Carbon monoxide is a compound of carbon and oxygen in which the ratio of the two elements is one atom of carbon to one atom of oxygen. Its formula is CO. Carbon monoxide is a colorless, odorless, tasteless, poisonous gas. Most people have heard about carbon monoxide because of its toxic effects. People who live or work in crowded urban areas may become ill with headaches and nausea because of exposure to carbon monoxide in polluted air. In higher concentrations, the gas can even cause death.
The early history of gases such as carbon monoxide is sometimes difficult to trace. Until the early 1600s, scientists did not realize that the material we call air is actually a mixture of gases. As early as the late thirteenth century, Spanish alchemist Arnold of Villanova (c. 1235–1311) described a poisonous gas formed by the burning of wood; this gas was almost certainly carbon monoxide.
Flemish scientist Jan Baptista van Helmont (c. 1580–1644; some sources give death date as 1635) nearly died as a result of inhaling gas carbonum , apparently a mixture of carbon monoxide and carbon dioxide. Credit for the discovery of carbon monoxide, however, is usually given to English chemist and theologian Joseph Priestley (1733–1804). During the period between 1772 and 1799, Priestley gradually recognized the difference between carbon dioxide and carbon monoxide and correctly stated the properties of the latter gas.
Like carbon dioxide, carbon monoxide is formed naturally during the combustion (burning) of wood, coal, and other naturally occurring substances. Huge quantities of carbon monoxide are produced, for example, during a forest fire or a volcanic eruption.
Combustion: Oxidation that occurs so rapidly that noticeable heat and light are produced; burning.
Hemoglobin: An complex iron-containing molecule that transports oxygen through the circulatory system.
Incomplete combustion: Combustion that occurs in such a way that fuel is not completely oxidized ("burned up"). The incomplete combustion of carbon-containing fuels (such as coal and oil) always results in the formation of some carbon monoxide.
Reducing agent: A substance that removes oxygen from some other material.
The relative amounts of carbon monoxide or carbon dioxide that form during combustion depend on two factors: the amount of oxygen present and the combustion temperature. When a large supply of oxygen is present and when the combustion temperature is high, carbon dioxide is more likely to be formed. With limited supplies of oxygen and at lower temperatures, carbon monoxide is produced.
Carbon monoxide is not extracted from the air very easily but is produced commercially by the controlled oxidation of carbon. For example, producer gas is a product made by blowing air across very hot coke (nearly pure carbon). Producer gas consists of three gases: carbon monoxide, carbon dioxide, and nitrogen in the ratio of 6 to 1 to 18. Water gas is made by a similar process—passing steam over hot coke. The products in this case are hydrogen, carbon monoxide, carbon dioxide, and other gases in the ration of 10 to 8 to 1 to 1.
The poisonous character of carbon monoxide has been well known for many centuries. At low concentrations, carbon monoxide may cause nausea, vomiting, restlessness, and euphoria (a feeling of well-being). As exposure increases, a person may lose consciousness and go into convulsions. Death is a common final result. The U.S. Occupational Safety and Health Administration has established a limit of 35 parts per million of carbon monoxide in workplaces where a person may be continually exposed to the gas.
Scientists now know how carbon monoxide poisoning occurs. Normally, oxygen is transported from the lungs to cells by means of red blood cells. This process occurs when oxygen atoms bond to an iron atom in the middle of a complex molecule known as oxyhemoglobin. Oxyhemoglobin is a fairly unstable molecule that breaks down to release free oxygen and hemoglobin for use by the body's cells. The oxygen is then available to carry out reactions in cells from which the body gets energy.
If carbon monoxide is present in the lungs, this sequence of reactions is disrupted. Carbon monoxide bonds with iron in hemoglobin to form carbonmonoxyhemoglobin, a complex somewhat similar to oxyhemoglobin. Carbonmonoxyhemoglobin, however, is a more stable compound than oxyhemoglobin. When it reaches cells, it has little tendency to break apart; instead, it continues to circulate in the bloodstream in its bound form. As a result, cells are unable to obtain the oxygen they need for energy production, and the symptoms of carbon monoxide poisoning begin to appear.
Carbon monoxide poisoning—at least at moderate levels—is so common in everyday life that carbon monoxide detectors, similar to smoke alarms, are found in many businesses and homes. Poorly ventilated charcoal fires, improperly installed gas appliances, and exhaust from automobiles and trucks are the most common sources of the gas. In fact, levels of carbon monoxide in the air can become dangerously high in busy urban areas that have large numbers of cars and trucks. Cigarette smokers may also be exposed to harmful levels of the gas. Studies have shown that the one-to-two pack-a-day smoker may have up to 7 percent of the hemoglobin in her or his body tied up in the form of carbonmonoxyhemoglobin.
Carbon monoxide is used in industry primarily as a source of energy and as a reducing agent. Both producer and water gas are burned as fuels for a variety of industrial operations. As a reducing agent, carbon monoxide is used to convert the naturally occurring oxide of a metal to the pure metal. When carbon monoxide is passed over hot iron oxides, for example, the oxides are converted to metallic iron.