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The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O

Geochimica et Cosmochimica Acta

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DOI: 10.1016/0016-7037(82)90056-4

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Abstract

Calculations based on approximately 350 new measurements (CaT-PCO2) of the solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C indicate the following values for the log of the equilibrium constants KCKA, and KV respectively, for the reaction CaCO3(s) = Ca2+ + CO2−3

 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 CaCO03 ion pairs, revised analytical expressions for CO2-H2O equilibria, and extended Debye-Hückel individual ion activity coefficients. Using this aqueous model, the equilibrium constant of aragonite shows no PCO2-dependence if the CaHCO+3 association constant is between 0 and 90°C, corresponding to the value logKCahco+3 = 1.11 ± 0.07 at 25°C. The CaCO03association constant was measured potentiometrically to be  between 5 and 80°C, yielding logKCaCO03 = 3.22 ± 0.14 at 25°C.

The CO2-H2O equilibria have been critically evaluated and new empirical expressions for the temperature dependence of KHK1 and K2 are and logK2 = −107.8871 − 0.03252849T + 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-HCO3 solutions at 25°C and 0.956 atm PCO2 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 CaCO3-CO2-H2O 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 PCO2-dependence of aragonite solubility between 317 ppm CO2 and 100% CO2.

Previous literature on the solubility of CaCO3 polymorphs have been critically evaluated using the aqueous model and the results are compared.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O
Series title:
Geochimica et Cosmochimica Acta
DOI:
10.1016/0016-7037(82)90056-4
Volume
46
Issue:
6
Year Published:
1982
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
1011
Last page:
1040
Number of Pages:
30
Online Only (Y/N):
N
Additional Online Files(Y/N):
N