Name Carbon monoxide
Chemical formula CO
Appearance Colourless 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)
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.
Inhalation Very dangerous, can be fatal.
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.

Table of contents
1 Toxicity
2 Physiological Role
3 Chemistry


Carbon monoxide is emitted by cars, because the temperature is too low to effect complete oxidation of the CO to 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.

CO binds very strongly to the iron atom in hemoglobin, (the principal oxygen-carrying compound in blood); this renders the hemoglobin incapable of releasing oxygen. 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. Hemoglobin acquires a bright red colour when bound to carbon monoxide, so a casualty of CO poisoning can actually look abnormally pink-cheeked and healthy. First aid for carbon monoxide poisoning is to immediately remove the patient from the exposure without endangering yourself, call for help, and if able to do so apply oxygen therapy.

Town gas, used for illumination and heating from the 19th century, 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 Town gas has now been replaced by natural gas (methane).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. The exhaust of the internal combustion engine, when burning a carbon based fuel (i.e. almost any fuel except pure hydrogen) contains carbon monoxide as well.

As other poisons such as cyanide or 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.

During World War II, the Nazis devised a portable gas chamber for their murder of Jews and other minorities: the so-called `gas van', a Diesel engine van, with the CO-laden exhaust piped into the back of the van.

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. There are a number of deaths every year from this cause.

Physiological Role

Carbon monoxide is produced naturally by the body. The breakdown-product of hemoglobin, heme, is a substrate for the enzyme heme 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.


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:

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.

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.

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