Persulphuric Anhydride, S₂O₇

In 1878, Berthelot, by submitting a mixture of sulphur dioxide or trioxide with excess of oxygen under compression to a silent electric discharge of high potential, obtained a product (Persulphuric Anhydride, Sulphur Heptoxide, S₂O₇) which he regarded as sulphur heptoxide:

4SO₃ + O₂ = 2S₂O₇.

At the ordinary temperature this was in the form of a viscous liquid which could be frozen to a crystalline solid of m.pt. 0° C. Although the heptoxide is an exothermic substance with respect to its elements, it is endothermic as regards sulphur trioxide and oxygen. As might be expected, therefore, it is very unstable, and after a few days gradually decomposes into sulphur trioxide and oxygen, the process being accelerated by rise in temperature and also by contact with platinum black.

According to Berthelot the substance fumed in moist air, owing to formation of sulphur trioxide, and reacted vigorously with water with partial decomposition, oxygen being liberated. A portion, however, dissolved in the manner of an ordinary acid anhydride, with the production of perdisulphuric acid, which can undergo further successive decomposition into permonosulphuric acid and hydrogen peroxide. Baryta water similarly gives rise to barium sulphate and barium perdisulphate, the latter being soluble in water.

Sulphur dioxide is oxidised by the heptoxide with formation of sulphuric anhydride:

S₂O₇ + SO₂ = 3SO₃.

Berthelot's experiments and conclusions have not been fully confirmed, however. According to Meyer and his co-workers, the crystalline product described is only formed when sulphur trioxide is present in excess, and may be regarded as an equimolecular mixture of sulphur trioxide and sulphur tetroxide, SO₄. There is evidence that the latter compound is produced during the oxidation of cold solutions of sulphuric acid (2.35 molar) or of alkali sulphates.

Maisin, on repeating Berthelot's experiment with a mixture of sulphur dioxide and oxygen, observed that the pressure in the discharge tube rapidly fell and an opaque solid formed on the walls of the vessel. Sometimes an oil was obtained. The product was relatively stable, did not fume in moist air, and dissolved in water without any vigorous action, forming a strongly oxidising solution. Unlike Berthelot's product, it was insoluble in sulphuric acid. The residual gases in the tube gave the reactions of the sulphate ion. Maisin concluded that the product was not the heptoxide, nor did its behaviour conform with Meyer's opinion that it was a mixture of the trioxide and tetroxide; he suggested, however, that it appeared to be a definite compound of composition S₃O₁₁, which might be regarded as a mixed anhydride of permono- and perdi-sulphuric acids.