Chemical elements
    Amorphous Sulphur
    Colloidal Sulphur
    Physical Properties
    Chemical Properties
      Hydrogen Sulphide
      Metal Polysulphides
      Hydrogen Polysulphides
      Hydrogen Pentasulphide
      Hydrogen Trisulphide
      Hydrogen Disulphide
      Sulphur Monofluoride
      Sulphur Tetrafluoride
      Sulphur Hexafluoride
      Sulphur Monochloride
      Sulphur Dichloride
      Sulphur Tetrachloride
      Sulphur Monobromide
      Thionyl Fluoride
      Sulphuryl Fluoride
      Fluorosulphonic Acid
      Thionyl Chloride
      Sulphuryl Chloride
      Sulphur Oxytetrachloride
      Pyrosulphuryl Chloride
      Chlorosulphonic Acid
      Thionyl Bromide
      Sodium Sulphoxylate
      Sulphur Dioxide
      Sulphurous Acid
      Sulphur Trioxide
      Pyrosulphuric Acid
      Sulphuric Acid
      Persulphuric Anhydride
      Persulphuric Acid or Perdisulphuric Acid
      Permonosulphuric Acid
      Amidopermonosulphuric Acid
      Thiosulphuric Acid
      Polythionic Acids
      Dithionic Acid
      Trithionic Acid
      Tetrathionic Acid
      Pentathionic Acid
      Wackenroders Solution
      Hexathionic Acid
      Polythionic Acids
      Sulphur Sesquioxide
      Hydrosulphurous Acid
      Nitrogen Sulphide
      Nitrogen Persulphide
      Nitrogen Pentasulphide
      Nitrogen Chlorosulphide
      Trithiazyl Chloride
      Thiotrithiazyl Chloride
      Dithiotetrathiazyl Chloride
      Nitrogen Bromosulphide
      Thiotrithiazyl Bromide
      Thiotrithiazyl Iodide
      Thiotrithiazyl Nitrate
      Thiotrithiazyl Hydrogen Sulphate
      Thiotrithiazyl Thiocyanate
      Sulphonic Acids
      Amidosulphonic Acid
      Imidosulphonic Acid
      Nitrilosulphonic Acid
      Hydroxylamine-monosulphonic Acid
      Nitrososulphonic Acid
      Hydroxylamine-disulphonic Acid
      Hydroxylamine-isodisulphonic Acid
      Hydroxylamine-trisulphonic Acid
      Dihydroxylamidosulphonic Acid
      Sulphazinic Acid
      Sulphazotinic Acid
      Dehydrosulphazotinic Acid
      Nitrosulphonic Acid
      Nitrosulphonyl Chloride
      Nitrosulphonic Anhydride
      Nitrosulphuric Acid
      Nitrosodisulphonic Acid
      Sulphonitronic Acid
      Sulphates of Hydroxylamine
      Hydroxylamine Dithionate
      Hydrazine Dithionate
      Hydrazine Amidosulphonate
      Carbon Subsulphide
      Carbon Monosulphide
      Carbon Disulphide
      Thiocarbonic Acid
      Ammonium thiocarbonate
      Thiolcarbonic Acid
      Xanthic Acid
      Perthiocarbonic Acid
      Sodium perthiocarbonate
      Carbonyl Sulphide
      Thiocarbonyl Chloride
      Thiocarbonyl Tetrachloride or
      Carbon Hexachlorosulphide
      Trichloromethyl Disulphide
      Thiocarbonyl Sulphochloride
      Carbon Bromosulphide
      Amino-derivatives of Thiocarbonic Acid
      Dithiocarbamic Acid
      Azidodithiocarbonic Acid
      Cyanogen Monosulphide
      Cyanogen Trisulphide
      Sulphur Thiocyanate
      Disulphur Dithiocyanate
      Thiocyanic Acid
      Dithiocyanic Acid
      Trithiocyanuric Acid
      Perthiocyanic Acid

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 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.

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