182

OXIDATION OF SULPHUROUS ACID.

of oxygen when burned in the air, or in oxygen gas, it is nevertheless a matter of no very great difficulty to cause it to take up a third atom.

In presence of water it gradually absorbs oxygen from the air, and is converted into sulphuric acid. Hence the aqueous solution of sulphurons acid (Exp. 96) cannot be preserved for any great length of time, unless it be kept in very tight vessels.

224. If a mixture of sulphurous acid gas and oxygen, or air, be brought in contact with hot platinum sponge, the sulphurous acid will unite with oxygen, and sulphuric acid will be formed; the same union occurs when the mixed gases are brought in contact with various other substances, such as pumice-stone, clay, and the oxides of chromium, iron, and copper. Several attempts

have been made to put these methods in practice for manufacturing sulphuric acid, but they have been found to be too slow, and in the case of platinum and clay, it has been observed that these substances soon lose their power and cease to convert the mixed gases into sulphuric acid.

Sulphurous acid is, indeed, a deoxidizing agent of very considerable power; and is much employed in the laboratory as a reducing agent. It decomposes iodic acid with separation of iodine, and nitric acid with evolution of hyponitric acid, sulphuric acid being formed in both cases.

1,0, + 5H,0 + 580,

5

=

=

5(H,0,80) + 21. H2O,SO, + 2NO,. Exp. 99.-Charge a dry bottle, of the capacity of a litre Fig. 45. or more, with sulphurous acid gas, by burning in it a bit of sulphur, as shown in Fig. 45. Fasten a shaving, or, better, a tuft of gun-cotton, upon a glass rod or tube bent at one end in the form of a hook; wet the shaving in concentrated nitric acid, and hang it in the bottle of sulphurous acid. Red fumes of hyponitric acid will immediately form about the nitric acid, and will gradually fill the bottle.

In presence of a mixture of water and chlorine, sulphurous acid takes up an atom of oxygen from the water, while the hydrogen of the water unites with chlorine.

SO, 2H,0 + 201 = H2O,SO, + 2HC1.

3

A similar reaction occurs between iodine and sulphurous acid, if

a very large amount of water be present; in spite of the fact, already mentioned, § 140, that iodohydric acid is readily decomposed by concentrated sulphuric acid with liberation of iodine, sulphurous acid, and water.

225. Sulphurous acid, though a weak acid, forms numerous well-defined salts by uniting with metallic oxides. These salts, called sulphites, are of two classes,-simple or normal sulphites, such as the sulphite of potassium K,SO, (or, dualistic, K,O,SO), and double or acid sulphites, such as the acid sulphite of potassium KHSO, (or, dualistic, KHO,SO,). All these salts are decomposed by strong acids, such as chlorhydric, nitric, or sulphuric, sulphurous acid being expelled; but they are not decomposed by carbonic acid. On the contrary, the salts of carbonic acid are decomposed by sulphurous acid; and hence it happens that the impure sulphurous acid gas obtained by heating a mixture of charcoal and sulphuric acid can be used for preparing the sulphites. If, for example, this gas be conducted into an aqueous solution of carbonate of sodium, there will be obtained a solution of sulphite of sodium, and carbonic acid will be set free.

226. Besides the solution of sulphurous acid, such as was prepared in Exp. 96, there is a definite crystalline compound of water and the acid, which can be obtained by passing a current of sulphurous acid gas into ice-water. This compound is very unstable, and is destroyed at temperatures but little above 0°; but by collecting it upon a cooled filter and then pressing the crystals repeatedly between folds of cold blotting-paper, it has been found possible to remove most of the mother-liquor which adheres to them at first, and to obtain the compound in a condition of tolerable purity. The composition of the crystals appears to be SO,+15H,0.

227. Sulphuric Acid.-The term sulphuric acid is applied somewhat indiscriminately to three or more distinct substancesnamely, to a compound of one atom of sulphur and three atoms of oxygen, SO,, which we shall call anhydrous sulphuric acid, and to certain compounds of sulphur, oxygen, and hydrogen, which have been usually regarded as compounds of the anhydrous sulphuric acid, just mentioned, and water. Of these hydrates the most important are those of the composition H,SO

(dualistic, H2O, SO,) [oil of vitriol], and H,S,O, (dualistic, H,0,280,) [Nordhausen or fuming sulphuric acid]. The body, whose formula is H2SO,, is one of the most important of chemical substances, and is usually the thing meant when sulphuric acid is spoken of. We will therefore proceed to study its properties before touching upon those of the other substances abovementioned.

Sulphuric acid is one of the most important products of chemical manufacture, and is made in enormous quantities. In the same way that the metal iron may be said to be the basis of all mechanical industries, sulphuric acid lies at the foundation of the chemical arts. By means of sulphuric acid, the chemist either directly or indirectly prepares almost everything with which he has commonly to deal.

Sulphuric acid might be prepared by passing sulphurous acid gas into boiling nitric acid, until all of the latter had been reduced, and finally distilling off the last traces of the lower oxides of nitrogen which would be formed. Even if sulphur itself were boiled in concentrated nitric acid, it would gradually be oxidized and converted into sulphuric acid. But neither of these processes would be economical. It can be very cheaply prepared, however, by the action of either of the high oxides of nitrogen, nitrous, hyponitric, or nitric acids, upon sulphurous acid, in presence of air and moisture; and this method is the one actually followed in the preparation of sulphuric acid on the large scale. A mixture of the gases above mentioned is effected in enormous chambers constructed of sheet lead, a metal upon which cold sulphuric acid has little or no action.

