The standard enthalpies of ionization of sulfurous acid and the standard enthalpies of solution of sodium sulfite and sodium metabisulfite in water at 298.15 K

Standard enthalpies of ionization of sulfurous acid and the standard enthalpies of solution of Na 2 SO 3 (s) and Na 2 S 2 O 5 (s) in water at 298.15 K were measured by solution-reaction calorimetry. All operations were carried out under N 2 with particular care to exclude O 2 from the reaction system. Steps in the corrections to the standard states are outlined in detail. The results (mean ± uncertainty interval) are: Δ ion H m ∘ (H 2 SO 3 , aq) = −(17.82±0.18) kJ·mol −1 ; Δ ion H m ∘ (HSO 3 − , aq) = −(3.67±0.06) kJ·mol −1 ; Δ sol H m ∘ (Na 2 SO 3 , s) = −(13.26±0.06) kJ·mol −1 ; and Δ sol H m ∘ (Na 2 S 2 O 5 , s) = (21.34±0.07) kJ·mol −1 for the process: Na 2 S 2 O 5 (s)+∞H 2 O(l) = 2NaHSO 3 (aq). For the equilibrium: 2HSO 3 − (aq) = H 2 O(l)+S 2 O 5 2− (aq), we selected K ∘ = 0.032 and Δ r H m ∘ = −4.2 kJ·mol −1 from the litrature.