Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/516

Rh CHEMISTRY [SULPHURIC ACID. nitrate and sulphuric acid, which continue to generate nitric acid for some time ; thus NaNO Sodium nitrate. Sulphu H 2 S0 4 = HNO 3 lphuric acid. Nitric acid. + NaHS0 4. Sodium hydrogen sulphate. The exact changes which occur among the various sub stances thus introduced into the chambers are but im perfectly understood ; the nitric acid, however, is appa rently decomposed by the sulphurous acid in such a manner that finally nitric oxide is produced 2HN0 3 + 3S0 2 + 2H 2 = 2NO + 3H 2 S0 4. But nitric oxide combines directly with oxygen at ordinary temperatures, forming the oxides N 2 O 3 an d N 2 O 4, and it is generally supposed that the nitric oxide thus formed acts as a carrier of oxygen, that on coming in contact with the oxygen of the air introduced into the chamber, it is converted into a higher oxide, which, however, is no sooner produced than it is reduced again to nitric oxide by the sulphurous acid, which is oxidized to sulphuric acid ; supposing the oxide formed to be nitric peroxide, we may represent the change by the following simple equation : 2S0 2 + 2H 2 O + N 2 O 4 = 2H 2 SO 4 + 2NO. The nitric oxide again absorbs oxygen to form the higher oxide, which again enters into reaction with sulphur dioxide and water, with liberation of nitric oxide, and so on con tinuously. Theoretically, therefore, a limited quantity of nitric oxide should suffice to produce an indefinitely large amount of sulphuric acid, but practically this is not the case, chiefly owing to the loss occasioned by the dissolution of a certain quantity in the sulphuric acid which collects on the floor of the chambers ; a certain amount also escapes from the chambers with the nitrogen of the air, but this is usually absorbed by passing the escaping vapours through a tower filled with pieces of coke, kept moistened with sulphuric acid, or over which water is constantly pouring ; when acid is employed it is afterwards pumped into a similar scrubber, and deprived of its nitric oxide gas by means of the sulphur dioxide gas from the burning sulphur or pyrites, and when water is used it is pumped into, and distributed on the floors of, the leaden chambers. In practice the amount of sulphuric acid produced usually approximates very closely to the theoretical quantity. It is beyond question that the reactions which occur in the formation of sulphuric acid are not all of the simple character represented by the equations above given. Thus, the formation of nitric oxide from nitric acid is probably the final result of three distinct reactions. In the first place, from the sulphur dioxide and nitric acid a compound which is known as the lead chamber crystals, on account of its forming crystals, is probably produced ; thus T. S0 N0 2 .OH = S0 2 {*. Sulphur dioxide. Nitric acid. Lead chamber crystals. This compound, it will be observed, is very closely related to sulphuric acid, and may be regarded as formed from it by the displacement of one of the OH groups by the group NO 2. It is decomposed by water, yielding sulphuric and nitrous acids : II. SOo NO., OH~ HOH = Lead chamber crystals. Water. Sulphuric acid. Nitrous acid. From the nitrous acid thus formed nitric oxide may be produced in two ways, either by its spontaneous decom position in accordance with the equation 3HN0 2 = 2X0 + HNO, + H 2 O ; Nitroos acid. Nitric oxide. Nitric acid. Water. or by the action of sulphur dioxide 2HN0 2 + SO 2 = 2NO + H 2 SO 4. Nitrous acid. Sulphur dioxide. Nitric oxide. Sulphuric acid. The latter reaction appears to be the more probable on account of the large amount of sulphur dioxide present, but it is not unlikely that both occur. With regard to the part which the nitric oxide plays in the conversion of sulphurous into sulphuric acid, it is highly probable that, by the conjoint action of the oxygen and water present, it is converted into nitrous acid, from which nitric oxide is reproduced by the action of the sulphur dioxide in the manner above pointed out 4NO + O 2 + 20H 2 = 4HN0 2. If this be the case, nitrous acid is the active agent in the conversion of sulphur dioxide into sulphuric acid. It is known to effect the oxidation of sulphur dioxide with the greatest readiness, whereas dilute nitric acid has very little action. On this account, it is probable also that the nitric oxide is not converted into the peroxide, as sometimes is supposed, and that the latter is the active agent in the formation of sulphuric acid, for our general experience appears to warrant the conclusion that, as sulphur dioxide enters so readily into reaction with nitrous acid, this acid would &quot;be produced from the nitric oxide almost entirely to the exclusion of other oxidation products. Moreover, nitric peroxide in presence of water forms nitrous and nitric acids : N 2 4 + H 2 = HNO 2 + HNO 3 ; so that, even supposing that the nitric oxide is converted into nitric per oxide, the formation of sulphuric acid would still in a large measure be due to the action of nitrous acid. Although sulphur dioxide acts very slowly on dilute nitric acid, it readily reduces it to nitric oxide in presence of moderately concentrated sulphuric acid ; hence any nitric acid formed in the process, when carried down to the floor of the chamber and mixed with the moderately concentrated sulphuric acid there collected, would also undergo reduc tion by the sulphur dioxide. The acid from the leaden chambers has generally a specific gravity of about 1 55 ; by concentration in shallow leaden pans this is raised to above 1 7. The further con centration is effected in glass or platinum retorts. The commercial acid, known as oil of vitriol, has a specific gravity of about 1 84 ; it is frequently of a more or less brown colour from the presence of organic matter, and always contains lead. Other impurities, such as arsenic and nitrous or nitric acid, are not unfrequently present. Pure sulphuric acid, H 2 S0 4, is an oily, colourless, in odorous liquid, of specific gravity 1 842 ; it solidifies at 35 C. It cannot be distilled unchanged, and when heated gives off the anhydride until a liquid remains containing about 1 5 percent, of water and 98 &quot;5 per cent, of sulphuric acid, which boils at 338 C. ; acid of this strength may always be obtained by boiling down acid of any other degree of concentration. When the acid is vaporized, dis sociation takes place very rapidly, the vapour at tempera tures above 400 C. consisting entirely of a mixture of water and sulphuric anhydride. Sulphuric acid chars most organic substances containing oxygen and hydrogen on account of its great affinity for water, causing the separation of these elements in the form of water and the liberation of carbon ; when exposed to moist air it will even double its weight in the course of a few days, and on this account it is much employed as a desiccating agent. Much heat is developed when it is mixed with water, as will be evident from the following table, which represents the amounts of heat developed on mixing one molecule of the acid (98 grammes) with n molecules of water at about 18 C., according to Thomsen s determinations :