Nitrogen dioxide may be present in the form of a yellowish-brown liquid or a reddish-brown gas above 21.1 °C (70 °F) with a pungent acrid odor. It reacts with water to form nitric and nitrous acid and has a vapor pressure of 720 mmHg. It is also a noncombustible liquid or gas that accelerates the burning of combustible materials. Nitrogen dioxide is more toxic than nitrogen oxide.
Nitrogen dioxide may be prepared by several methods. It is produced when an electric discharge is passed through air. It is made commercially from nitric oxide and air. Nitric oxide made by various processes (See Nitric Oxide) rapidly oxidizes to nitrogen dioxide. It is formed by decomposing nitric acid or by oxidizing ammonia with air:
HNO3 → NO2 + H2O
4NH3 + 7O2 → 4NO2 + 6H2O
Also, nitrogen dioxide can be made by heating copper with nitric acid.
In the laboratory, nitrogen dioxide is formed by heating lead nitrate or nitrate of another heavy metal:
2Pb(NO3)2 → 2PbO + 4NO2 + O2
Gaseous mixture of nitrogen dioxide and oxygen is passed through a U-tube placed in a freezing mixture. Nitrogen dioxide condenses and is collected as liquid.
The oxidation state of nitrogen in nitrogen dioxide is +4. The molecule has an unpaired electron. Both these factors contribute to its reactivity. Nitrogen dioxide readily converts to other forms of nitrogen oxides. It coexists in equilibrium with its dimeric form, N2O4. The latter is more stable at ordinary temperatures.
When heated above 150°C, nitrogen dioxide dissociates to nitric oxide and oxygen:
2NO2 → 2NO + O2
Nitrogen dioxide dissolves in cold water, forming a mixture of nitrous acid and nitric acid:
2NO2 + H2O → HNO2 + HNO3
Nitrous acid readily decomposes to nitric acid and nitric oxide:
3HNO2 → HNO3 + NO + H2O
The overall reaction is as follows:
3NO2 + H2O → 2HNO3 + NO
When dissolved in warm water, no nitrous acid forms.
Nitrogen dioxide is a strong oxidizing agent. It oxidizes both nonmetals and metals, forming their oxides and itself reduced to nitrogen. Thus, sulfur, phosphorus and charcoal burn in nitrogen dioxide to yield oxides of these elements and nitrogen:
2NO2 + 2S → 2SO2 + N2
2NO2 + 2C → 2CO2 + N2
Copper, zinc, iron and many other metals are similarly converted to their oxides when heated with nitrogen dioxide:
2NO2 + 2Cu → 2CuO + N2
2NO2 + 4Zn → 4ZnO + N2
Nitrogen dioxide oxidizes an aqueous solution of iodide to iodine, hydrogen sulfide to sulfur, and carbon monoxide to carbon dioxide. In such reaction, it is reduced to nitric oxide, rather than nitrogen:
NO2 + 2I¯ + H2O → I2 + NO + 2OH¯
NO2 + H2S → NO + H2O + S
NO2 + CO → NO + CO2
With stronger oxidizing agents, nitrogen dioxide acts as a reducing agent.
Thus, it reduces per manganate, MnO4¯, to Mn2+ ion, decolorizing its solution. In this reaction, it is oxidized to nitrate ion:
MnO4¯ + 5NO2 + H2O → Mn2+ +2H+ + 5NO3¯
Reaction with fluorine forms nitryl fluoride, NO2F:
2NO2 + F2 → 2NO2F
Nitrogen dioxide reacts with alkalies, giving a mixture of nitrite and nitrate:
2NO2 + 2OH¯ → NO2¯ + NO3¯ + H2O
nitrogen dioxide is a reddish-brown
gas (or yellow liquid) with a strong, acrid odor. Nitrogen dioxide readily dimerizes to produce
N2O4.nitrogen dioxide are nonfl ammable, toxic gases.The federal government has established air quality standards for nitrogen dioxide
at 0.053 partsper million (ppm), which equals 100μg (micrograms) per cubic meter.Nitrogen dioxide is highly soluble in water and forms nitric acid (HNO3), and nitric oxide
is slightly soluble and forms nitrous acid (HNO2).
