Hydrochloric Acid

Hydrochloric acid (Acidum muriaticum, Acid hydrochloricum), a solution of hydrogen chloride (HCl) in water. It occurs in nature in the water of some volcanic springs and is also found in gastric juice (up to 0.5%). Hydrogen chloride can be obtained by the direct combination of H₂ with Cl₂ under the influence of light, electric discharge, spongy platinum, and other catalysts. The most common method of obtaining hydrogen chloride, both in laboratory practice and in industrial conditions, consists in the action of concentrated sulfuric acid on common salt: H₂SO₄ + 2NaCl → Na₂SO₄ + 2HCl. Hydrogen chloride is a colorless suffocating gas that smokes in the air. Specific weight 18.1 in relation to hydrogen and 1.2592 in relation to air. At -4° and a pressure of 25 atm, it condenses into a liquid. Water very energetically absorbs hydrogen chloride (1 liter at 20° dissolves 460 liters of gaseous HCl), forming hydrochloric acid. To obtain HCl in industry, common salt is treated with sulfuric acid in closed vessels first at room temperature, and finally heated in muffle or refractory furnaces at high temperature. The hydrogen chloride released through cooling pipes enters a tower with wet refractory bricks, irrigated with a solution of barium chloride, for purification from sulfuric acid, then, passing through a series of condensers, enters a tower filled with coke, where it meets water flowing counter-current. Thanks to the counter-current principle, complete saturation of water with hydrogen chloride is achieved and the so-called raw HCl is obtained with a content of 32-35% HCl in the form of a yellowish liquid that smokes in the air and contains impurities of iron, sulfuric and sulfurous acids, chlorine, arsenic, and selenium. After purification from the mentioned impurities by treating HCl with barium salts, H₂S, copper shavings, etc., and redistillation, pure HCl is obtained in the form of a colorless, sharply smelling, smoky liquid in the air, specific weight 1.18-1.19. The content of HCl at different specific weights is given in the following table. ¹⁵ Specific weight at - Baumé degrees Percent HCl 1.010 2.14 1.030 4 1 6.15 1.050 6.7 10.17 1.070 9.4 14.17 1.090 11.9 18.11 1.110 14.2 21.92 1.130 16.5 25.75 1.150 18.8 28.57 1.170 20.9 33.46 1.190 23.0 37.23 1.200 24.0 39.11 When concentrated HCl is heated, hydrogen chloride is released until a 25% HCl solution is formed, after which HCl is distilled without decomposition at 110°. The composition of the acid distilled at 110° is HCl·6H₂O. HCl belongs to the class of highly dissociated mineral acids and can be distinguished from weakly dissociated organic acids by the reaction with congo, methyl violet, and Hüzburg's reagent (see). A specific reagent for the chloride ion is silver ion, which in the presence of HCl gives a white curd-like precipitate of AgCl, insoluble in water and strong acids (nitric), soluble in ammonia. HCl dissolves metals and their oxides, forming chlorides. HCl in a mixture with nitric acid (1 mole HNO₃, 3 moles HCl) gives the so-called aqua regia, which dissolves even gold and platinum. Chemically pure HCl after dilution 10 times should satisfy the following requirements: 1) not to stain iodine-starch paper, 2) not to change when passing H₂S through it, 3) not to become turbid from a solution of barium nitrate, 4) not to give a weighable residue after evaporating 10 g, 5) not to give a blue coloration with ferrocyanide, 6) not to give Marsh's test for arsenic (see). HCl finds extensive application in a number of chemical productions, for example, in obtaining chlorine, bleaching powder, sal ammoniac, and many other salts, in the production of dyes, pharmaceutical preparations, etc. 3- and 10% aqueous or alcoholic solution of HCl is used in histological technique for removing lime from tissues (see Decalcination). Fuming HCl is used in dental practice; HCl with specific weight 1.124 is occasionally used in medicine as a caustic and tissue-destructive agent. Diluted HCl (Ac. hydroch. dilutum) with specific weight 1.040-1.061 is taken internally 0.25-1.0 pro dosi, up to 5.0 pro die, in mixtures in dilution 1:100 in chronic gastritis, dilatation of the stomach, dyspepsia, gastralgia; in dilution 5:100 with the addition of mucilage decoctions or sugar water, a tablespoonful every 2 hours in febrile diseases, children's diarrhea, cholera nostras; in dilution 2:100 it is used as a refreshing drink. HCl, being in gastric juice, plays an important biochemical role: 1) it creates acidity optimal for the action of pepsin, 2) it is a good antiseptic, 3) it is a strong natural stimulator of pancreatic juice secretion (see Gastric juice and Acidity).

L. Kuzin. Discovery of HCl in forensic chemical cases and hydrogen chloride in the air in professional poisonings. Objects of forensic chemical research can be vomit, stomach contents and parts of it, with an acid reaction not only on litmus, but also on "congo" paper, dimethylaminoazobenzene, tropaeolin 00 and methyl violet. Objects are extracted with water. The discovery of the chloride ion with silver nitrate leads to the isolation of hydrogen chloride. The aqueous extract is subjected to distillation, placing the distillation flask in a bath of cylinder oil. First, water is distilled, then, when the content of hydrogen chloride reaches 10%, HCl begins to distill (the liquid is evaporated to dryness). In the distillate, HCl is detected with silver nitrate and by the release of chlorine with potassium chlorate. To discover hydrogen chloride in the air, the latter is drawn through absorption bottles with water. In one part of the liquid, the amount of acid is determined by titration with methyl orange, in the other - the amount of chloride ion by titration with Folhard's solution.

A. Stiganoi. Hydrochloric acid in histological technique has found wide application in the preparation of many staining solutions and reagents. Due to its harmful effect on tissue structure, especially nuclei, it is usually used in the form of weak solutions. See "Iron, Berlin Blue, Hemosiderin, Hematoxylin, Histological technique, Weigert's staining methods, Decalcification."