| 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 KC, KA, and KV respectively, for the reaction CaCO3(s) = Ca2+ + CO2-3: Log KC = -171.9065 - 0.077993T + 2839.319 T + 71.595 log TLog KA = -171.9773 - 0.077993T + 2903.293 T +71.595 log TLog KV = -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 CaCO03 ion pairs, revised analytical expressions for CO2-H2O equilibria, and extended Debye-Hu??ckel individual ion activity coefficients. Using this aqueous model, the equilibrium constant of aragonite shows no PCO2-dependence if the CaHCO+3 association constant is Log KCahco+3 = 1209.120 + 0.31294T - 34765.05 T - 478.782 log T between 0 and 90??C, corresponding to the value log KCahco+3 = 1.11 ?? 0.07 at 25??C. The CaCO03 association constant was measured potentiometrically to be log KCaCO03 = -1228.732 - 0.299444T + 35512.75 T + 485.818 log T between 5 and 80??C, yielding log KCaCO03 = 3.22 ?? 0.14 at 25??C. The CO2-H2O equilibria have been critically evaluated and new empirical expressions for the temperature dependence of KH, K1 and K2 are log KH = 108.3865 + 0.01985076T - 6919.53 T - 40.45154 log T + 669365. T2, log K1 = -356.3094 - 0.06091964T + 21834.37 T + 126.8339 log T - 1684915. T2 and log K2 = -107.8871 - 0.03252849T + 5151.79/T + 38.92561 log T - 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. ?? 1982. |
| Genre: | Article |
| ProdID: | 70011789 |
| Citation Author: | Plummer, L. N.; Busenberg, E. |
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| Citation End Page: | 1040 |
| Citation Issue: | 6 |
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| Citation Language: | English |
| Citation Larger Work Title: | Geochimica et Cosmochimica Acta |
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| Citation Number Of Pages: | 30 |
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| Citation Search Results Text: | 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; 1982; Article; Journal; Geochimica et Cosmochimica Acta; Plummer, L. N.; Busenberg, E. |
| Citation Start Page: | 1011 |
| Citation Volume: | 46 |
| Citation Year: | 1982 |
| Type: | citation/reference |
| Text: | 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; 1982; Article; Journal; Geochimica et Cosmochimica Acta; Plummer, L. N.; Busenberg, E. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Fri, 1 Jan 1982 00:00 -0600 |
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