SULPHURIC ACID, FI,S0,. A well-known acid compound of hydrogen, sulphur, and oxygen. When pure and free from water, it is a. colorless, oily liquid of specific gravity I.S4 (approximate ly) at 0° C. While it may readily be under cooled, it solidifies normally at 6.79° C. (44.3° F.). At about 290° C. (554° F.) it begins to boil, breaking up into sulphur trioxide and water vapor, and as some of the latter is retained by the boiling acid, the temperature rapidly rises to about 33S° (640° F.). It is highly scopic, absorbing about 30 per cent, of its weight of water. It readily chars organic matter, and is poisonons, not only on account of its power ful corrosive action. but also on account of a specific effect on the blood. Chemically it is a dibasic acid, either one or both of its hydrogen atoms being replaced by metals. with the forma tion. respectively. of either acid or neutral salts (sulphates). Its chemical constitution is gen erally assumed to be represented by the formula II—O—S-0-0-0—H. By adding to it a little water, it may he caused to form the hydrate which crystallizes out when the mixture is cooled and may then be readily isolated from the mother-liquor by suction.
another well-defined hydrate is formed, having the composition II,S0,.211,0.
The term 'sulphuric acid,' however, as tech nically and commercially understood, seldom if ever refers to the actual monohydrate or to any of the other recognized hydrates of sulphur tri oxide.
It refers either to a series of solutions of the monohydrate in water or to a series of solutions of sulphur trioxide in the monohydrate.
SO,.) The first series are known mercially as 'chamber acid'—`oil of vitriol'— `concentrated acid' or in terms of their specific gravity as indicated by a hydrometer scale, as 60° Baume acid, 144° Twaddell acid and so forth. The second series are termed 'fuming' or Woodhansen acids, and are always estimated al kali-metrically according to the percentage of free sulphur trioxide contained.
Technically, therefore, 'sulphuric acid' might he considered to he the generic name of a series of solutions of sulphur trioxide in water, some of which solutions are distinguished by variations of properties which occur uniformly with uniform percentage mixtures of sulphur trioxide and water and are therefore chemical* hydrates of sulphur trioxide—most of which are, however,. merely solutions of convenient strength for use in the arts.
The manufacture of sulphuric acid is one of the greatest chemical industries. The processes involved include the production of sulphur di oxide, the transformation of this into sulphur trioxide, and the transformation of the trioxide into sulphuric acid. Of these, the oxidation of the dioxide is the most important, the dioxide it self being readily produced by burning sulphur, Glass bottles containing commercial sulphuric acid often burst in winter, owing to the separa tion of crystals of this hydrate, whose melting point is about S° C. (about 46° F.). and which decomposes into sulphuric acid and water above 205° C. (about 370° F.). If pure sulphuric acid is mixed with water in the proportion of 49 parts of the former to IS parts of the latter, mineral sulphides, or sulphureted hydrogen. with free access of air, the resulting gases, tech nically termed 'burner-gas,' including from 5 to S per cent. of the dioxide (the remainder is con stituted by the nitrogen and excessive oxygen of the air and by impurities). The two processes devised to effect the oxidation of the dioxide are alike catalytic in their action, in so far as the substances used, while producing the required chemical reaction, remain themselves in the end unchanged. These two processes are termed, re spectively, the chamber process and the con tact process.