The solubilities of calcite, aragonite and vaterite in CO 2-H 2O solutions between 0 and 90°C, and an evaluation of the aqueous model for the system CaCO 3-CO 2-H 2O

Calculations based on approximately 350 new measurements (Ca _T-PCO ₂) of the solubilities of calcite, aragonite and vaterite in CO ₂-H ₂O solutions between 0 and 90°C indicate the following values for the log of the equilibrium constants K_C, K_A, and K_V respectively, for the reaction CaCO ₃(s) = Ca ²⁺ + CO ^2-₃: Log K _C = -171.9065 - 0.077993T + 2839.319/T + 71.595 log T Log K _A = -171.9773 - 0.077993T + 2903.293/T +71.595 log T Log K _V = -172.1295 - 0.077993T + 3074.688/T + 71.595 log T where T is in ^oK. At 25°C the logarithms of the equilibrium constants are -8.480 ± 0.020, -8.336 ± 0.020 and -7.913 ± 0.020 for calcite, aragonite and vaterite, respectively. The equilibrium constants are internally consistent with an aqueous model that includes the CaHCO ⁺₃ and CaCO ⁰₃ ion pairs, revised analytical expressions for CO ₂-H ₂O equilibria, and extended Debye-Hückel individual ion activity coefficients. Using this aqueous model, the equilibrium constant of aragonite shows no PCO ₂-dependence if the CaHCO ⁺₃ association constant is Log K _{Cahco ⁺3} = 1209.120 + 0.31294T - 34765.05/T - 478.782 log T between 0 and 90°C, corresponding to the value logK_Cahco⁺3 = 1.11 ± 0.07 at 25°C. The CaCO ⁰₃ association constant was measured potentiometrically to be log K _{CaCO ⁰3} = -1228.732 - 0.299444T + 35512.75/T + 485.818 log T between 5 and 80°C, yielding logK_CaCO⁰3 = 3.22 ± 0.14 at 25°C. The CO ₂-H ₂O equilibria have been critically evaluated and new empirical expressions for the temperature dependence of K_H, K₁ and K₂ are log K _H = 108.3865 + 0.01985076T - 6919.53/T - 40.45154 log T + 669365./T ², log K ₁ = -356.3094 - 0.06091964T + 21834.37/T + 126.8339 log T - 1684915./T ² and logK₂ = -107.8871 - 0.03252849 T + 5151.79/ T + 38.92561 logT - 563713.9/ T² which may be used to at least 250°C. These expressions hold for 1 atm. total pressure between 0 and 100°C and follow the vapor pressure curve of water at higher temperatures. Extensive measurements of the pH of Ca-HCO ₃ solutions at 25°C and 0.956 atm PCO ₂ using different compositions of the reference electrode filling solution show that measured differences in pH are closely approximated by differences in liquid-junction potential as calculated by the Henderson equation. Liquid-junction corrected pH measurements agree with the calculated pH within 0.003-0.011 pH. Earlier arguments suggesting that the CaHCO ⁺₃ ion pair should not be included in the CaCO ₃-CO ₂-H ₂O aqueous model were based on less accurate calcite solubility data. The CaHCO ⁺₃ ion pair must be included in the aqueous model to account for the observed PCO ₂-dependence of aragonite solubility between 317 ppm CO ₂ and 100% CO ₂. Previous literature on the solubility of CaCO ₃ polymorphs have been critically evaluated using the aqueous model and the results are compared.