An aqueous thermodynamic model for Ca²⁺−SO ²⁻₃ ion interactions and the solubility product of crystalline CaSO₃·1/2H₂O

Abstract

The solubility of CaSO₃·1/2H₂O(c) was studied under alkaline conditions (pH>8.2), in deaerated and deoxygenated Na₂SO₃ solutions ranging in concentration from 0.0002 to 0.4M and in CaCl₂ solutions ranging in concentration from 0.0002 to 0.01M, for equilibration periods ranging from 1 to 7 days. Equilibrium was approached from both the over- and the under-saturation directions. In all cases, equilibrium was reached in <1 days. The aqueous Ca²⁺−SO ²⁻₃ ion interactions can be satisfactorily modeled using either ion-association or ion-interaction aqueous thermodynamic models. In the ion-association model, the log K°=2.62±0.07 for Ca²⁺+SO ²⁻₃ ⇆CaSO ⁰₃ . In the Pitzer ion-interaction model, the binary parameters β(⁰) and β(¹) for Ca²⁺−SO ²⁻₄ were used, and the value of β(²) was determined from the experimental data. As expected given the strong association constant, the value of β(⁰) was quite small (about −134). We feel a combination of the two models is most useful. The logarithm of the thermodynamic equilibrium constant (K°) of the CaSO₃·1/2H₂O(c) solubility reaction (CaSO₃·1/2H₂O(c)⇆Ca²⁺+SO ²⁺₃ +0.5H₂O) was found to be −6.64±0.07.