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

Abstract

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 0 3 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 log K Cahco + 3 = 1.11 ± 0.07 at 25°C. The CaCO 0 3 association constant was measured potentiometrically to be log K CaCO 0 3 = −1228.732 − 0.299444T + 35512.75 T + 485.818 log T between 5 and 80°C, yielding log K CaCO 0 3 = 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 , K 1 and K 2 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 log K 2 = −107.8871 − 0.03252849 T + 5151.79/ T + 38.92561 log T − 563713.9/ T 2 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.