The solubility of BaCO 3(cr) (witherite) in CO 2-H 2O solutions between 0 and 90°C, evaluation of the association constants of BaHCO 3+(aq) and BaCO 30(aq) between 5 and 80°C, and a preliminary evaluation of the thermodynamic properties of Ba 2+(aq)

Abstract

One hundred and fifty new measurements of the solubility of witherite were used to evaluate the equilibrium constant of the reaction BaCO 3( cr) = Ba 2+( aq) + CO 3 2−( aq) between 0 and 90°C and 1 atm total pressure. The temperature dependence of the equilibrium constant is given by log K = 607.642 + 0.121098 T − 20011.25/ T − 236.4948 log T where T is in degrees Kelvin. The log K of BaCO 3(cr), the Gibbs energy, the enthalpy and entropy of the reaction at 298.15 K are −8.562, 48.87 kJ · mol −1, 2.94 kJ · mol −1 and −154.0 J · mol −1 · K −1, respectively. The equilibrium constants are consistent with an aqueous model that includes the ion pairs BaHCO 3 +(aq) and BaCO 3 0(aq) Three different methods were used to evaluate the association constant of BaHCO 3 +(aq), and all yielded similar results. The temperature dependence of the association constant for the reaction Ba 2+( aq) + HCO 3 −( aq) = BaHCO 3 +( aq) is given by log K BaHCO 3 + = −3.0938 + 0.013669 T. The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15°K are 0.982, −5.606 kJ · mol −1, 23.26 kJ · mol −1 and 96.8 J · mol −1 · K −1, respectively. The temperature dependence of the equilibrium constant for the reaction Ba 2+( aq) + CO 2− 3( aq) = BaCO 0 3( aq) is given by log K BaCO 3 0 = 0.113 + 0.008721 T. The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15° K are 2.71, −15.49 kJ · mol −1, 14.84 kJ · mol −1 and 101.7 J· mol −1 · K −1. The above model leads to reliable calculations of the aqueous speciation and solubility of witherite in the system BaCO 3-CO 2-H 2O from 0 to more than 90°C. Literature data on witherite solubility were re-evaluated and compared with the results of this study. Problems in the thennodynamic selections of Ba compounds are considered. Newer data require the revision of Δ fH° and Δ fG° of Ba 2+(aq) to −532.5 and −555.36 kJ · mol −1, respectively, for agreement with solubility data.