Thermodynamic Consistency in the Modeling of Speciation in Self-Complexing Electrolytes

Speciation in aqueous solutions of complexing electrolytes is an important topic in environmental, toxicological, and sometimes industrial issues. Its determination requires an estimation of the deviations from ideality that originate from association effects, and excluded volume and electrostatic interactions between the various species. Modeling of such solutions is often based on the use of commonly accepted values for the complex formation constants and the use of the Davies equation to compute the activity coefficients of the species in solution. In this work, it is shown that this treatment may result in a thermodynamic inconsistency when moderately concentrated solutions of multiply self-complexing salts are considered. This observation casts some doubt on the determination of the speciation in such solutions. Occurrence of this shortcoming is illustrated in the case of zinc and cadmium halides. This finding suggests that the description of deviations from ideality (besides association) should be improved. It also reveals that available complexity constants for some common salts might not always have optimum values in the literature.