Carbon monoxide



Name Carbon monoxide
Chemical formula CO
Appearance Colorless gas


Formula weight 28.0 amu
Melting point 68 K (-205 °C)
Boiling point 81 K (-192 °C)
Density 8.0 ×103 kg/m3 (liquid)
1.145 kg/m3 (gas at 298K) - lighter than air
Solubility 0.0026 g in 100g water


ΔfH0gas -110.53 kJ/mol
ΔfH0liquid  ? kJ/mol
ΔfH0solid  ? kJ/mol
S0gas, 1 bar 197.66 J/mol·K
S0liquid, 1 bar  ? J/mol·K
S0solid  ? J/mol·K


Ingestion May cause nausea and vomiting.

Very dangerous, can be fatal.

Levels of approximately 100-800 ppm cause headaches of varying severity. Levels higher than these can cause dizziness, nausea, unconsciousness, brain damage and death.

Skin Inhalation may cause skin lesions. Avoid contact with cryogenic liquid.
Eyes Inhalation can cause long-term problems with vision.
More info Hazardous Chemical Database (

SI units were used where possible. Unless otherwise stated, standard conditions were used.

Disclaimer and references

Carbon monoxide, chemical formula CO, is a colourless, odourless, flammable and highly toxic gas. It is a major product of the incomplete combustion of carbon and carbon-containing compounds.



Carbon monoxide has many common sources. The exhaust of the internal combustion engine, when burning a carbon-based fuel (i.e. almost any fuel except pure hydrogen) contains carbon monoxide, especially when the temperature is too low to effect complete oxidation of the hydrocarbons in the fuel to water and CO2, because the time (i.e., the residence time) available in the combustion chamber is too short, or because there is insufficient oxygen present. Usually, it is more difficult to design and operate a combustor for very low CO than for very low unburned hydrocarbons. Carbon monoxide is also present in small but significant concentrations in cigarette smoke. In the home, CO gas forms when fuels like natural gas, oil or wood do not burn completely in appliances such as furnaces and stoves, water heaters, ranges and ovens. Thus, common sources of carbon monoxide include leaky heat exchangers in furnaces; improperly or blocked vents for gas appliances, fireplaces and stoves; idling vehicles in attached or underground garages; or large collections of idling vehicles. Carbon monoxide gases can percolate through concrete hours after vehicles have left a garage.

In the past a significant source of CO was Town gas, used for illumination and heating from the 19th century. Town gas was made by passing steam through red-hot coke; the resultant reaction between the water and carbon generated a mixture of hydrogen and carbon monoxide. Town gas has now been replaced by natural gas (methane). Wood gas, the result of the incomplete combustion of wood, also contains carbon monoxide as a major component.

Sociological notes

As other poisons such as cyanide and arsenic had their availability placed under more and more stringent legal restrictions, the use of carbon monoxide in town gas became the principal method of suicide by poisoning. Later, suicide was often committed by inhaling exhaust fumes of running engines. Air-quality regulations have begun to reduce suicide by this route, as catalytic converters designed to clean up the exhausts remove all but a trace of CO.


CO binds very strongly to the iron atoms in haemoglobin, the principal oxygen-carrying compound in blood. The affinity between CO and haemoglobin is 200 times stronger than the affinity between haemoglobin and oxygen. As CO binds to the haemoglobin, it cannot be released nearly as readily as oxygen would be. Thus, the body's haemoglobin becomes saturated with CO and is rendered incapable of carrying oxygen to the body. A sufficient exposure to carbon monoxide can reduce the amount of oxygen taken up by the brain to the point that the victim becomes unconscious, and can suffer brain damage or even death from anoxia. The brain regulates breathing based upon carbon dioxide levels in the blood, rather than oxygen, so a victim can succumb to anoxia without ever noticing anything up to the point of collapse. Haemoglobin acquires a bright red colour when bound to carbon monoxide, so a casualty of CO poisoning can actually look abnormally pink-cheeked and healthy.

A major problem of accidental CO poisoning that still exists is the use of heaters, particularly gas water heaters and gas fires which are improperly vented. A number of deaths occur every year from this cause. CO poisoning can occur in SCUBA diving due to faulty or badly sited diving air compressors.

First aid for carbon monoxide poisoning is to immediately remove the victim from the exposure without endangering oneself, call for help, apply CPR and if possible apply oxygen first aid. Hyperbaric oxygen therapy is a treatment for carbon monoxide poisoning.

Physiological Role

Carbon monoxide is produced naturally by the body. The breakdown-product of haemoglobin, haeme, is a substrate for the enzyme haeme oxygenase which produces CO and biliverdin. The biliverdin can then be reduced to bilirubin which is excreted by the liver. The CO produced in the brain might act as a neurotransmitter.


Carbon monoxide was first prepared by the French chemist de Lassone in 1776 by heating zinc oxide with coke but thought it to be hydrogen by mistake as it burned with a blue flame. It was identified as a compound containing carbon and oxygen by the english chemist William Cruikshank in the year 1800.


The structure of the CO molecule is best described using molecular orbital theory. The length of the bond (0.111 nm) indicates that it has a partial triple bond character. The molecule has a small dipole moment and is often represented by three resonance structures:

<math>\mbox{C}^-\equiv \mbox{O}^+ \quad\leftrightarrow\quad \mbox{C}=\mbox{O} \quad\leftrightarrow\quad \mbox{C}^+-\mbox{O}^-<math>

Note that the octet rule is violated for the carbon atom in the two structures on the right.

The metal nickel forms a volatile compound with carbon monoxide, known as nickel carbonyl. The carbonyl decomposes readily back to the metal and gas, and this was used as the basis for the industrial purification of nickel.

Many other metals may form carbonyl complexes containing covalently attached carbon monoxide, these can be made by a range of different methods for instance boiling ruthenium trichloride with triphenyl phosphine in methoxyethanol (or DMF) the complex [RuHCl(CO)(PPh3)3] can be obtained. Nickel carbonyl is special as it can be formed by the direct combination of carbon monoxide and nickel metal at room temperature.

As in nickel carbonyl and other carbonyls, the electron pair on the carbon bonded to the metal. In this case carbon monoxide is regarded as a the carbonyl ligand.

Carbon monoxide and methanol are reacted together using a homogenous rhodium catalyst to form acetic acid in the Monsanto process which is responsible for most of the industrial production of acetic acid.

The CAS registry number of carbon monoxide is 630-08-0.

Carbon monoxide in the atmosphere

Missing image
MOPITT 2000 global carbon monoxide

Carbon monoxide has an indirect radiative forcing effect by elevating concentrations of methane and tropospheric ozone through chemical reactions with other atmospheric constituents (e.g., the hydroxyl radical, OH) that would otherwise destroy them. Carbon monoxide is created when carbon-containing fuels are burned incompletely. Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide. Carbon monoxide concentrations are both short-lived in the atmosphere and spatially variable.cs:Oxid uhelnat de:Kohlenstoffmonoxid es:Monxido de carbono eo:Karbona monooksido fr:Monoxyde de carbone id:Karbon monoksida it:Monossido di carbonio he:פחמן חד חמצני nl:Koolmonoxide ja:一酸化炭素 pl:Tlenek węgla pt:Monxido de carbono ru:Монооксид углерода fi:Hiilimonoksidi sv:Kolmonoxid zh:一氧化碳


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