SULPHURIC ACID, or, more correctly, hydrated sulphuric acid (80,,H0), is the chemical name of the liquid commercially and popularly known as oil of vitriol.* It is a dense, colorless, oily liquid,without smell, of a spec. gray. of 1.846 at a temperature of 60°, and of an intensely acid taste and reaction. It has a powerful caustic action, and chars and destroys organic matters from its strong affinity for water; and in consequence of this destructive property, it must always be handled with the greatest caution. So powerful is this affinity, that if the acid be exposed for a few days to the air in a shallow dish, so as to present a large surface, it often doubles its weight by absorbing aqueous vapor from the air; and in consequence of its possessing this property it is extensively used in laboratory operations as a desiccating agent. It mixes completely with water in all proportions, and as great heat is given out at the moment of mixture, the dilution should be performed by very gradually adding the acid to the water. When cold the mixture occupies less bulk than the two components previously occupied. This acid freezes at a temperature of 15°, and boils at 620° (or according to Marignac, at 640°), and just above the boiling-point it assumes the form of a vapor, with a spec. gray. of 2.15. Oil of vitriol, or the protohydrate, is not the only hydrate of sulphuric acid. Three others arc known to exist. When the fuming oil of vitriol of Nordhausen is ex posed to a low temperature, a white crystalline substance separates, which is a hydrate, containing half as much water as the common liquid acid. Its formula is 2S0,,H0, or Its point is 95°. Then, again, a mixture of 49 parts of strong liquid acid and 9 parts of water (SO,,2H0) freezes at 47° and crystallizes into splendid rhombic prisms, from which property it is often termed glacial sulphuric acid. It boils at 435°, and its spec. gray. is 1.780. Lastly, when a very dilute acid is concentrated by evapora tion in roacuo, at 212°, till it ceases to lose weight, there will be a resulting compound, consisting of 40 parts of the real acid, and 27 of water, and represented by the formula, S03,3H0. It boils at 348°, and its spec. gray, is 1.602. There are thus no less than four hydrates of sulphuric acid—viz.: (1) 2803,110; the ordinary protohy drate, (3) the bihydride. S02,2110; and (4) the terhydrate, S03,31I0. The compound formerly known as anhydrous sulphuric acid possesses none of the character istic properties of an acid. See SULPHURIC ANHYDRIDE; also CHEMISTRY.
Sulphuric acid in its free state is a very rare natural product; although, in combina tion with bases, it is common in the animal and vegetable, and abundant in the inor ganic kingdom. The only cases in which it is known to occur free are certain Ameri can rivers, especially the rio Vinagre, and some lakes in Tennessee and in Java; and it has been found to be a normal constituent of the saliva of dolium galia, a species of snail found in Sicily. In all these cases the acid is, of course, in an extremely diluted form. In plants it exists in the juices, and in animals in the blood and its derivates chiefly in the form of sulphates of the alkalies; while in the mineral kingdom it occurs as gypsum (sulphate of lime), heavy spar (sulphate of baryta), celestine (sulphate of strontia), etc.
Sulphuric acid may be prepared on a small scale by boiling sulphur in aqua regia, or in nitric acid, the sulphur becoming gradually oxidized into sulphuric acid. As a gen eral rule, however, the commercial acid is employed even for laboratory experiments. See below.
In order to obtain the acid in a pure form, suitable for medical use or medico-legal analyses, it must be redistilled with sulphate of ammonia in a retort containing a few slips of platinum foil, the first and last portions being rejected. The distillation is attended with violent concussions, partly owing to the high specific gravity of the acid, and partly owing to its high boiling-point, and this convulsive action is moderated mechanically by the platinum slips. Sulphuric acid thus pre pared according to the direction of the British pharmacopoeia may be regarded as perfectly pure, presuming arsenic is not present. Strong sulphuric acid has compara tively little action on the metals except at a high temperature, when it dissolves them, and, at the same time, undergoes partial decomposition; the metal being oxidized by a portion of the acid which becomes decomposed into oxygen and sulphurous acid, and then uniting with a portion of acid to form a sulphate. Silver, copper, mercury, arsenic, antimony, bismuth, tin, lead, and tellurium are thus acted on. Gold, platinum, rhodium, and iridium are not affected by the acid even at a boiling tempera ture. The more oxidizable metals, such as zinc, iron, nickel, and manganese, are read ily soluble in the dilute acid, water beino. decomposed, and hydrogen liberated, while the oxygen of the water unites with the metal; and the metallic oxide, at the moment of its formation, combines with the sulphuric acid to form a sulphate.