SULPHURIC ACID. (FR., Acide sulphurique ; GER., Schwefelsdure.) Formula of the an hydrous acid of the hydrated acid, H.,SO4.
This acid being an essential means employed in the manufacture of all the important acids which follow in their alphabetical order of treatment, it has been judged desirable to displace it from that order, for the purpose of rendering subsequent descriptions of processes more simple and easier to be understood.
Tho varieties of sulphuric, acid met with in commerce are three in number, viz. anhydrous sulphuric acid, or sulphuric anhydride ; Nordhausen, or fuming acid, a distillation from an hydrous acid, or subhydrate mixed with monohydrate ; and ordinary sulphuric acid, known on the Continent as English acid, composed of monohydrate and a varying proportion of water or higher hydrates, and which when fully concentrated is usually called "oil of vitriol." Sulphuric anhydride is a viscous, crystalline, fibrous, asbestos-like mass, which can be kneaded between the fingers. Its sp. gr. is 1.97 at 20' C. (GP F.). It is colourless and odourless, and burns on the tongue, destroying the moist skin. It melts at 25° C. (77° F.), and boils at 35° C. (95° F.), yielding a colourless vapour, which in the presence of moist atmosphere forme a white mist owing to the formation of sulphuric hydrate. On this account also it fumes at ordinary E 2 temperatures in damp air, and dissolves very rapidly in it. In bright red heat the vapour decomposes into sulphurous acid and oxygen.
Nordhausen acid is an oily, thick-flowing liquid, generally brownish-hued from the presence of organic matters, having a sp. gr. of 1'850 to and containing at 1.854 sp. gr. per cent. by weight of anhydrous acid, and 891 per cent. of monohydrate. It fumes and evaporates rapidly iu a moist atmosphere. Cooled to 0° C. (32° F.), it gives crystals of subhydrate.
Ordinary English sulphuric acid is a combination of sulphur, oxygen, and hydrogen, and may be expressed by any of the following formulae: or Its density or specific gravity is and 100 parts should contain parts anhydrous acid and 18.5 parts
water, or 92-93 per cent. by weight of monohydrate and 8-7 per cent. of water. It behaves very like the sesquihyd•ate; boils at 327° (620° F.), solidifies at a temperature 3 degrees below the freezing point of water, and remelts at 10° (50° F.).
Sulphuric acid is a powerful solvent, nearly all metals being dissolved by it, even in a dilute state. Also other reducing agents, such as carbon, phosphorus, sulphur, exert a deoxidizing action upon concentrated acid, but with more difficulty and at a higher temperature. It combines very energetically with free bases to form sulphates, and is very difficult of elimination from such salts again. It is exceedingly hygroscopic, being capable of absorbing fifteen times its bulk df water. It combines with water most powerfully, and with great evolution of heat and genera tion of steam if mixed suddenly. With water it forms several compounds capable of assuming definite crystalline shapes.
Sulphuric acid exerts a severe carbonizing or dehydrating influence upon organic substances, and then assumes a more or less darkened colour. In a pure state it should be quite colourless and inodorous, and in that condition it presents an oily appearance, whence its familiar name oil of vitriol (often shortened to o.v.) is derived.
The two first-named varieties of commercially known sulphuric acid never occur in a natural state. The last named is seldom met with in a free state on account of its great solvent powers. Nevertheless, an Andean river, the Rio Vinagre, contains 0.1 per cent. of the acid, and is calculated to produce some 38 tons daily. Besides some smaller Andean streams which are acidulous, a volcanic spring in New Grenada contains upwards of 0.25 per cent. of this acid. It is, however, commonly found in intimate combination with bases forming sulphates. The principal sulphates are those of aluminium, barium, calcium, copper, iron, lead, potassium, and strontium ; alunite, heavy spar, gypsum, blue vitriol, green vitriol, anglesite, kainite, and celestine being the names by which these natural salts are generally known.