Hydrochloric acid gas (HC1; 36.4) dissolved in water, and forming 31.8 B.P., 31.9 U.S.P., per cent. by weight of the solution.

Properties. - A nearly colourless liquid, sp. gr. 1.16. It emits white vapours having a pungent odour, and has a strongly acid taste.

Preparation. - By warming chloride of sodium with sulphuric acid, washing the evolved HC1, and conducting it into cold water by which it is absorbed. Excess of sulphuric acid is employed if glass vessels are used in the preparation either of this or of nitric acid, as the bisulphate of potassium left behind is more soluble than the neutral sulphate, and thus the vessels are more easily cleaned. NaCl + H.2SO4 = NaHSO4 + HC1.

Reaction. - It gives with nitrate of silver a curdy white precipitate soluble in excess of ammonia, insoluble in nitric acid.

Impurities. - Salts; sulphuric acid, with its impurities lead and arsenic; chloride of sodium or chlorine; sulphurous acid formed from sulphuric by organic substances; iron from the apparatus in which it is made commercially.

Arsenic is of importance as an impurity because hydrochloric acid is sometimes used in testing for arsenic by the formation of arseniuretted hydrogen. When testing for arsenic in cases of suspected poisoning both the acid and the zinc must be tested first, in order to ascertain their purity before the suspected substance is added.

Tests. - When diluted with four times its volume of distilled water it gives no precipitate with solution of chloride of barium (absence of sulphuric acid), or with sulphuretted hydrogen (absence of lead or arsenic), and does not tarnish or alter the colour of bright copper foil when boiled with it

(absence of arsenic). When diluted with five volumes of water it should not liberate iodine from iodide of potassium (absence of chlorine); and when 1 c.c. is diluted to 10 c.c. with water and supersaturated with ammonia, the addition of two drops of ammonium sulphide causes no black colour (absence of iron). If a fluid drachm of it mixed with half an ounce of distilled water be put into a small flask with a few pieces of granulated zinc, and while the effervescence continues a slip of bibulous paper wetted with solution of sub-acetate of lead, B.P., or nitrate of silver, U.S.P., be suspended in the upper part of the flask above the liquid for about five minutes, the paper will not become discoloured (absence of sulphurous or arsenious acid, SO2 + 6H = H2S + 2H2O). When evaporated it leaves no residue (no sodium chloride or other fixed impurity).

Preparations Containing Free Hydrochloric Acid

B.P.

Dose.

Acidum Hydrochloricum Dilutum (acid 8, diluted with water up to 26 1/2 by measure)......................................

10-30 m.

Acidum Nitro-hydrochloricum Dilutum......................................

10-30 m.

Liquor Antimonii Chloridi...........................................................

" Arsenici Hydrochloricus.................................................

" Morphinae Hydrochloratis.............................................

U.S.P.

Acidum Hydrochloricum Dilutum (acid 6, water 13 by weight; 5 1/2 and 14 by measure)...........................................

10-30 m.

Acidium Nitro-hydrochloricum...................................................

" " " Dilutum.....................................

10-30 m.

Action and Uses. - It produces symptoms of poisoning like those of sulphuric acid. The stains which it leaves upon the mucous membrane are white. It is rarely used externally. It may be employed to quench thirst in fevers, and to lessen phos-phatic deposits in the urine; it is sometimes useful in cases of sore-throat. As it is the acid of the gastric juice, it may be given after meals in cases of indigestion, where we suspect deficiency of acid (p. 568), and to aid the digestion of food, as well as to relieve thirst in febrile conditions (pp. 360 and 569).