Consistency issues of aqueous solubility data and solution thermodynamics of electrolytes

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.

Some highs and lows (and in-betweens) of solubility measurements of solid electrolytes

“…1 in ref. [5]) that at 25 °C the solubility recommended by CODATNNIST [6] is -0.75 % higher than the IUPAC value (4], while that given by Archer's well-known model [7] is -1.2 % lower. Other models, often parameterized on the same data, fared equally badly; only the value of Pitzer et al [8] agreed within the (1 a) limit of the experimental uncertainty given above.…”

“…The numerical values given in those reviews are often encapsulated in the fonn of an appropriate (usually semi-empirical) mathematical model such as a Pitzer equation. De Visscher and Yanderdeelen [5] have compared the results obtained via these models. They showed (see Fig.…”

“…The agreement between these recent studies offers hope that the solubilities of simple salts in amenable solvents can, with due care, be determined routinely with a relative accuracy of better than ±0.1 %, at least at 25 oc. Data with this level of accuracy will provide a very stern test of the reliability of any modeling equation [5].…”

IUPAC-NIST Solubility Data Series. 95. Alkaline Earth Carbonates in Aqueous Systems. Part 1. Introduction, Be and Mg

“…Solubility of CO 2 . Critical reviews of the solubility of CO 2 have been made by Crovetto 32 and by Carroll et al 33 Compilations are available from Scharlin et al 34 De Visscher and Vanderdeelen 18 found that the evaluation of Crovetto 32 at 0-80 C is more consistent with the thermodynamic data of CODATA 35 than the evaluation of Carroll et al 33 Thermodynamic properties derived from different models are given in Table 1. A more recent review of CO 2 solubility data is by Fernández-Prini et al 36 However, their correlation has poor consistency with CODATA.…”

“…To that effect, an equation that mimics some thermodynamic aspects of alkaline earth carbonate solubility was selected. De Visscher and Vanderdeelen 18 argued that the solubility (s) of an alkaline earth carbonate is approximately proportional to the cubic root of the CO 2 fugacity:…”