The product on dissolving sulphur trioxide in water is sulphuric acid, H2SO4; if smaller quantities of water be used than are necessary for the formation of H2SO4, various pyro- or anhydro-sulphuric acids are produced, the simplest of which is the acid, HO-(SO2)-0-(SO2)-OH, analogous to some extent in formula to (OH)2=PO-0-PO=(OH)2, pyrophosphoric acid, but more closely resembling potassium dichro-mate. It, too, is a fuming liquid, evolving much heat on addition of water.
Sulphuric acid, however, is ordinarily made by bringing together sulphur dioxide in presence of steam with nitric peroxide, NO2, and oxygen. For this purpose, sulphur or iron pyrites is burned in air ; the products of combustion are passed through a flue provided with a chamber in which it is possible to place, when required, a pot containing a mixture of sodium nitrate and sulphuric acid ; the product is nitric acid, which is at once attacked by the sulphur dioxide, yielding sulphuric acid and nitric peroxide, thus: 2HNO3 + SO2 = H2SO4 + 2NO2. The gases next pass up a tower, termed the " Glover tower," after its inventor. In this tower they meet a spray of dilute sulphuric acid, the decomposition product with water of a compound which will afterwards be alluded to, hydrogen nitrosyl sulphate. The hot gases, in contact with the dilute acid, evaporate much of its water, which as steam finds its way along with them up the tower. From the Glover tower the gases enter the first of a series of leaden chambers, in which a reaction occurs between the sulphur dioxide, the nitric peroxide, and the steam, thus: SO2 + NO2 + H2O = H2SO4 -f NO. Excess of air is admitted along with the sulphur dioxide, so that there is present in the leaden chamber a considerable excess of oxygen. By its aid, the nitric oxide is re-oxidised to peroxide, which is again attacked by the sulphur dioxide, so that the nitric oxide serves as a carrier of oxygen to the sulphur dioxide. The nitrogen of the air conveys the gases from chamber to chamber ; and when it has passed through a sufficient series (from nine to thirteen) of chambers, all the sulphur dioxide has been converted into sulphuric acid, and deposited on the floors of the chambers, whence it is run off from time to time; it is called " chamber-acid." Formerly, the nitric oxide and peroxide used to escape into the air and be lost, besides causing a nuisance ; to save it, Gay-Lussac devised a means of trapping it by passing the escaping gases up a tower which bears his name; a stream of concentrated sulphuric acid flows down this tower, moistening the coke or flint with which it is filled. On coming into contact with the mixture of nitric oxide and peroxide, a salt of sulphuric acid is formed-hydrogen nitrosyl sulphate, HO-SO2-O-N=0, the group - N=0 having replaced one of the hydrogen atoms of the sulphuric acid. This compound dissolves in the excess of sulphuric acid; it is conveyed by means of a special pump to the Glover tower, where it is mixed with water, and is decomposed, thus : 2HO-SO2-0-NO + H2O = 2HO-SO2-OH = NO + NO2. Although this compound is formed by the action of concentrated sulphuric acid on a mixture of NO and NO2, yet excess of water causes the action to proceed in the opposite direction ; this affords a good example of the action of mass.
After the chamber acid has been evaporated in leaden vessels until a portion of the water is expelled, it is further concentrated in vessels of platinum, glass, or iron. The dilute acid is without action on lead, and the concentrated acid does not attack platinum or iron, although iron is at once dissolved by dilute acid. The heavy oily liquid remaining after evaporation still goes by its old name of M oil of vitrol." Its composition is not quite expressed by the formula H2SO4, however, for that substance is unstable, and parts with a trace of sulphuric anhydride when heated, leaving a trace of water in the oil of vitrol. It can be made by dissolving the right amount of anhydride in the acid to combine with that water; the resulting acid melts at 10.50; oil of vitrol has a much lower melting-point. The molecular weight of liquid sulphuric acid, determined by its rise in a capillary tube, is very high, and appears to correspond to about 3oH2SO4; on dilution it is no doubt considerably lowered, and in dilute solution it is mostly in the state of ions.
When heated to about 250°, sulphuric acid, as oil of vitrol is usually termed, begins to emit fumes of anhydride; apparent ebullition takes place at about 350°, and the acid distils over. This is, however, really dissociation into anhydride and water ; for the density of the vapour is not, as might be expected, half the molecular weight, 98, but only 24.5, one quarter of that number. And this agrees with the theoretical density of a mixture in equal proportions of the vapours of the anhydride and water, for (40 + 9) / 2 = 24.5. A considerable rise of temperature takes place on mixing sulphuric acid with water ; it is not improbable that the first anhydride of the true ortho-acid is formed ; the compound of the formula HgSCX-HgO, which may be 0=S=(OH)4, melts at 8°. The point of maximum contraction of a mixture of sulphuric acid and water occurs when the proportion corresponds to H9SO4.2H2O ; this is possibly S(OH)(;, but it does not easily solidify. Water can be withdrawn from sulphuric acid by distilling it with phosphorus pentoxide, when sulphuric anhydride is formed and distils over.