Spectroscopic characterization of the blue and red complexes in reduced SO2 solutions in HMPA

Abstract

Polythionite solutions Li(SO₂)_n–HMPA, where SO₂ is reduced chemically by lithium, have been investigated using ESR and spectrophotometry. The variation of the Li/SO₂ ratio allows us to modify the distribution of the reduced species, which has led to the identification and characterization of these species. It is shown that the 595 nm absorption band must be assigned to a new species: S₄O₈^2-, the dimer of S₂O₄^-•. In previous works, this absorption band was assigned to S₂O₄^-•, named the blue complex for this reason. We show that the blue complex is not S₂O₄^-•, but its dimer. The equilibrium constants for the reactions between the reduced species: the dimerization of SO₂^-•, that of S₂O₄^-•, and the formation of S₂O₄^-• by reaction between SO₂ and SO₂^-•, were determined, thus allowing the concentrations of all the species in a given solution to be calculated. In the presence of a supporting electrolyte, the red complex S₃O₆^2- is observed and it is shown, from spectroelectrochemical experiments, that this species is formed by reaction between SO₂^-• and S₂O₄^-•.