(also carbonic anhydride, carbonic acid gas), CO2, carbon (IV) oxide, the highest oxide of carbon. In 1756, J. Black demonstrated that a gas, which he called “fixed” air, is liberated upon decomposition of magnesium carbonate. The gas’s composition was established in 1789 by A. Lavoisier.
Carbon dioxide is a colorless gas with a faintly pungent odor and acid taste; it has a density of 0.0019 g/cm3 (at 0°C and 0.1 meganewtons per sq m [MN/m2]), a melting point of -56.6°C, a boiling point of -78.5°C, a critical temperature of 31°C, and a critical pressure of 7.62 MN/m2, or 76.2 kilograms-force per sq cm (kgf/cm2). At atmospheric pressure and a temperature of -78.5°C, carbon dioxide hardens into a white, snowlike mass known as dry ice, thus bypassing the liquid state. Liquid carbon dioxide exists at room temperature only when the pressure exceeds 5.85 MN/m2 (58.5 kgf/cm2). The density of liquid CO2 is 0.771 g/cm3 at 20°C, while that of the solid form is 1.512 g/cm3. In the gaseous phase, the carbon dioxide molecule has the symmetrical form O=C=O, with a distance between the carbon and oxygen atoms of 1.162 angstroms (Å). Solid CO2 crystallizes in a face-centered cubic lattice, with a = 5.62 Å.
Carbon dioxide is thermostable, dissociating into carbon monoxide and oxygen only at a temperature above 2000°C. The compound is noticeably soluble in water, forming solutions of 0.335 percent (by weight) at 0°C and 0.169 percent at 20°C; it partially reacts with water to form carbonic acid (H2CO3). Carbon dioxide dissolves in the following organic solvents: acetone, benzene, chloroform, and alcohols. It reacts vigorously with bases to yield carbonates. CO2 does not burn and does not support combustion. It is reduced only by very highly reactive metals at high temperatures, for example, by magnesium at 600°C and by calcium at 700°C. Carbon dioxide reacts with red-hot coal: CO2 + C = 2CO, a reaction having great importance in metallurgy. It also reacts with ammonia at a temperature of 160°-200°C and a pressure of 10–40 MN/m2 (100–400 kgf/cm2): CO2 + 2NH3= CO(NH2)2 + H2O. Carbon dioxide reacts with hydrogen in the presence of cupric oxide, forming methane.
Carbon dioxide is a component of air, constituting 0.03 percent by volume; the total content is 2.3 × 1012 tons. In the hydrosphere, there are 1.4 × 1014 tons of carbon dioxide in equilibrium with the atmosphere. Carbon dioxide is formed and introduced into the atmosphere upon the combustion of fuels, the decay of organic matter, the process of fermentation, and the respiration of humans and animals. As a result of industrial pollution, the amount of carbon dioxide in the air of industrial cities greatly exceeds permissible levels. Measures have been taken to reduce this level in a number of industrially developed countries, including the USSR. Carbon dioxide is necessary for the growth of plants, which absorb the compound from the atmosphere in the process of photosynthesis. The atmospheres of the planets Mars and Venus contain carbon dioxide as their major component.
Carbon dioxide is produced industrially mainly by roasting limestone at 900°-1300°C, a process that also yields lime; the compound is purified through its absorption by solutions of soda, potash, or ethanolamine. It is stored and transported in the liquefied state under a pressure of 6 MN/m2 (60 kgf/cm2) in steel cylinders. In the laboratory, CO2 is usually obtained by the reaction of hydrochloric acid with marble.
Carbon dioxide is used in the production of soda water, beer, and sugar. Dry ice is used for the preservation and transport of perishable food products. In the chemical industry, CO2 is consumed in producing soda, urea, and hydroxycarboxylic acids; in graphite-moderated reactors, it functions as a heat carrier. Carbon dioxide is also used in extinguishing fires and transporting flammable substances.
B. A. POPOVKIN
In agriculture, carbon dioxide is used as a fertilizer. An insufficiency of carbon dioxide in the air, which frequently occurs when the ground is shielded, as is especially the case with hydroponic cultivation, lowers the rate of photosynthesis and the crop yield. Gaseous carbon dioxide (from cylinders) or purified products (containing up to 15 percent CO2) of the catalytic combustion of natural gas and solid fuel are introduced during the daytime into hothouses and greenhouses to improve the carbon supply to plants. Solid carbon dioxide (dry ice) can be used as a source of gaseous carbon dioxide; here, pieces of the solid are distributed about an area. Organic and mineral fertilizers that liberate carbon dioxide upon decomposition may also be used as sources. The efficiency of carbon dioxide fertilizers depends on the mineral supply available to plants, the illumination, and the temperature of the soil and air.
In humans and animals, carbon dioxide, together with bicar-bonates, forms an important buffer system of the blood. An increase in the partial pressure of carbon dioxide in the blood strengthens the bond of oxygen to hemoglobin. By acting, both directly and indirectly, on the centers of the medulla oblongata, carbon dioxide figures in the regulation of respiration and blood circulation. A mixture of 95 percent oxygen and 5 percent carbon dioxide (Carbogen) is used in medicine in the case of toxic dosages of narcotics and carbon monoxide poisoning. In high concentrations, carbon dioxide is toxic, inducing hypoxia. Breathing carbon dioxide for a period of several days, even in concentrations of 1.5–3 percent, causes headache, vertigo, and nausea. At concentrations greater than 6 percent (critical level), a person becomes drowsy and unable to work, and there is a weakening of respiratory and cardiac activity, posing a threat to life. An accumulation of carbon dioxide in the air with a concomitant decrease in the oxygen content is seen in enclosed, poorly ventilated spaces, for example, areas in mines and sewers, and in places, such as breweries, where fermentation is occurring. First aid calls for removing the victim into the fresh air and applying artificial respiration. Carbon dioxide does not reach critical levels in the air in residential and public buildings. The concentration of carbon dioxide serves as an environmental indicator of air purity.
V. F. KIRILLOV