m³ with a limit value of 40 µg / m³. France was convicted in October 2019 and Germany in June 2021.

## Health effects

Nitrogen dioxide exposure poses a significant health risk according to the World Health Organization . It is consistently associated with adverse health effects in the scientific literature.

In an epidemiological study commissioned by the Federal Environment Agency , around 6,000 premature deaths due to cardiovascular diseases that were associated with NO 2 were statistically determined in 2014 . This corresponds to around 1.8 percent of all cardiovascular deaths in Germany. The study also establishes eight percent of the existing diabetes mellitus diseases in Germany in 2014. This corresponds to around 437,000 cases. In the case of existing asthma illnesses, the percentage of illnesses that can be traced back to the exposure is around 14 percent. This corresponds to around 439,000 cases of illness. The causal relationship between NO 2 pollution and lower life expectancy has been questioned by the WHO due to the fact that other toxins often occur together with nitrogen dioxide (fine dust, ozone and others).

At the beginning of 2019 in Germany a supposedly scientific debate about the health dangers of air pollution and nitrogen oxides. This was triggered by a small group of pulmonologists and engineers around the physician Dieter Köhler , who questioned the scientific evidence for pollutant limit values ​​and called for limit values ​​to be softened . Although the scientific evidence on the harmfulness of air pollution had previously grown significantly and negative health effects were shown to occur well below the limit values, the claims of this group were quickly spread via regular media and social media and were welcomed by politicians, industry associations and parts of the public. Using numerous communication channels, including interviews and talk shows , distorted and manipulative claims about the state of research were presented to the public in order to make unsubstantiated false claims and question the findings of scientific studies. The distorted and massive reporting in the media, where the debate was presented in an apparently balanced manner as a supposedly scientific discussion with two (equal) sides, ultimately led to great doubts about the scientific state of affairs being sown among the population. In fact, the supposedly scientific debate bore all the hallmarks of other misinformation campaigns , such as those to deny global warming or the tobacco industry's campaigns to defend their products.

The question of the effect of nitrogen dioxide on health is, however, often only politically motivated, but undisputed among most scientists - it corresponds to the state of science . The climate reporter writes that a wrong balance in the media (cf. controversy over global warming ) would turn the issue into a question of faith .

## Effect on plants

There are numerous plants that are sensitive to nitrogen dioxide - more sensitive than humans. Even small proportions of volume can influence normal biochemical processes and lead, for example, to a reduction in dry weight, leaf growth and a loss of yield in the plants. These include B. apple and pear trees, white birch, barley, lettuce. With these plants, a volume fraction of 3 ppb is sufficient to allow the first external changes to appear after less than an hour after a single exposure. (ppb, stands for “parts per billion” i.e. for the number 10 −9 one billionth.

## Metrological proof of nitrogen dioxide

### Emission measurement

In emission measurements, nitrogen dioxide is often measured together with nitrogen monoxide as a sum parameter. In the chemiluminescence process, a representative partial flow is taken from the exhaust gas and, after flowing through a converter, which converts any nitrogen dioxide that may be present into nitrogen monoxide, is brought into contact with ozone . The light emitted during the reaction is converted by a photomultiplier into an electrical signal that provides information about the nitrogen oxide concentration.

In the ion chromatography process, a defined volume of exhaust gas is converted into nitric acid using ozone or hydrogen peroxide in an aqueous solution. The analytical determination of the nitrate concentration is carried out by ion chromatography.

### Immission measurement

The Saltzman method can be used to measure the immission of nitrogen dioxide , in which the sample air is passed through a reaction solution that reacts with the gas component to be detected to form a red azo dye . The color intensity of the reaction solution is determined photometrically and is a measure of the mass of nitrogen dioxide. This method is also used for the determination of nitrogen dioxide in indoor air.

As with emission measurement, chemiluminescence methods can also be used for immission measurement. The corresponding procedure is described in the EN 14211 standard .

Another possibility of measuring the immission of nitrogen dioxide is the use of passive collectors . A wire mesh prepared with triethanolamine on which nitrogen dioxide is deposited is located in a glass tube that is open at the bottom . At the end of the collection time, the wire mesh is treated with a combination reagent in order to analyze the resulting discoloration photometrically. Passive collectors can also be used in remote locations due to their independence from the network.

## literature

• Alexandra Schneider, Josef Cyrys, Susanne Breitner, Ute Kraus, Annette Peters, Volker Diegmann, Lina Neunhäuserer: Quantification of the environmental burden of disease due to nitrogen dioxide exposure in Germany . In: Umweltbundesamt (Ed.): Environment & Health . tape 2018 , no. 01 , March 5, 2018, ISSN  1862-4340 , ZDB -ID 2732263-4 ( umweltbundesamt.de [PDF; 5.0 MB ]).
• Dustin Grunert: Diesel Engine Emissions: A Health Hazard . Ed .: Federal Medical Association (Association of German Medical Associations) and National Association of Statutory Health Insurance Physicians. tape 115 , no. 10 . Deutscher Ärzteverlag, March 9, 2018, ISSN  0012-1207 , ZDB -ID 1453475-7 , p. A-430 - A-432 ( [1] [PDF; 299 kB ]).

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