Effect of HCl on Spectral Properties of Sulfur Dioxide and its Derivatives Dissolved in Water

A number of studies on sulfur dioxide (SO2) in toxicology and pharmacology have been reported, however spectral properties of SO2 and its derivatives were seldom investigated. We investigated the absorption spectra of SO2, sodium sulfite (Na2SO3), sodium bisulfite (NaHSO3) and sodium metabisulfite (Na2S2O5) in aqueous solution. In the meanwhile, the effects of HCl on spectral properties of SO2 and its derivatives were also investigated. We found that gaseous SO2 in ethanol, n-butyl-alcohol and glycerol had a characteristic absorption peak at 276 nm. Na2S2O5 and NaHSO3 exhibited an absorption peak at 257 nm. Absorption of SO2 at 276 nm was strongly enhanced in the presence of HCl. NaHSO3, Na2SO3 and Na2S2O5 also exhibited absorption at 276 nm with the addition of HCl, which was enhanced with the increase of HCl concentration. Importantly, two conclusions have been reached on the basis of our results. First, we attributed the absorbing power of SO2 to SO2 molecule, rather than hydrated sulfur dioxide. Second, absorption of SO2 strongly enhanced by HCl at 276 nm was due to H+, instead of the formation of a complex SO2Cl-. Primary studies also indicated that NaHSO3 and Na2S2O5 with HCl had a similar effect as SO2 did in rat thoracic aortic rings, which prompted us believe that NaHSO3 and Na2S2O5 with HCl may be acted as a donor of SO2 in biology and other area.