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hydrochloric acid:see hydrogen chloridehydrogen chloride,
chemical compound, HCl, a colorless, poisonous gas with an unpleasant, acrid odor. It is very soluble in water and readily soluble in alcohol and ether. It fumes in moist air. It is not flammable, and the liquid is a poor conductor of electricity.
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HCI a strong monobasic acid; a solution of hydrogen chloride in water. Hydrochloric acid is a colorless liquid with the sharp odor of hydrogen chloride. The technical grade has a yellowish green color caused by admixtures of chlorine and iron salts. The maximum concentration of hydrochloric acid is approximately 36 percent; this solution, which has a density of 1.18 g/cm3, fumes in the air because the escaping HCl forms tiny droplets with water vapor.
Hydrochloric acid was known to alchemists in the late 16th century, who obtained it by heating table salt with either clay or green vitriol. In the mid-17th century, J. R. Glauber prepared hydrochloric acid, which he called spirit of salt, by the reaction of NaCl with H2SO4. Glauber’s method is still used.
Hydrochloric acid is one of the strongest acids. It dissolves all metals above hydrogen in the electromotive force series, and the process is accompanied by the evolution of H2 and the formation of salts known as chlorides. Chlorides are also formed when hydrochloric acid reacts with oxides and hydroxides of metals. With strong oxidizers, hydrochloric acid acts as a reducing agent. For example,
MnO2 + 4HC1 = MnCl2 + Cl2 + 2H2O
The two steps involved in the industrial production of hydrochloric acid are the derivation of HCl and the absorption of the substance by water. The principal method of obtaining HCl is synthesis from Cl2 and H2. Large quantities of HCl are formed as a by-product in the chlorination of organic compounds:
RH + Cl2 = RC1 + HCl
where R is an organic radical.
The technical-grade hydrochloric acid currently produced has a strength of not less than 31 percent (synthetic) or 27.5 percent (from NaCl). Commercial hydrochloric acid is called dilute if it contains, for example, 12.2 percent HCl and concentrated if it contains 24 percent or more HCl. In laboratory practice, a 2N solution of HCl (7 percent; density, 1.035) is usually called dilute hydrochloric acid.
Hydrochloric acid is an extremely important product of the chemical industry. It is used to obtain chlorides of various metals and to synthesize chlorine-containing organic products. Other uses include the pickling of metals and the removal of carbonates, oxides, and other sediments and contaminants from various vessels and from the casings used in petroleum engineering. In metallurgy, hydrochloric acid is used to process ores, and in the leather industry, to treat leather before tanning. Hydrochloric acid is an important reagent in laboratory practice. It is transported in glass bottles or in rubber-lined metal containers.
Gaseous HCl is toxic. Prolonged work in an HCl atmosphere causes catarrhs of the respiratory tract, damage to the teeth, ulcération of the nasal mucous membrane, and gastrointestinal disturbances. The HCl content of the air in working areas must not exceed 0.005 mg//. Protective devices include respirators, goggles, rubber gloves, footwear, and aprons.
I. K. MALINA
Hydrochloric acid is contained in gastric juice (approximately 0.3 percent); it promotes digestion and kills pathogenic bacteria.
In medicine, dilute hydrochloric acid is used in drops and mixtures in combination with pepsin for diseases accompanied by insufficient acidity of the gastric juice, such as gastritis, and for hypochromic anemia. In the latter case, the drops and mixtures are used with iron preparations to improve absorption.