Calorimetric determination of the standard enthalpy of the first ionization of aqueous sulfur dioxide or ‘sulfurous acid’ at 25°C

Previous investigations of the first ionization of aqueous sulfur dioxide or ‘sulfurous acid’ have led to ΔH ₁ ⁰ values ranging from ?15 to ?24.5 kJ-mol^?1. To help with selection of a ‘best’ value for this enthalpy of ionization we have made calorimetric measurements of enthalpies associated with adding dilute perchloric acid to dilute aqueous sulfur dioxide and sodium bisulfite. Results have led to ΔH ₁ ⁰ =?17.4₀ kJ-mol^?1, which we have combined with K₁=0.0139 and the related ΔG ₁ ⁰ =10.60 kJ-mol^?1 to obtain ΔS ₁ ⁰ =?93.₉ J-K^?1-mol^?1 for the first ionization, all referring to 25°C.