One hundred and fifty new measurements of the solubility of witherite were used to evaluate the equilibrium constant of the reaction BaCO₃(cr) = Ba²⁺(aq) + CO₃²⁻(aq) between 0 and 90°C and 1 atm total pressure. The temperature dependence of the equilibrium constant is given by logK = 607.642 + 0.121098T − 20011.25/T − 236.4948 logT where T is in degrees Kelvin. The logK of BaCO₃(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⁻¹and −154.0 J · mol⁻¹ · K⁻¹, respectively. The equilibrium constants are consistent with an aqueous model that includes the ion pairs BaHCO₃⁺(aq) and BaCO₃⁰(aq) Three different methods were used to evaluate the association constant of BaHCO₃⁺(aq), and all yielded similar results. The temperature dependence of the association constant for the reaction Ba²⁺(aq) + HCO₃⁻(aq) = BaHCO₃⁺(aq) is given by logK_BaHCO3⁺ = −3.0938 + 0.013669T.

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 Ba²⁺(aq) + CO²⁻₃(aq) = BaCO₀³(aq) is given bylogK_BaCO3⁰ = 0.113 + 0.008721T.

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 BaCO₃-CO₂-H₂O 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 Ba²⁺(aq) to −532.5 and −555.36 kJ · mol⁻¹, respectively, for agreement with solubility data.