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Carbon monoxide
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.
Sources
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 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 Town gas has now been replaced by natural gas (methane).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
Toxicity
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.
History
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 compund containing carbon and oxygen by the english chemist William Cruikshank in the year 1800.
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