228. The essential points of the process are, first, that SO, when in presence of much moisture, can take oxygen from either NO,, NO,, or N,O,, and reduce them to nitric oxide, NO, while it is itself converted into sulphuric acid, and, secondly, that NO can take oxygen from the air and become NO2.

In practice, the sulphurous acid is obtained by burning crude sulphur, or more commonly a compound of sulphur and iron, known as iron-pyrites, FeS2; the gas, together with a large excess of atmospheric air, is then conducted into the first of a series of leaden chambers into which steam is admitted. Nitrous fumes are supplied either

MANUFACTURE OF SULPHURIC ACID.

185

by allowing nitric acid to fall in fine streams through the incoming current of sulphurous acid and air, or from the decomposition of a mixture of salt, nitrate of sodium, and sulphuric acid, as described in § 105, or by heating a vessel charged with nitrate of sodium and sulphuric acid, by means of the burning sulphur.

In conformity with the principles above stated, the sulphurous acid, as soon as it comes in contact with the steam, reacts upon the nitrous fumes; there is formed nitric oxide gas and hydrated sulphuric acid, which falls to the floor. But, as there is present in the chamber an excess of air, the nitric oxide immediately unites with a portion of the oxygen therein contained, and is converted into hyponitric acid. This hyponitric acid immediately reacts upon a new portion of sulphurous acid, and the process thus goes on through a whole series of leaden chambers, the very small portion of nitric acid at first taken being sufficient to prepare a large quantity of sulphuric acid. In reality, the oxygen employed in converting the sulphurous into sulphuric acid, all comes from the air, excepting a very little at first; the nitrous fumes serve only as a conveyer of oxygen. The nitric oxide takes oxygen from the air and transfers it to the sulphurous acid, which, as has been stated in § 223, is, by itself and unaided, incapable of combining with oxygen. It will, of course, be understood that, although we trace out these reactions as if they were consecutive, they are really, so far as we know, simultaneous.

Theoretically, a single portion of hyponitric acid would be sufficient to effect the conversion of an unlimited amount of sulphurous into sulphuric acid; but practically this power is qualified by a variety of circumstances. It is found to be impossible, for example, to mix new portions of air with the mixture of sulphurous acid and nitric oxide for an indefinite period; for at a certain point these gases become so loaded down with nitrogen derived from the air already consumed, that they are as good as lost in it. In general the flow of gases is so regulated that all the sulphurous acid shall be oxidized, and that nothing but nitric oxide and waste nitrogen shall pass out of the last leaden chamber.

229. The process of manufacturing sulphuric acid can readily be illustrated upon the small scale.

A large glass balloon, or receiver, of the capacity of several litres, placed in a vertical position, is closed with a cork pierced with five holes, through four of which are passed small glass tubes. All of these glass tubes reach nearly to the bottom of the balloon, and are bent at a right angle above the cork; one of the tubes is connected at the top with a flask containing copper-turnings and sulphuric acid, for the

186

MANUFACTURE OF SULPHURIC ACID.

generation of sulphurous acid (see Exp. 96), another with a flask containing copper-turnings and furnished with a thistle-tube, through which nitric acid can be poured, for the generation of nitric oxide (see Exp. 37), and the third with a flask containing water for the evolution of steam; the fourth tube and the fifth hole are both left open.

Everything being in readiness, nitric oxide is generated in the small flask fitted for this purpose; as the gas passes over into the large balloon it unites with oxygen from the air, and red fumes of hyponitric acid are formed. Sulphurous acid is now made to pass into the balloon; this will have no action upon the red fumes, so long as there is no water present, but the moment steam is thrown in from the third small flask, a reaction occurs, the hyponitric acid is reduced, and the sulphurous acid oxidized. By means of bellows, air must, from time to time, be blown into the balloon, through the fourth glass tube, the waste nitrogen passing off through the fifth hole in the cork.

If but little steam be employed in this experiment, a solid compound, formed by the union of nitrous and anhydrous sulphuric acids, is liable to be deposited upon the walls of the balloon; the appearance of this body always indicates that the supply of steam is insufficient; it is never formed when the proper proportion of moisture is present.

230. The sulphuric acid which collects at the bottom of the leaden chambers is necessarily dilute, because of the large amount of water which must be present, in order that the reactions above described may freely occur; moreover it would not be advantageous to allow an acid more concentrated than that of specific gravity 1.4 to form in the chambers, since a stronger acid would absorb and retain a considerable quantity of nitric oxide. To make it fit for the purposes for which sulphuric acid is usually employed, the dilute acid of the chambers must be concentrated by expulsion of the water; to this end, it is run off into shallow leaden pans, and there evaporated until it is of specific gravity 1.71 to 1.75. The concentration cannot safely be carried beyond this point in ordinary leaden vessels, since the strong, hot acid begins to attack the metal, and the temperature at which the liquid boils is so high as to approach the melting-point of lead. This acid of 1.72 specific gravity is somewhat extensively employed, for a variety of purposes, at the factories where it has been prepared, but is still too dilute for transportation. It is therefore transferred from the leaden pans to large glass retorts set in deep sand baths, or to platinum stills, and there evaporated further, until it is nearly of the composition H.SO..