Sulphuric Acid, the hydrate of sulphuric anhydride, or teroxide of sulphur, S03 + H20 = H2SO4. It may also be regarded as a salt of hydrogen, this element holding the place of a basyle to the radical sulphion, S04. (See Salts, vol. xiv., pp. 582, 583.) The discovery of sulphuric acid is ascribed to Basil Valentine, a monk of Erfurt in Saxony, about 1440. He obtained it by distilling green vitriol or the sulphate of iron, and as the liquid product had an oily appearance when poured out, it was called oil of vitriol. He also obtained it by burning sulphur under a bell glass containing moisture, calling the product oleum sulphuris per campanum, or oil of sulphur by the bell. This was the germ of the present process of manufacture, which consists in producing sulphurous acid and carrying it to a higher state of oxidation by nitrous and hy-ponitrous acids. The old process of distillation from green vitriol is still employed in some parts of Germany, particularly in the neighborhood of Nordhausen in Prussian Saxony, and near Prague in Bohemia. Sulphate of iron, chiefly produced by the oxidation of iron pyrites, first has its water of crystallization expelled, when it is subjected to a high red heat in earthen retorts placed in galleries in a furnace, as shown in fig. 1. As soon as the acid begins to distil over, the necks of the retorts are passed into receivers.
The product is a brown oily liquid having a density of about 1.9, and fumes in the air, for which reason it is also called fuming sulphuric acid. Its composition may be expressed by the formula H2S04,S03. When gently heated it breaks up into sulphuric anhydride, S03, and sulphuric acid, II2SO4. If fuming Nordhausen acid is distilled into a receiver cooled by ice, white fumes will solidify on its sides into white silky needles. This product was formerly called anhydrous sulphuric acid. It does not however possess acid properties like the residue in the retort, but requires to be united with water to enable it to combine with bases. It is tough and ductile, and can be moulded in the fingers for a short time if they are drv. It has a specific gravity of 1'946 at 55.4°, fumes in the air, and when thrown into water hisses like red-hot iron, and forms sulphuric acid. It melts at 65° and boils at about 95°, forming a colorless vapor, which is decomposed in highly heated porcelain tubes into two volumes of sulphurous anhydride and oxygen. The common way of preparing sulphuric acid at present, known as the English process, is to oxidize sulphurous acid.
It is said to have been introduced by Dr. Roebuck about the middle of the 18th century, but the invention is also claimed for a calico printer at Rouen, with improvements by Chaptal, such claimants giving credit to Dr. Roebuck only for the invention of the leaden chambers in which the process was carried on. A large and long chamber, divided into sections by partitions which alternately leave open spaces at the top and bottom, has at one end a small furnace in. which the flame of sulphur heats a crucible containing a mixture of nitre and oil of vitriol. The chamber is lined with sheet lead, and its floor is covered with a thin stratum of water. Jets of steam are also introduced. The sulphur in burning produces sulphurous anhydride, SO2, which in the presence of moisture becomes sulphurous acid (SO2 + H2SO3H2SO3), and this again, by the action of NO5, becomes sulphuric acid, H2S04, the nitric acid being at the same time reduced to a lower oxide. According to the researches of Weber and Winkler, the following is the rationale of the process: The oxidation of the sulphurous acid to sulphuric acid takes place in the leaden chambers under the influence of the vapor of water, at the expense of the oxygen of the nitric or nitrous acid, which is converted into deutoxidc of nitrogen.
It is necessary however that the nitrous acid be first absorbed in plenty of water, which takes up the free nitrous acid and decomposes the deutoxidc of nitrogen, a process greatly promoted by the presence in the chamber of sulphurous acid purposely introduced. The water, usually in the form of steam (practical experience proving that a certain elevation of temperature is required), acts in this process as in others wherein sulphurous acid effects reduction. By the presence of atmospheric air in the chamber the deutoxide of nitrogen is oxidized into hyponitric or nitrous acid, and this acid again is decomposed by sulphurous acid. A peculiar crystalline substance sometimes forms, having the formula H2SO4 + N2O3,S03, and formerly thought to play an important part in the transformation; but according to R. Weber this substance only appears when the process is not well managed, and is chiefly due to want of water. The process of forming sulphuric acid may be illustrated on a small scale by means of the apparatus shown in fig. 2. A flask, 7), furnishes sulphurous anhydride, and the bottle e deutoxide of nitrogen, to the large glass balloon 7*, and the flask w supplies steam when it is required.
