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Pure Appl. Chem., 2005, Vol. 77, No. 3, pp. 619-629

Consistency issues of aqueous solubility data and solution thermodynamics of electrolytes

Alex De Visscher and Jan Vanderdeelen

Department of Applied Analytical and Physical Chemistry, Ghent University, Coupure links 653, B-9000 Ghent, Belgium

Abstract: For the calculation of aqueous solubility of electrolytes, thermodynamic data from different sources are required. This can lead to errors if the data are inconsistent. This study reveals that some often-used semi-empirical equations for CO2 solubility and other equilibria are inconsistent with CODATA (Committee on Data for Science and Technology) key thermodynamic data. The equations were recalculated to make them consistent with CODATA. Combining standard thermodynamic data of NaCl with the Pitzer model leads to significant deviations from the accepted experimental value at 25 °C. Owing to the inadequacy of the Pitzer model at molalities exceeding 6 mol kg–1, this model leads to poor predictions of the HCl vapor pressure in equilibrium with highly concentrated aqueous HCl solutions. A long-standing inconsistency problem is related to the solubility of calcium carbonate. The main problem is disagreement on the existence of the CaHCO3+ ion pair. It is shown that the inconsistency largely disappears if it is assumed that the CaHCO3+ exists at low ionic strength, but becomes less stable at higher ionic strength.