HYDROCHLORIC, or CHLORHYDRIC ACID. (Fa., acide chlorhydrique ; (;1,11., saure.) Constituents, hydrogen and chlorine; formula, IICI ; molecular weight, specific gravity, 18.25.
This most important body, tho only known compound of hydrogen and chlorine, was first obtained in a gaseous state by Priestley in 1772; its solution in water, however, has been used from very early times under the names of muriatic acid—muria, sea salt—spirit of salt, and acidunt hydrochloricum. The ancient alchemists were acquainted with its mixture with nitric acid, which they obtained by distilling nitre, sal-ammoniac, and vitriol together. Mention is made of "spiritue sails," prepared from vitriol and "sal cominunis," in the chemical treatises of the fifteenth century. Glauber pre pared the acid about the year 1648, by heating salt with oil of vitriol, and further allusion, in the writings of Stephen Hales, about the year 1727, is made to a gas which was evolved when sal-ammoniao and sulphuric acid were heated together, and which was soluble in water. This gas Priestley collected over mercury, naming it "marine-acid air." Hydrochloric acid exists in nature in considerablo quantities. It is evolved during volcanic eruptions—notably, those of Vesuvius—and probably, with this origin, is found to an appreciable extent in the hot springs of Iceland and South America, and in the streams which have their sources in volcanio regions and in the craters of extinct volcanoes.
Boussingault gives the following analysis of water taken from the torrent called Rio Vinagrc, which rises in the old volcanoes of Puree, in the Andes : Sulphuric acid .. 0.00110 Magnesia .. traces Hydrochloric acid .. 0.00091 Oxide of iron .. .. traces Alumina .. Soda .. Lime 0.00012 Although the percentage of hydrochloric acid appears small upon analysis—not exceeding part—it is worth noting that something like 12,000 tons per annum are annually carried down by the river cited.
An analysis of water from an acid spring gives the following result :— Sulphuric acid .. 0.005181 Magnesia .. . 0.000320
Hydrochloric acid .. 0.000851 Soda .. 0.000360 Alumina .. 0 . 000500 Oxide of iron .. 0'000365 Limo .. 0•000140 Silica .. 0 . 000183 Hydrochloric acid to the amount of 0.447 per cent. has been found in the lava flowing from Heide. Its presence under all theso circumstances is probably due to the decomposition of the chlorine compounds of the lava by the action of the silicates and steam of a high temperature.
In its gaseous form hydrochloric acid is colourless, 1.267 times heavier than air, with a stronz acid taste and reoetion and powerful suffocating odour.
It is peculiarly irritating to the nostrils, eyes, and skin, and, as the districts where tho manufac ture is carried on readily testify, extremely injurious to all kinds of vegetation. It has not been solidified but at a temperature of 10° (50° F.), and under a pressure of forty atmospheres is obtainable as a clear liquid of 1.27 speeifio gravity, possessing acid properties, but not so strongly marked as in the case of the ordinary commercial acid—the solution of the gas in water. Thus the anhydrous liquid is incapable of dissolving zinc, magnesium, and various metallic sulphides ; it attacks sodium, lead, tiu, and potassium, but no evolution of gas takes place during the operation. Hydrochloric acid supports combustion only to a very limited extent, and is not inflammable. One of the most remarkable properties of the gas is its extreme solubility in water, the enormous volume of 490 times its own bulk being taken up by the liquid at 40° (39° F.), with evolution of consi derable heat. The energy of the operation may be shown by opening a cylinder of the dry gas under water ; it will be noticed that the liquid immediately rushes up, as into a vacuum, the vessel becoming perceptibly warm.
The aqueous saturated solution has a density of 1.2265, and contains 44 per cent. of the dry acid. The following tables give the specific gravity at various degrees of concentration, as delivered by Ur° and Davy. It will be noticed that the results differ slightly.