Nitrogen dioxide

Nitrogen dioxide (NO2) is one of several nitrogen oxides. Nitrogen oxides represent a mixture of gases designated by the formula NOX. The mixture includes nitric oxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (N2O3), nitrogen tetroxide (N2O4), and nitrogen pentoxide (N2O5).

Nitrogen dioxide is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent air pollutant. Nitrogen dioxide is a paramagnetic bent molecule with C2v point group symmetry.

Nitrogen dioxide is irritating to the upper respiratory tract and lungs even at low concentrations. Only one or two breaths of a very high concentration can cause severe toxicity. Nitrogen dioxide is heavier than air, so that exposure in poorly ventilated, enclosed, or low-lying areas can cause asphyxiation.

The most hazardous of the nitrogen oxides are nitric oxide and nitrogen dioxide; the latter exists in equilibrium with its dimer, nitrogen tetroxide. Nitrogen dioxide is a colorless to brown liquid at room temperature and a reddish-brown gas above 70oF poorly soluble in water. Nitrogen dioxide is toxic by inhalation, but it is acrid and easily detectable by smell. One potential source of exposure is fuming nitric acid, which spontaneously produces NO2 at temperatures above 0C.

The excess air required for complete combustion of fuels in these processes introduces nitrogen into the combustion reactions at high temperatures and produces nitrogen oxides (NOX). Limiting NOX production demands the precise control of the amount of air used in combustion.

Major sources of NO2 are internal combustion engines, thermal power stations, and, to a lesser extent, pulp mills, and butane gas heaters and stoves.


Gas Nitrogen dioxide Gas Density 1.44
Chemical Symbol NO2 Detection Principle Electrochemical
PEL (ppm) 0.5 LEL (%)
IDHL (ppm) 1 UEL (%)
Industries Parking Garages, Tunnels, Power Generation, HVAC description


Industrial Applications


The main use of nitric acid is for the production of fertilizers; other important uses include the production of explosives, etching and dissolution of metals, especially as a component of aqua regia for the purification and extraction of gold, and in chemical synthesis.

Rocket fuel

Nitric acid has been used in various forms as the oxidizer in liquid-fueled rockets. These forms include red fuming nitric acid, white fuming nitric acid, mixtures with sulfuric acid, and these forms with HF inhibitor.

Chemical reagent

In elemental analysis by ICP-MS, ICP-AES, GFAA, and Flame AA, dilute nitric acid (0.5 to 5.0 %) is used as a matrix compound for determining metal traces in solutions. Ultrapure trace metal-grade acid is required for such determination, because small amounts of metal ions could affect the result of the analysis.

It is also typically used in the digestion process of turbid water samples, sludge samples, solid samples and other types of unique samples which require elemental analysis via ICP-MS, ICP-OES, ICP-AES, GFAA and FAA. Typically these digestions use a 50% solution of the purchased HNO3 mixed with Type 1 DI Water.


In a low concentration (approximately 10%), nitric acid is often used to artificially age pine and maple. The color produced is a grey-gold very much like very old wax or oil finished wood (wood finishing)


“Conspec not only supplied detailed installation and operation instructions with my gas monitor, but anytime I had a question they were only a phone call away. I had several of my maintenance staff receive training on calibration over the phone.”


Find Gas Detectors