When sulphur trioxide is dissolved in anhy drous sulphuric acid in the proportion of one molecule of the trioxide to one of the acid, a definite compound having the formula H2S202 (or H,SO..SO,) is obtained. When pure, this substance is known as upyrosulphuric acid.° It is a dibasic acid, forming salts which are termed °pyrosulphates,° but which are of com paratively little practical importance. Pyro sulphuric acid forms large crystals, which melt at F., and it is easily decomposed by heat into ordinary sulphuric acid and free sulphur trioxide. Fuming sulphuric acid (now com monly known in the arts as but for merely called °Nordhausen oil of con sists of a mixture of pyrosulphuric acid and ordinary sulphuric acid and may be regarded as a solution of sulphur trioxide in sulphuric acid, the trioxide not being present in sufficient quantity to convert the ordinary acid entirely into pyrosulphuric acid. It fumes strongly in the air, gives off sulphur trioxide when heated, and is prepared by the °contact process,* described in this article.
The uses of sulphuric acid in chemistry and in the arts are past enumeration ;. for this acid is one of the most important chemical sub stances known, and it is employed is so many industrial processes that it has been said that the wealth and prosperity of a nation can be estimated from its consumption of sulphuric acid. About half of .the total quantity manu factured in the United States is consumed in the preparation of fertilizers. The two general methods now in use for manufacturing the acid are described in this article.
The sulphur used for the manufacture of sulphuric acid is obtained (1) from the gases generated by burning iron pyrites (FeS2), (2) from the sulphur deposits of Louisiana, Sicily and elsewhere, and (3) to a very limited ex tent from the waste gases given off by sul phide smelters. At the present time pyrites is the most important source, but it is probable that smelter gases will be utilized to a greatly increased extent in the future, and they may eventually compete with pyrites. The sulphur
dioxide that is now wasted by discharging the fumes from smelters into the air would far more than supply the United States with sul phuiic acid. Moreover, the fumes are exceed ingly objectionable, and they are destructive to vegetation.
The fundamental principles in the manu facture of sulphuric acid are (1) to oxidize sul phur or a suitable sulphide, so as to obtain sul phur dioxide, SO,; (2) to further oxidize this to the trioxide, S02; and (3) to effect •the combination of the trioxide with water, in ac cordance with the equation S02-1-H20=H2SO4. In attempting to carry out the second of these processes, however, we are confronted by the fact that sulphur dioxide does not readily.take tip oxygen, so as to become completely con verted into the trioxide. To effect this oxida tion we are in fact compelled to resort to one or the other of two expedients: (1) To mix a certain amount of an oxide of nitrogen with the sulphur dioxide and air —the oxide of nitro gen then acting as a sort of °carrier*, taking up oxygen from the air and passing it on to the sulphur dioxide; or (2) to subject a mixture of air and sulphur dioxide to the action of a suitable catalyzer. The first of these expe dients is used in the °chamber process* and the second in the °contact process.* The first step in the manufacture of sul phuric acid is to provide a suitable supply of sulphur dioxide gas. This is usually obtained by burning sulphur or iron pyrites in a special furnace and considerable skill and judgment are required in this part of the operation, to obtain gases of proper composition. It is also important to minimize the quantity of dust that the gases carry over into the later parts of the process. Settling chambers, baffle plates, centrif ugal separators, parallel-plate separators, and filtration through piles of marbles or other loosely aggregated solid lumps are among the devices used for the removal of the• dust.