Carbon Dioxide

CARBON DIOXIDE (carbonic anhydride, carbon dioxide, carbonic acid gas, CO₂) was known as early as the 16th century to Paracelsus and van Helmont as a component of air and as a product formed during fermentation, respiration, etc. Carbon dioxide is colorless, has a faint odor, and a slightly acidic taste. Its density is almost 22 times greater than the density of hydrogen and 1.5 times greater than the density of air. CO₂ is known in three states. Carbon dioxide transitions to a liquid state at 0° at a pressure of about 36 atm; the critical temperature is +31.4°; liquid carbon dioxide is colorless, does not mix with water, and dissolves in alcohol, ether, and oils; at 0° its specific gravity is 0.9. When flowing into the air from a pressurized vessel, carbon dioxide turns into a solid snow-like mass, which, by rapidly evaporating and bypassing the liquid state, lowers the temperature to -78°; water, mercury, and other substances freeze upon contact with this mass. In solid form, especially after compression into a dense mass ("dry ice"), carbon dioxide persists for quite a long time in the air, since it requires even more heat to turn into gas than liquid CO₂. Dry ice has great application in technology, in laboratories, and in transport for obtaining low temperatures. Carbon dioxide dissolves well in water, alcohol, and other liquids. At 0°, one volume of alcohol dissolves 4.3 volumes of CO₂; water at 0° and 760 mm pressure dissolves 1.5 volumes, and at 10-12°, 1 volume. The hydrate (H₂CO₃) formed upon dissolution in water, carbonic acid (carbon dioxide), is unknown in free form, since as it forms, it decomposes into CO₂ and H₂O. Carbon dioxide possesses weak acidic properties (pH approx. 6.1); its solutions turn blue litmus red, which disappears upon standing; as a dibasic acid, it gives neutral and acidic salts, carbonates and bicarbonates; salts with alkali metals—both acidic and neutral—dissolve well in water; alkaline-earth salts: acidic ones dissolve, neutral ones do not dissolve. Carbon dioxide does not burn and does not support combustion; only a few substances are capable of removing oxygen from it, for example, Mg and K burn in carbon dioxide, with carbon being released. Carbon dioxide is very widespread B. M. E. Vol. XXXIII. BATHS

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in nature, found in free form and in various compounds; in free form it is found in the air, in water; in the form of compounds (MgCO₃, FeCO₃, limestone) it is part of the earth's crust. The presence of CO₂ in water gives it the ability to dissolve various salts (calcium phosphate, calcium carbonate), as a result of which the destruction of rock occurs. Carbon dioxide is emitted by volcanoes, from mountain fissures, in some caves, by springs, etc. Carbon dioxide is formed during the respiration of animals and plants, during decay and fermentation, and during the combustion of organic substances. Green parts of plants in the light absorb CO₂ and form carbohydrates from it (see Assimilation, Carbohydrates). In the animal organism, carbon dioxide is found as a product of the oxidation of organic substances, in tissues and fluids—both in a free state and in a bound state, in the form of carbonates. Carbon dioxide is a respiratory hormone that acts as an irritant on the respiratory center (see Respiration, Blood Gases). The release of carbon dioxide from the blood in the lungs requires the preliminary conversion of H₂CO₃ into the anhydride, CO₂. Meldrum and Roughton believe that this process (as well as the hydration of CO₂ into H₂CO₃) is catalyzed by a special enzyme (carbonic anhydrase). Carbonic acid and its salts are one of the main buffers of the blood, maintaining the constancy of its active reaction (see). The intensity of metabolic processes in the organism is judged by the amount of carbon dioxide released. To determine the carbon dioxide released during respiration, the methods of Pettenkofer, Pettersson-Palmquist, etc., are used. The Pettenkofer method is based on the absorption of carbon dioxide by a solution of caustic baryta and the determination of the excess of the latter by titration. The Pettersson-Palmquist method is simpler and faster. Carbon dioxide and its compounds have wide practical application. Carbonated waters are used for medicinal purposes in the form of baths and internally. It is believed that carbon dioxide has an excitatory effect on gastric secretion. Carbonated water quenches thirst well, which is why it is recommended to drink it at work at high temperatures. Carbon dioxide is used in chemical industries in the preparation of soda, white lead, etc. Carbon dioxide is obtained in factories either by burning coal or by the action of acids on lime; it can also be obtained by calcining dolomite (MgCO₃) and other compounds. In laboratories, Kipp's apparatus is used to obtain carbon dioxide by acting on marble with hydrochloric acid.