Comparison of the Pitzer and Hückel Equation Frameworks for Activity Coefficients, Osmotic Coefficients, and Apparent Molar Relative Enthalpies, Heat Capacities, and Volumes of Binary Aqueous Strong Electrolyte Solutions at 25 °C

Abstract

Detailed numerical analysis of literature data for five key thermodynamic properties (activity coefficients, osmotic coefficients, apparent molar relative enthalpies, heat capacities, and volumes) of well-characterized aqueous strong electrolyte solutions shows that equally good fits are generally obtained from the Pitzer and Huckel equations when both frameworks have the same number of adjustable parameters. Fifty-seven strong electrolyte systems with published values at 25 °C have been examined: the differences found between the two frameworks are never large and are only significant with the activity and osmotic coefficients of eight systems. Most of these eight exceptions involve a lanthanide salt that has been characterized by just one research group. For describing the solution thermodynamics of binary (single) strong electrolytes, the Pitzer equations are found to possess no fundamental theoretical advantage over an extended Huckel framework. On the other hand, slight additional flexibility inherent in the Pitzer formalism makes it more susceptible than the Huckel equations to the numerical effects of experimental error. Comparing the fits achieved by the Pitzer and Huckel equation frameworks can help to identify (a) the simple behavior characteristic of strong electrolytes even at high concentration and (b) signs of error in experimental data sets lacking confirmation by independent measurement.