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

Polythionite solutions Li(SO2)(n)-HMPA, where SO2 is reduced chemically by lithium, have been investigated using ESR and spectrophotometry. The variat ion 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: S4O82-,the dimer of S2O4-.. In previous works, this absorptio n band was assigned to S2O4-., named the blue complex for this reason. We s how that the blue complex is not S2O4-., but its dimer. The equilibrium con stants for the reactions between the reduced species: the dimerization of S O2-., that of S2O4-., and the formation of S2O4-. by reaction between SO2 a nd SO2-., were determined, thus allowing the concentrations of all the spec ies in a given solution to be calculated. In the presence of a supporting e lectrolyte, the red complex S3O6 (2-) is observed and it is shown, from spe ctroelectrochemical experiments, that this species is formed by reaction be tween SO2-. and S2O4-..