SULPHURIC ACID, or OIL OF VIT RIOL, a common and exceedingly important oxy-acid of sulphur, having the chemical formula H2SO4. It was first prepared by Geber, in the 8th century, by distilling alum; and in the 15th century it was manufactured by burning sulphur with saltpetre, though the identity of the product so obtained with that described by Geber was not established until near the end of the 16th century. Considerable quantities of sulphuric acid were formerly manufactured by the distillation of ferrous sulphate, the prac tice of this method dating from the early part of the 18th century. At the present time prac tically all of the sulphuric acid that is used is prepared from sulphur dioxide gas, either by the °chamber process,* or by the more recently perfected °contact process,° both of which are described in this article.
When pure and free from water, sulphuric acid is a colorless liquid with an oily appear ance, and a specific gravity of 1.89. It may be readily frozen, the solidified acid melting again at 50.9° F. It exhibits the phenomenon of surfusion to a marked extent, and the liquid acid can be cooled, much below the melting point here given, without inducing solidifica tion; but if a crystal of the solid acid, or a small amount of sulphur trioxide, be added to the supercooled fluid, crystallization begins at once, and the temperature rises until it becomes 50.9° F., after which no further solidification occurs. The presence of a trace of water in the acid lowers the freezing point nearly to 32° F. If five parts (by weight) of sulphuric acid be mixed with nearly one part of water, and the solution is cooled by a freezing mixture, a definite hydrate of sulphuric acid, having the composition H2SO4-1- H20, crystallizes out at 45° F. Another solid hydrate, having the com position HiS0.+ 4H2O, may also be prepared by cooling, to a much lower temperature, a mixture of sulphuric acid and water, containing 57.6 per cent of water. Several other hydrates are also believed to exist, and special study has been expended upon them, on account of their importance in the illustration of the °hydrate° theory of solutions. (Consult Mendeleeff, of Chemistry,' Vol. II). Sulphuric acid has no really definite boiling point. It begins to boil at about 550° F., the distillate containing sulphuric acid, water and sulphur trioxide. (See SULPHUR). The temperature of the liquid may be raised to 640° F., how ever, before a state corresponding in definite ness to the boiling point of water is attained, the vapor that passes off then consisting en tirely of water vapor and free sulphur trioxide.
At •higher temperatures the decomposition is even more complete. Thus if a stream of the acid be allowed to flow over redhot bricks, it is broken up into sulphur dioxide (SO,), free oxygen and water-vapor. If the gases re sulting from this decomposition are passed through cool water, and the steam that they contain is condensed and the sulphur dioxide removed by solution, a supply of pure oxygen gas is obtained. Pure anhydrous sulphuric acid has a specific heat, at ordinary temperatures, of about 0.34, and a coefficient of expansion (Fahrenheit scale) of about 0.000310. The con centrated acid is a powerfully corrosive poison, destroying organic tissues rapidly, and even charring paper and wood. It is also poisonous (though far less violently so) when adminis tered in any considerable quantity in a highly dilute form. Concentrated sulphuric acid has a powerful affinity for water, its combination with water being attended by the evolution of a large amount of heat. The strong acid is used as a drying agent, for removing moisture from gases. For this purpose it is sometimes suffi cient to allow the gas to stand for a time in a receiver containing a dish of the concentrated acid i but a more effective mode of procedure consists in passing the gas through tubes that are partially filled with fragments of pumice that have been wetted with the acid.
Chemically, sulphuric acid is dibasic, either or both of its hydrogen atoms 'being replace able by metals or other bases, the compounds that are thus formed being termed °sulphates.° With the metals of the alkalis (which are monovalent), sulphuric acid therefore forms two kinds of sulphates, which may be sufficiently illustrated by the potassium silts. If one of the hydrogen atoms of the acid is replaced by potassium, the resulting salt, HKSO4, is called °hydrogen potassium sulphate,° or °acid po tassium while if both are replaced, the resulting salt has the. formula K,SO4, and is known as °normal potassium sulphate.° Many of the sulphates of the metals occur native in large quantities, and many of them are of great value in the arts. Those that are of especial importance are described, in this en cyclopedia, under the metals (or other bases) with which the acid is combined. The sulphate of barium is perhaps the most insoluble salt known. It is formed whenever a soluble barium salt (such as the chloride) is added to a solution of a soluble sulphate; and its formv don constitutes a valuable test for sulphuric acid and the sulphates. See CHEMICAL