619 Pure Appl. Chem., Vol. 77, No. 3, pp. 619–629, 2005. DOI: 10.1351/pac200577030619 © 2005 IUPAC 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 CO 2 solubility and other equilib- ria 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 inade- quacy of the Pitzer model at molalities exceeding 6 mol kg –1 , this model leads to poor pre- dictions of the HCl vapor pressure in equilibrium with highly concentrated aqueous HCl so- lutions. A long-standing inconsistency problem is related to the solubility of calcium carbonate. The main problem is disagreement on the existence of the CaHCO 3 + ion pair. It is shown that the inconsistency largely disappears if it is assumed that the CaHCO 3 + exists at low ionic strength, but becomes less stable at higher ionic strength. Keywords: solubility; electrolytes; modeling; Pitzer parameters; thermodynamics. INTRODUCTION For the calculation of the aqueous solubility of electrolytes, thermodynamic data are needed for the pure compound and for the species formed in aqueous solution. As most of these dissolved species are ions, in the case of a salt, a model is needed to account for the nonideality of the solution. Unless the ionic strength of the solution is low, a model accounting for specific ion interactions like the model of Pitzer [1] with experimentally derived parameters [2] will be required for accurate calculations. As data from different sources is needed to calculate the solubility, internal consistency of the data used is not guar- anteed. In 1989, the Committee on Data for Science and Technology (CODATA)published a set of in- ternationally agreed values for the thermodynamic properties of a number of key chemical substances [3]. This data set is considered to be highly internally consistent, but it is limited to about 150 species only. Furthermore, the reference data refer to 298.15 K, so additional information on heat capacities is required for calculations at other temperatures. As a practical result, other sources of information are often used for solubility calculations, and even for the data analysis of primary solubility data. This can lead to consistency issues. *Paper based on a presentation at the 11 th International Symposium on Solubility Phenomena (11 th ISSP), Aveiro, Portugal, 25–29 July 2004. Other presentations are published in this issue, pp. 513–665. ‡ Corresponding author