Conversion of parameters between different variants of Pitzer’s ion-interaction model, both with and without ionic strength dependent higher-order terms

Abstract

In this article we present a method for converting the parameters between different variants of Pitzer’s ion-interaction model for electrolyte solutions. The original version of Pitzer’s ion-interaction model contains an ionic-strength dependent virial coefficient for two-ion interactions, but the virial coefficient for three-ion interactions is set to an empirically determined constant value. Extended versions of the ion-interaction model are now in common use, where the virial coefficient for three-ion interactions is also allowed to depend on the ionic strength, and terms for higher-order interactions may also be included. These extended ion-interaction equations may have one or more additional ionic-strength dependent terms, and are particularly useful for representing the thermodynamic activity values of highly soluble electrolytes. However, the parameters of the original Pitzer model are often needed to supplement existing databases based on this model. Analytical equations presented here allow the parameters of these extended ion-interaction models to be directly transformed to the parameters of the original Pitzer model. The parameters of the original Pitzer model derived by this approach are compared against parameter values obtained by directly fitting the parameters to values of the osmotic coefficients calculated using the equations and parameters of the extended Pitzer models, with excellent agreement. This approach is demonstrated by application to Rb 2SO 4(aq), Cs 2SO 4(aq), NaNO 3(aq), and Ca(NO 3) 2(aq).