The solubility of strontianite (SrCO₃) in CO₂-H₂O solutions between 2 and 91°C, the association constants of SrHCO⁺₃(aq) and SrCO⁰₃(aq) between 5 and 80°C, and an evaluation of t

Seventy new measurements (Sr_T-P_co2 of the solubility of strontianite were used to evaluate the equilibrium constant for the reaction SrCO₃(cr) = Sr²⁺(aq) + CO²⁻₃(aq) between 2 and 91 °C. The temperature dependence of the equilibrium constant is given by the expression Log K = 155.0305 − 7239.594/T − 56.58638 log T where T is in degrees Kelvin. The log K of strontianite, the Gibbs energy, enthalpy and entropy of the reaction at 25°C are −9.271 ± 0.020, 52.919 ± 0.08 kJ · mol⁻¹, −1.67 ± 1.30 kJ · mol⁻¹, and −183.1 ± 4.0 J · mol⁻¹ · K⁻¹, respectively. The equilibrium constants are consistent with an aqueous model that includes the ion pairs SrHCO⁺₃(aq) and SrCO⁰₃(aq) which were evaluated by potentiometric methods between 5 and 80°C. The equilibrium constant for the association reaction Sr²⁺(aq) + HCO⁻₃(aq) =SrHCO⁺₃aq) is given by the expression Log K_SrHCO⁺3 = −3.248 + 0.014867T. The log of the association constant, the Gibbs energy, enthalpy and entropy of the reaction at 25°C are 1.18, −6.76 kJ · mol⁻¹, 25.30 kJ · mol⁻¹, and 107.5 J · mol⁻¹ · K⁻¹, respectively. The equilibrium constant for the association reaction Sr²⁺(aq) + CO²⁻₃(aq) = SrCO⁰₃aq) is given by the expression Log K_SrCO⁰3 = −1.019 + 0.012826T. The log of the association constant, the Gibbs energy, enthalpy, and entropy of the reaction at 25°C are 2.81, −16.01 kJ · mol⁻¹,21.83 kJ · mol⁻¹, and 126.9 J · mol⁻¹ · K⁻¹, respectively. These results lead to reliable calculation of the aqueous speciation and solubility of strontianite in the system SrCO₃-CO₂-H₂O from 0 to more than 90°C. Literature data on the solubility of strontianite have been evaluated and compared with these results.

Our new data for strontianite have been used in an evaluation of the thermodynamic properties of Sr²⁺(aq), SrCO₃(cr) and related compounds. The following values are recommended for the standard enthalpy (kJ · mol⁻¹), Gibbs energy (kJ · mol⁻¹), and entropy (J · mol⁻¹ · K⁻¹), respectively, of Sr²⁺aq): −550.90 ± 0.50, −563.83 ± 0.8 and −31.50 ± 2.0, and for SrCO₃(cr): −1225.77 ± 1.1, −1144.73 ± 1.0 and 97.2.