At one time it was thought that all the salts of thiosulphuric acid only existed in combination with water, and it was then supposed that their composition was H4S2O4, or H2SO2, but Popp obtained the anhydrous salts.

[43] Nordhausen sulphuric acid may serve as a very simple means for the preparation of sulphuric anhydride. For this purpose the Nordhausen acid is heated in a glass retort, whose neck is firmly fixed in the mouth of a well-cooled flask. The access of moisture is prevented by connecting the receiver with a drying-tube. On heating the retort the vapours of sulphuric anhydride will pass over into the receiver, where they condense; the crystals of anhydride thus prepared will, however, contain traces of sulphuric acid—that is, of the hydrate. By repeatedly distilling over phosphoric anhydride, it is possible to obtain the pure anhydride, SO3, especially if the process be carried on without access of air in a closed vessel.

The ordinary sulphuric anhydride, which is imperfectly freed from the hydrate, is a snow-white, exceedingly volatile substance, which crystallises (generally by sublimation) in long silky prisms, and only gives the pure anhydride when carefully distilled over P2O5. Freshly prepared crystals of almost pure anhydride fuse at 16° into a colourless liquid having a specific gravity at 26° = 1·91, and at 47° = 1·81; it volatilises at 46°. After being kept for some time the anhydride, even containing only small traces of water, undergoes a change of the following nature: A small quantity of sulphuric acid combines by degrees with a large proportion of the anhydride, forming polysulphuric acids, H2SO4,nSO3, which fuse with difficulty (even at 100°, Marignac), but decompose when heated. In the entire absence of water this rise in the fusing point does not occur (Weber), and then the anhydride long remains liquid, and solidifies at about +15°, volatilises at 40°, and has a specific gravity 1·94 at 16°. We may add that Weber (1881), by treating sulphuric anhydride with sulphur, obtained a blue lower oxide of sulphur, S2O3. Selenium and tellurium also give similar products with SO3, SeSO3, and TeSO3. Water does not act upon them.

[44] Pyrosulphuric chloranhydride, or pyrosulphuryl chloride, S2O5Cl2, corresponds to pyrosulphuric acid, in the same way that sulphuryl chloride, SO2Cl2, corresponds to sulphuric acid. The composition S2O5Cl2 = SO2Cl2 + SO3. It is obtained by the action of the vapour of sulphuric anhydride on sulphur chloride: S2Cl2 + 5SO3 = 5SO2 + S2O5Cl2. It is also formed (and not sulphuryl chloride, SO2Cl2, Michaelis) by the action of phosphorus pentachloride in excess on sulphuric acid (or its first chloranhydride, SHO3Cl). It is an oily liquid, boiling at about 150°, and of sp. gr. 1·8. According to Konovaloff (Chapter [VII.]), its vapour density is normal. It should be noticed that the same substance is obtained by the action of sulphuric anhydride on sulphur tetrachloride, and also on carbon tetrachloride, and this substance is the last product of the metalepsis of CH4, and therefore the comparison of SCl2 and S2Cl2 with products of metalepsis (see later) also finds confirmation in particular reactions. Rose, who obtained pyrosulphuryl chloride, S2O5Cl2, regarded it as SCl6,5SO3, for at that time an endeavour was always made to find two component parts of opposite polarity, and this substance was cited as a proof of the existence of a hexachloride, SCl6. Pyrosulphuryl chloride is decomposed by cold water, but more slowly than chlorosulphuric acid and the other chloranhydrides.

The relation between pyrosulphuric acid and the normal acid will be obvious if we express the latter by the formula OH(SO3H), because the sulphonic group (SO3H) is then evidently equivalent to OH, and consequently to H, and if we replace both the hydrogens in water by this radicle we shall obtain (SO3H)2O—that is, pyrosulphuric acid.

[45] The removal of the water, or concentration to almost the real acid, H2SO4, is effected for two reasons: in the first place to avoid the expense of transit (it is cheaper to remove the water than to pay for its transit), and in the second place because many processes—for instance, the refining of petroleum—require a strong acid free from an excess of water, the weak acid having no action. When in the manufacture of chamber acid, both the Gay-Lussac tower (cold, situated at the end of the chambers) and the Glover tower (hot, situated at the beginning of the plant, between the chambers and ovens for the production of SO2) are employed, a mixture of nitrose (i.e. the product of the Gay-Lussac tower) and chamber acid containing about 60 p.c. H2SO4, is poured into the Glover tower, where under the action of the hot furnace gases containing SO2, and the water held in the chamber acid (1) N2O3 is evolved from the nitrose; (2) water is expelled from the chamber acid; (3) a portion of the SO2 is converted into H2SO4; and (4) the furnace gases are cooled. Thus, amongst other things, the Glover tower facilitates the concentration of the chamber acid (removal of H2O), but the product generally contains many impurities.

[46] The difficulty with which the last portions of water are removed is seen from the fact that the boiling becomes very irregular, totally ceasing at one moment, then suddenly starting again, with the rapid formation of a considerable amount of steam, and at the same time bumping and even overturning the vessel in which it is held. Hence it is not a rare occurrence for the glass retorts to break during the distillation; this causes platinum retorts to be preferred, as the boiling then proceeds quite uniformly.

[47] According to Regnault, the vapour tensions (in millimetres of mercury) of the water given off by the hydrates of sulphuric acid, H2SO4,nH2O, are—

t=5°15°30°
n=10·1 0·1 0·2
20·4 0·7 1·5
30·9 1·6 4·1
41·3 2·8 7·0
52·1 4·210·7
73·2 6·215·6
94·1 8·019·6
114·4 9·022·2
175·510·626·1

According to Lunge, the vapour tension of the aqueous vapour given off from solutions of sulphuric acid containing p per cent. H2SO4, at t°, equals the barometric pressure 720 to 730 mm.