Note 44 — Chapter XX · SULPHUR, SELENIUM, AND TELLURIUM

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.