Nitrogen dioxide is
a strong oxidizing agent and causes corrosion.Nitrogen dioxide is used as an oxidizing agent, a catalyst in oxidation reactions, an inhibitor,
as a nitrating agent for organic reactions, as a flour bleaching agent, and in increasing the
wet strength of paper.
Red to brown gas above 21.1C, brown
liquid below 21.1C; colorless solid approximately
?11C.The pressurized liquid is nitrogen tetroxide (dinitrogen tetroxide) because of admixture of
N
2O4 with NO2,Noncombustible but supports combustion.
Nitrogen dioxide (and nitrogen tetroxide, the
solid dimer) is a dark brown gas (above 21 C) or a yellow,
fuming liquid or colorless solid with a pungent, acrid odor.
The solid form is colorless below about 11 C; it is found
structurally as N2O4.
Reddish-brown gas; pungent irritating odor; liquefies to a yellow liquid at 21.2°C; liquefies under pressure to a brown fuming liquid, commercially known as nitrogen tetroxide which actually is an equilibrium mixture of nitrogen dioxide and dinitrogen tetroxide, N2O4; converts to a colorless crystalline solid at -11.2°C; refractive index 1.40 at 20°C; density of gas in air 1.58 (air=1); density of liquid 1.449 g/mL at 20°C; critical temperature 158.2°C; critical pressure 99.96 atm; decomposes in water forming nitric acid; reacts with alkalies; soluble in concentrated nitric and sulfuric acids; soluble in chloroform and carbon disulfide.
Nitrogen dioxide is an intermediate in producing nitric acid. It also is used in the lead chamber process for making sulfuric acid. It is used as a nitrating and oxidizing agent, in rocket fuels, in the manufacture of hemostatic cotton and other oxidized cellulose compounds, and in bleaching flour. Nitrogen dioxide occurs in trace concentrations in the atmosphere due to oxidation of nitric oxide in air. It also is found in exhaust gases of internal combustion engines, in industrial waste gases from plants using nitric acid, and in cigarette smoke. Brown color of smog in many industrial urban areas is attributed to nitrogen dioxide.
nitrogen dioxide was prepared in 1772 by Joseph Priestley
(1733–1804) and described in his volumes Experiments and Observations of Different Kinds
of Air published between 1774 and 1786. Priestley called nitric oxide nitrous air, nitrogen
dioxide nitrous acid vapor, and nitrous oxide phlogisticated nitrous air, but also referred to the
dioxide. Priestley prepared nitric oxide by reacting nitric acid with a metal such as copper:
3Cu(s) + 8HNO3(aq) → 2NO(g) + 3Cu(NO3)2(aq) + 4H2O(l).
Nitrogen dioxide has been used as a catalyst in
certain oxidation reactions, as an inhibitor to
prevent polymerization of acrylates during distillation, as a nitrating agent for organic compounds, as an oxidizing agent, and as an oxidizer for rocket fuel. It is also used as a flour
bleaching agent in the manufacture of liquid
explosives and for increasing the wet strength of
paper.
Nitrogen dioxide is produced by the reactionof nitric acid with metals or other reducingagents; decomposition of nitrates; when airis heated to high temperatures; and duringfire. It occurs in the exhausts of internalcombustion engines and in cigarette smoke.It is used as an intermediate in the productionof nitric and sulfuric acids, in rocket fuels,as a nitrating and oxidizing agent, and inbleaching flour.
Nitrogen dioxide is an intermediate in producing nitric acid. It also is used in the lead chamber process for making sulfuric acid. It is used as a nitrating and oxidizing agent, in rocket fuels, in the manufacture of hemostatic cotton and other oxidized cellulose compounds, and in bleaching flour. Nitrogen dioxide occurs in trace concentrations in the atmosphere due to oxidation of nitric oxide in air. It also is found in exhaust gases of internal combustion engines, in industrial waste gases from plants using nitric acid, and in cigarette smoke. Brown color of smog in many industrial urban areas is attributed to nitrogen dioxide.
Nitric oxide (nitrogen monoxide, mononitrogen monoxide;
NO) and nitrogen dioxide [nitrogen peroxide, nitrogen
tetroxide (NTO); NO2] are often found in dynamic equilibrium.
