RAMAN SPECTRA OF THE H 2 SO 4 –SO 3 AND D 2 SO 4 –SO 3 SYSTEMS: EVIDENCE FOR THE OCCURRENCE OF POLYSULPHURIC ACIDS

Abstract

The Raman spectra of oleums, i.e. mixtures of sulphur trioxide and sulphuric acid, have been re-examined. Similar measurements on the sulphur trioxide – deuterosulphuric acid (D 2 SO 4 ) system are also reported. The experimental results and conclusions of previous similar work on oleums are discussed. By comparison of the spectra of oleums with those of the polysulphuryl halides it is shown that the polysulphuric acids H 2 S 2 O 7 and H 2 S 3 O 10 are present in this system. The increase in the frequency of the SO 2 stretching vibrations with increasing concentration of sulphur trioxide gives evidence for the existence of higher polysulphuric acids such as H 2 S 4 O 13 at high concentrations of sulphur trioxide. In relatively concentrated oleum, sulphur trioxide monomer and trimer are also present. It is shown that the self-dissociation of liquid H 2 S 2 O 7 gives mainly molecular H 2 S 2 O 10 and H 2 SO 4 and not ionic species. The conclusions reached from the interpretation of the Raman spectra of the D 2 SO 4 –SO 3 system are similar to those arrived at for sulphuric acid oleums. The spectra of solutions of NaHSO 4 in oleums were also examined, and are discussed.