Page:The American Cyclopædia (1879) Volume IX.djvu/117

HYDROCHLORIC ACID 109 over mercury. If a piece of ice be introduced into a jar containing the gas, the ice is in- stantly liquefied, and the gas disappears. If the jar be opened under water, the water rushes up as into a vacuum. Water at 40 F. absorbs newly its own weight, or about 480 times its bulk of hydrochloric acid gas, in- creasing in volume about one third, and ac- quiring a density of 1-2109; at this strength it contains nearly 43 per cent, of acid. The aqueous solution is the form in which the acid is commonly known. It is of various degrees of strength, the strongest readily obtained having 6 equivalents of water to 1 of acid, 40'66 per cent, of real acid, and being of spe- cific gravity 1-203. This loses acid by evapo- ration, coming, according to Prof. Graham, to 12 equivalents of water to 1 of acid, this con- taining 25-52 of real acid, and being of spe- cific gravity 1-1197. When reduced by dis- tillation till it changes no more, it contains 16'4 equivalents of water and 20 per cent, of real acid, and is of specific gravity 1-0947. The fol- lowing table by Mr. E. Davy gives its strength at different densities : Sp.gr. 1-21... 1-20... 1-18... 1-18... 1-17... 1-16... 1-15... 114.. 1-18... 1-12... 1-11... Quantity of acid per cent. .... 42-43 .... 40-40 86-86 8484 82-82 80-30 20-28 26-26 24-24 22-22 Quantity of 8p. gr. acid per cent. 1-10 20-20 1-09 18-18 1-08 16-16 1-OT 14-14 1-06 12-12 1-05 10-18 1-04 8-08 1-08 6-06 1-02 4-04 1-01 2-02 An approximate result is obtained by multiply- ing the decimal of the specific gravity by 200. The pure concentrated acid is colorless, and fuming when exposed to the air. It is conve- niently used for most purposes diluted to a spe- cific gravity of about 1-1, at which it does not fume. Though powerfully arid, it is not so corrosive as sulphuric acid. It is decomposed by substances which yield oxygen freely, as the manganese dioxide, and is thus made to furnish chlorine gas, its hydrogen combining with the oxygen of the metallic oxide. Ni- trate of silver, AgNO, (old AgO,NO 6 ), detects its presence by the formation of a white curdy precipitate of chloride of silver, AgCl, which is soluble in ammonia, but not in nitric acid. Ingredients used for preparing hydrochloric acid either upon a large or small scale are common salt, sulphuric acid, and water. Different pro- portions are adopted, the most usual being equal weights of concentrated acid and of salt, or in the large way 6 parts of salt to 5 of acid, being an equivalent of each, to which 5 parts of water are usually added. The acid mixed with about half water is poured when cool upon the salt contained in a large retort, and the remainder of the water is placed in the vessel serving as a condenser to receive the gas. Heat is applied to the retort, and the acid gas distils over ; the water in the condenser allows none of it to escape, so long as it is kept cool and is not saturated. The aqueous solu- tion obtained is of specific gravity about 1'17, and contains 34 per cent, of dry acid. The residuum is common sulphate of soda or Glau- ber's salt. The acid is so cheaply prepared in large chemical works, that it is seldom made in the laboratory. It is an incidental product in the manufacture of carbonate of soda, and was formerly allowed to go to waste. The com- mercial article is often contaminated with iron, which gives it a yellow color, though this is sometimes owing to organic matter, as cork or wood. Sulphuric acid is almost always present in it, and sometimes free chlorine and nitrous acid. Sulphurous acid, H s SOs, has also been found, to the amount of 7 to nearly 11 pel- cent. Sulphuric acid is detected by the forma- tion of a white precipitate of sulphate of bary- ta, produced when chloride of barium, BaClj, is added to a diluted portion of acid. Traces of sulphurous acid are detected by a mixture of perchlorideof iron and ferrocyanide of potas- sium, Prussian blue being formed by the re- ducing action of the acid on the mixture. Arse- nic and chloride of lead, PbCU, may sometimes be detected by a current of sulphuretted hydro- gen, H 2 S (PbCl, + HS 3 = 2HC1 + PbS). The common method of purifying is to dilute, add chloride of barium, and distil. Hydrochloric acid is largely employed in the arts, especially as a solvent for mineral substances. In combi- nation with nitric acid it makes the aquaregia, used for dissolving gold and platinum. It is used to furnish chlorine in the preparation of bleach- ing and disinfectant salts, and in the production of sal ammoniac ; and is employed to extract gelatine from bones. When neutralized with basic oxides, it does not combine as an acid with these, but gives its hydrogen to their oxy- gen, and its chlorine unites with the metallic base of the oxide. In medicine hydrochloric acid may V employed with advantage, largely diluted, to assist the process of digestion, which it does by replacing the deficient portion of the normal acid and of the gastric juice. When administered with pepsine it forms a sort of ar- tificial gastricjuice. It has also been employed as a tonic in various diseases, and as an in- gredient of gargles, when sufficiently diluted. The strong acid may be used as an escharotic. It is much less corrosive than sulphuric acid. When poisoning has occurred from swallowing the strong acid, it should be neutralized by magnesia or soap, and the case then treated as other kinds of corrosive poisoning are. The principal indications for the therapeutic admin- istration of hydrochloric acid are to be found in calculous affections, in certain forms of dys- pepsia, in typhus and typhoid fevers, and in aphthous affections of the mouth and stomach. It may be given in the dose of from 10 to 30 drops three or four times a day, freely diluted with water. Its local application in cases of ul- cerated, putrid, and diphtheritic sore throat has often been attended with the happiest results.