Historically, these compounds sometimes have been
erroneously described as “nitrous fumes.” In air, NO is
readily oxidized to NO2, and liquefied NO2 (existing principally
as its dimer nitrogen tetroxide, N2O4) releases NO2 at
room temperature. Thus, these compounds are often
grouped as nitrogen oxides (NOx). Other nitrogen oxides
include nitrogen trioxide (NO3), dinitrogen trioxide (N2O3),
and dinitrogen pentoxide (N2O5). Of all the oxides of nitrogen,
NO2 is the most acutely toxic and has been most
extensively studied. Accordingly, much of this section
focuses on the toxicity of this compound.
Discoveries on the role forNOin biology and medicine led
to a 1998 Nobel Prize for Robert Furchgott, Louis J. Ignarro,
and Ferid Murad. Nitric oxide and NO2 occur
naturally by bacterial degradation of nitrogenous compounds
and to a lesser extent from fires, volcanic action, and fixation
by lightning. NO has been the subject of intense and extensive
research in a vast array of fields including chemistry,
molecular biology, pharmaceuticals, and gene therapy.
Formed endogenously, NO has a physiological role in
blood flow regulation, thrombosis, and neurotransmission,
and a pathophysiological role in inflammation, oxidative
stress, and host defense. NO is derived from the amino
acid L-arginine by five-electron oxidation catalyzed by NO
synthase (requiring reduced pyridine nucleotides, reduced
biopteridines, and calmodulin). The by-product, citrulline,
is recycled back to L-arginine. In the bloodstream,
NO binds primarily hemoglobin, is converted to NO3, and is
eliminated in the urine with a half-life of 5–8 h.
Nitric oxide is manufactured by passing air through an
electric arc or by oxidation of ammonia over platinum gauze.
A brown gas produced by the dissociation
of dinitrogen tetroxide (with which it is in
equilibrium), the dissociation being complete
at 140°C. Further heating causes dissociation
to colorless nitrogen monoxide
and oxygen:
2NO2(g) = 2NO(g) + O2(g)
Nitrogen dioxide can also be made by
the action of heat on metal nitrates (not the
nitrates of the alkali metals or some of the
alkaline-earth metals).
A reddish brown gas or yellowish-brown liquid when cooled or compressed. Shipped as a liquefied gas under own vapor pressure. Vapors are heavier than air. Toxic by inhalation (vapor) and skin absorption. Noncombustible, but accelerates the burning of combustible materials. Cylinders and ton containers may not be equipped with a safety relief device.
Combines with oxygen to form NITROGEN DIOXIDE, a brown gas that is deadly poisonous [Merck 11th ed. (1989]. Decomposes in water to form nitric acid and nitric oxide, reacts with alkalis to form nitrate and nitrites [Merck 11th ed. 1989]. The liquid nitrogen oxide is very sensitive to detonation, in the presence of water.
NITROGEN DIOXIDE (nitrogen peroxide) is a strong oxidizing agent. Powdered aluminum burns in the vapor of carbon disulfide, sulfur dioxide, sulfur dichloride, nitrous oxide, nitric oxide, or nitrogen peroxide [Mellor 5:209-212. 1946-47]. Boron trichloride reacts energetically with nitrogen peroxide, phosphine, or fat and grease [Mellor 5:132. 1946-47]. Nitrogen peroxide and acetic anhydride reacted to form tetranitromethane, but resulted in an explosion [Van Dolah 1967]. Nitrogen peroxide forms explosive mixtures with incompletely halogenated hydrocarbons [Chem. Eng. News 42(47):53. 1964]. During an experiment to produce lactic acid by oxidizing propylene with nitrogen peroxide, a violent explosion occurred. These mixtures (olefins and nitrogen peroxide) form extremely unstable nitrosates or nitrosites [Comp. Rend. 116:756. 1893]. Contact of very cold liquefied gas with water may result in vigorous or violent boiling of the product and extremely rapid vaporization due to the large temperature differences involved. If the water is hot, there is the possibility that a liquid "superheat" explosion may occur. Pressures may build to dangerous levels if liquid gas contacts water in a closed container [Handling Chemicals Safely 1980]. Corrosive to steel when wet, but may be stored in steel cylinders when moisture content is 0.1% or less.