Air is occasionally blown into the balloon through the bent tube t, the effete products passing out at 0. If but little vapor of water is present, the white crystalline solid above mentioned makes its appearance upon the sides of the globe; but when sufficient water is present the substance is not deposited, neither is it supposed to be formed as a necessary stage of the process. Gay-Lussac invented what is called a condenser as an attachment to the large leaden chambers, for the purpose of economizing the consumption of nitre, which formerly amounted to from one eighth to one twelfth of the weight of sulphur. The condenser consists of a leaden tower filled with fragments of coke, through which sulphuric acid of 66° Baume is constantly trickling. Through this condenser the spent gases are passed, and the oxides of nitrogen which they may contain are absorbed. The sulphuric acid which collects at the bottom of the chambers is too dilute for most purposes; it is not found advantageous to allow it to attain a specific gravity of quite 1.6, because at that strength it absorbs too much of the nitrous fumes. It may be used at this strength for the manufacture of salt cake (see Soda), but for other uses it must be further concentrated.
This is generally effected by two different stages, the first in leaden pans, the second in platinum or glass retorts. Some makers concentrate to 6O0 Baume (sp. gr. 1.71) in leaden pans; others use them only till the acid is raised to 55° Baume (sp. gr. 1.59). The leaden pans are rectangular, long and wide, supported by iron plates to protect the lead from immediate contact with the flame which is used for evaporation. The pans are generally arranged in steps, the acid being conveyed from the upper to the lower ones by syphons, the density of the acid increasing from one pan to the next lower. When it has attained a density of from 1.65 to 1/72 in the leaden pans, it is known as brown oil of vitriol, and may be used by bleachers, calico printers, dyers, etc.; but to raise it to the strength of commercial oil of vitriol, it is further concentrated in the glass or platinum retorts above mentioned. Glass retorts holding 20 gallons or more are often used, set in an iron pot, the bottom of which is covered with dry sand. The concentration requires from 12 to 16 hours; the vapors which distil over toward the last, carrying some acid with them, are passed into condensers and returned to the lead pans.
Platinum retorts are more costly, but are thought by many to be more economical in the end on account of their not being liable to break. Fig. 3 shows a section of a platinum retort. The syphon x is worked without a stopcock by the vessel c, in the following ingenious manner. When the vessel is lowered with its spout to the gutter d, the outer limb of the syphon, which is constantly full, becomes lengthened below n (the end of one of the leaden pans), and the acid flows out, fills the movable vessel, and runs out of the spout and through the gutter d into the retort 13. The head C communicates by moans of tubing, not shown in the engraving, with a worm, where the watery vapor and the very weak acid mechanically carried over with it are condensed. The fire under the retort communicates with the flue A, which passes under the leaden pans. The concentrated acid left in the retorts contains a slight excess of water beyond that required for the formula H20,S03 or H2S04. This formula gives 18.30 per cent, of the elements of water, while Marignac obtained 19.62 per cent, from the concentrated acid.
According to Playfair, if the concentration is conducted at a temperature not exceeding 500° F., the true compound, of sp. gr. 1.844, is obtained; but if heated to ebullition, there is partial decomposition. - Properties. The oil of vitriol of commerce is an oily-looking, colorless, and odorless liquid, of sp. gr. 1.842. It chars nearly all organic substances, in consequence of abstracting from them the elements of water, leaving a carbonaceous residue. It mixes with water in all proportions, with condensation of volume of the mixture, and consequent evolution of heat. Its attraction for moisture is so great, that if exposed to the air for a few days in a shallow vessel it frequently doubles its weight; and advantage is taken of this in the laboratory for drying various substances. The boiling point of sulphuric acid is 620.6° F., and it freezes at about - 29°, although when frozen it does not melt below 32°. Marignac finds that the true sulphuric acid when heated parts with a small quantity of vapor of the anhydride, and the remaining liquid boils at 640.4°. Sulphuric acid forms two definite hydrates, the monohydrate, H2 S04,H2O, and the dehydrate, H2S04,2H2O. The first, of sp. gr. 1.78, crystallizes at 47° in splendid rhombic prisms, of sp. gr. 1.951. From this property it is often called glacial sulphuric acid.
It boils at about 400°. The dehydrate may be formed by concentrating a dilute acid in vacuo at 212° till it ceases to lose weight. Its sp. gr. is 1.62; boiling point, 370°. - Uses. Sulphuric acid is the starting point of nearly all the great chemical manufactures. It is used to procure nitric acid from the nitrates of potassium and sodium, and hydrochloric acid from common salt, at the same time furnishing salt cake, from which the carbonates of soda are obtained. It is therefore used in the preparation of various bleaching compounds. Phosphate of lime in artificial manures is reduced to biphosphate by the action of sulphuric acid. In medicine, diluted with water or spirits of wine and known as acidum sulphu-ricum clilutum and acidum sulphuricum aromaticum, it is used as a tonic, refrigerant, and astringent. It is given in typhoid fevers, in convalescence from various fevers, and as an aid to digestion.
Fig. 1. - Distillation of Nordhausen Acid.
Fig. 2. - Class-room Apparatus for Sulphuric Acid.
Fig. 3. - Platinum Retort.