The disulphuric acid solvent system. Part I. Cryoscopic and conductimetric measurements on the system H2SO4–SO3 and solutions of some metal sulphates

Abstract

Freezing-point depressions produced by a number of non-electrolytes give a value for the cryoscopic constant of disulphuric acid of 5·9 deg. mole^–1 kg. It is shown that the self-dissociation of the solvent can be represented approximately by the equation, 2H₂S₂O₇⇌ H₂S₃O₁₀+ H₂SO₄⇌ H₃SO₄⁺+ HS₃O₁₀^– The molecular dissociation is extensive (ca. 20%) but the amount of ionic self-dissociation is small. Sulphuric acid is a very weak base, while sulphur trioxide forms not only H₂S₃O₁₀, but also appreciable amounts of tetrasulphuric acid H₂S₄O₁₃ and perhaps higher polysulphuric acid. Some tetrasulphuric acid is also formed in the solvent self-dissociation. The conductivities and freezing points of solutions of some metal sulphates have been studied and the results are discussed. There is evidence that the HS₂O₇^– and HS₃O₁₀^– ions conduct by a proton-transfer process.