Inhalation may be fatal. Can react strongly
with reducing materials. Lower respiratory tract
irritant. Questionable carcinogen.
The acute toxicity of nitrogen dioxide by inhalation is high. Inhalation may cause shortness of
breath and pulmonary edema progressing to respiratory illness, reduction in the blood's oxygen
carrying capacity, chronic lung disorders and death; symptoms may be delayed for hours and
may recur after several weeks. Toxic effects may occur after exposure to concentrations of 10
ppm for 10 min and include coughing, chest pain, frothy sputum, and difficulty in breathing.
Brief exposure to 200 ppm can cause severe lung damage and delayed pulmonary edema, which
may be fatal. Nitrogen dioxide at concentrations of 10 to 20 ppm is mildly irritating to the eyes;
higher concentrations of the gas and liquid NO2-N2O4 are highly corrosive to the skin, eyes, and
mucous membranes. Nitrogen dioxide can be detected below the permissible exposure limit by
its odor and irritant effects and is regarded as a substance with adequate warning properties.
Animal testing indicates that nitrogen dioxide does not have carcinogenic or reproductive effects.
It does produce genetic damage in bacterial and mammalian cell cultures; however, most studies
in animals indicate that it does not produce heritable genetic damage.
Nitrogen dioxide is a highly toxic gas. It is anirritant to the eyes, nose, and throat and to therespiratory system. The toxic symptoms arecough, frothy sputum, chest pain, dyspnea,congestion, and inflammation of lungs andcyanosis. Even a short exposure can causehemorrhage and lung injury. Death mayresult within a few days after exposure. Toxicsymptoms may be noted in humans followinga 10-minute exposure to a 10 ppm concentration in air. One or two minutes of exposureto 200 ppm can be lethal to humans.
Flammability and Explosibility
Nitrogen dioxide is not combustible (NFPA rating = 0) but is a strong oxidizing
agent and will support combustion. Cylinders of NO2 gas exposed to fire or intense
heat may vent rapidly or explode.
When dry (0.1 percent moisture or less), nitrogen dioxide is not corrosive to mild steel at ordinary temperatures and pressures. Numerous
metals and alloys such as carbon steel, stainless
steel, aluminum, nickel, and Inconel are satisfactory for handling and storage. Under wet
conditions, stainless steels resistant to about 60
percent nitric acid serve best.
Equipment parts, such as valve stems, which
are partly in contact with the atmosphere,
should be stainless steel with sufficient chromium content to resist corrosion caused by
leaks through stuffing boxes. Good quality ceramic bodies and Pyrex are satisfactory for handling wet or dry nitrogen dioxide.
Among the plastics, Teflon and Kel-F films
are most satisfactory. Koroseal and Saran are
useful but have a limited service life. In general,
the vinyl plastics do not hold up well with nitrogen dioxide. Asbestos and asbestos-graphite are
satisfactory for valve stuffing boxes. Koroseal
has given reasonably good service in this use.
For use on pipe threads, graphite-disodium silicate (waterglass) is recommended, and hydrocarbon lubricants should be avoided.
Experimental poison by
inhalation. Moderately toxic to humans by
inhalation. An experimental teratogen.
Other experimental reproductive effects.
Human systemic effects by inhalation:
pulmonary vascular resistance changes,
cough, dpspnea, and other pulmonary
changes. Mutation data reported. Violent
reaction with cyclohexane, F2,
formaldehyde, alcohols, nitrobenzene,
petroleum, toluene. When heated to
decomposition it emits toxic fumes of NOx.
See also NITRIC OXIDE.
Nitrogen dioxide is found in automotive
and diesel emissions. Nitrogen dioxide is an industrial
chemical used as an intermediate in nitric and sulfuric acid
manufacture; it is used in the nitration of organic compounds;
it is used as an oxidizer in liquid propellant rocket
fuel combinations. It is also used in firefighting, welding
and brazing.
