Absorption of sulfur dioxide in dilute aqueous solutions of sulfuric and hydrochloric acid

Spectrophotometric measurements of combined phase and chemical equilibrium have been performed for SO2 + H2SO4 + H2O and SO2 + HCl + H2O systems at 298 K and atmospheric pressure. The vapour and liquid phases have been analysed in situ using a fibre optic based technique described in an earlier publication. The measurements have been performed up to an equilibrium partial pressure of about 1 kPa for sulfur dioxide. The concentration range of the acids have been varied up to 0.5 M for H2SO4 and 1.0 M for HCl. A thermodynamic model is presented which allows the interpolation and the extrapolation of the experimental results. The calculated partial pressures of SO2 in the SO2 + H2SO4 + H2O system are compared with existing literature values. The experimental results for the SO2 + HCl + H2O system show an anomaly in the uv-spectrum of molecular dissolved sulfur dioxide. The absorption band for SO2 is shifted towards higher wavelength. This effect is interpreted in terms of the formation of a weak complex SO2Cl−.