Hydrochloric Acid, Or Chlorohydrie Acid, a gaseous compound of one equivalent of chlorine and one of hydrogen (HC1), of combining proportion 36'5, long known in its aqueous solution by the names of muriatic acid, marine salt, and spirit of salt, in reference to its being prepared from sea salt (murias). Priestley first obtained it as a gas in 1772, and Gay-Lussac, Thenard, and Davy long afterward showed that it consists of equal volumes of chlorine and hydrogen, and occupies the same space as the gases which produce it. Its elements mixed together slowly combine by the action of the light, but instantly with explosion if exposed to the direct rays of the sun, or if an electric spark is passed through the mixture, or a lighted taper is brought in contact with it. The gas is obtained by adding concentrated sulphuric acid to common salt placed in a retort, and collecting over mercury. The chlorine of the salt (chloride of sodium) unites with the hydrogen of the sulphuric acid, producing hydrochloric acid and acid sulphate of soda; or, by symbols, NaCl + H2SO4 = HCl + NaHSO4. The gas is colorless, but escaping in the air it instantly unites with moisture present, and forms a white cloud. It has a strongly acid taste and a pungent odor.

Taken into the lungs it is irrespirable, but when diluted with air is not so irritating as chlorine. It neither supports combustion nor is itself inflammable. Under a pressure of 40 atmospheres, at 50° F., it is condensed into a liquid of specific gravity 1.27, which dissolves bitumen. The density of the gas is 1269.5, air being 1000. Its affinity for water is such that it can be kept only in jars over mercury. If a piece of ice be introduced into a jar containing the gas, the ice is instantly 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 nearly its own weight, or about 480 times its bulk of hydrochloric. acid gas, increasing in volume about one third, and acquiring 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 specific gravity 1.203. This loses acid by evaporation, coming, according to Prof. Graham, to 12 equivalents of water to 1 of acid, this containing 25.52 of real acid, and being of specific gravity 1.1197. When reduced by distillation 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 following table by Mr. E. Davy gives its strength at different densities:

Sp. gr.

Quantity of acid per cent

Ml..,

42.43

1.20...

40.40

1.19..

38.38

1.18..

36.36

ML.

34.34

1.16..

32.32

1.15..

30.30

1 14.

20.28

113..

26-26

112..

24.24

1.11..

22.22

Sp. gr.

Quantity of acid per cent.

MO...

20.20

1.09..

18.18

1.08..,

16.16

1.07..

14.14

1.06..

12.12

1.05..

10.18

1.04...

8.08

1.03..

606

1.02..,

4.04

1.01...

2.02

An approximate result is obtained by multiplying the decimal of the specific gravity by 200. - The pure concentrated acid is colorless, and fuming when exposed to the air. It is conveniently used for most purposes diluted to a specific gravity of about 1.1, at which it does not fume. Though powerfully acid, 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. Nitrate of silver, AgN03 (old AgO,N06), 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 proportions 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 solution obtained is of specific gravity about 1.17, and contains 34 per cent. of dry acid. The residuum is common sulphate of soda or Glauber'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 commercial 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, H2SO3, has also been found, to the amount of 7 to nearly 11 per cent. Sulphuric acid is detected by the formation of a white precipitate of sulphate of baryta, produced when chloride of barium, BaCl2, is added to a diluted portion of acid. Traces of sulphurous acid are detected by a mixture of perchloride of iron and ferrocyanide of potassium, Prussian blue being formed by the reducing action of the acid on the mixture.

Arsenic and chloride of lead, PbCl2, may sometimes be detected by a current of sulphuretted hydrogen, H2S (PbCl2 + HS2 = 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 combination with nitric acid it makes the aqua regia, used for dissolving gold and platinum. It is used to furnish chlorine in the preparation of bleaching 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 oxygen, and its chlorine unites with the metallic base of the oxide. - In medicine hydrochloric acid may be 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 artificial gastric juice. It has also been employed as a tonic in various diseases, and as an ingredient 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 administration of hydrochloric acid are to be found in calculous affections, in certain forms of dyspepsia, 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 ulcerated, putrid, and diphtheritic sore throat has often been attended with the happiest results.