A major hazard regarding exposure to nitrogen
dioxide is that serious effects are not felt until
several hours after the exposure. Exposure to
nitrogen dioxide at levels of 90 ppm and higher
has resulted in delayed pulmonary edema occurring anywhere from a few hours to 72 hours
after exposure ceases. Symptoms include cyanosis, shortness of breath, restlessness, headache, and the production of a frothy yellow or
brown sputum. With appropriate treatment,
symptoms usually resolve rapidly, but can persist for several weeks.
Exposures to nitrogen dioxide of 10 minutes
or less at a level of about 150 ppm produces
cough, nose and throat irritation, tearing, headache, nausea, and vomiting. Exposures ranging
from 50 ppm to 150 ppm have been associated
with moderate irritation to the eyes and mucous
membranes. Permanent eye damage can occur, however, if exposures at these levels are prolonged.
Delayed pulmonary edema may follow exposure to 100 ppm to 150 ppm for only 30-60
minutes, while a few breaths at a concentration
of 200 ppm to 700 ppm will produce severe
pulmonary damage that may result in fatal pulmonary edema after 5 to 8 hours have elapsed.
Nitrogen dioxide in 10 ppm to 20 ppm concentrations in air is slightly irritating to mucous
membranes and the upper respiratory tract. The
odor is distinct in concentrations of 5 ppm.Concentrations above 100 ppm in air cause immediate distress. Exposure of the skin to liquid
nitrogen dioxide can cause severe bums.
ACGIH recommends the Threshold Limit
Value-Time-Weighted Average (TLV-TWA)
of 3 ppm (5.6 mg/m3) for nitrogen dioxide. The
TLV-TWA is the time-weighted average concentration for a normal 8-hour workday and a
40-hour workweek, to which nearly all workers
may be repeatedly exposed, day after day, without adverse effect. ACGIH also recommends
the Threshold Limit Value-Short Term Exposure Limit (TLV-STEL) of 5 ppm (904 mg/m3) for nitrogen dioxide. The TLV-STEL is the
IS-minute TWA exposure that should not be
exceeded at any time during a workday even if
the 8-hour TWA is within the TLV-TWA. Exposures above the TLV- TWA up to the STEL
should not be longer than 15 minutes and should
not occur more than 4 times per day. There
should be at least 60 minutes between successive exposures in this range.
OSHA lists a Ceiling Value of 5 ppm (9 mg/m3) for nitrogen dioxide. The Ceiling Value
is the exposure limit that shall not be exceeded
at any time during the working day. If instantaneous monitoring is not feasible, then the ceiling shall be assessed as a IS-minute TWA exposure that shall not be exceeded at any time
during the working day.
Cylinders of nitrogen dioxide should be stored and used
in a continuously ventilated gas cabinet or fume hood.
UN1067/124 Dinitrogen tetroxide, Hazard Class:
2.3; Labels: 2.3-Poisonous gas, 5.1-Oxidizer, 8-Corrosive
material, Inhalation Hazard Zone A. UN1975 Nitric oxide
and dinitrogen tetroxide mixtures or Nitric oxide and
nitrogen dioxide mixtures, Hazard Class: 2.3; Labels:
2.3-Poisonous gas, 5.1-Oxidizer, 8-Corrosive material,
Inhalation Hazard Zone A. Cylinders must be transported
in a secure upright position, in a well-ventilated truck.
Protect cylinder and labels from physical damage. The
owner of the compressed gas cylinder is the only entity
allowed by federal law (49CFR) to transport and refill
them. It is a violation of transportation regulations to refill
compressed gas cylinders without the express written permission
of the owner.
A strong oxidizer. Reacts violently with
combustible matter, chlorinated hydrocarbons; ammonia,
carbon disulfide; reducing materials. Reacts with water,
forming nitric acid and nitric oxide. Attacks steel in the
presence of moisture.
Destroy by incineration with
the addition of hydrocarbon fuel, controlled in such a way
that combustion products are elemental nitrogen, CO2, and
water. Consult with environmental regulatory agencies for
guidance on acceptable disposal practices. Generators of
waste containing this contaminant (≥100 kg/mo) must conform
with EPA regulations governing storage, transportation,
treatment, and waste disposal.
Nitrogen dioxide is available in grades of 99.5
percent or 99.995 percent.
