Rapidly Converging Activity Expansions For Representing The Thermodynamic Properties Of Fluid Systems: Gases, Non-Electrolyte Solutions, Weak And Strong Electrolyte Solutions

For dilute gases and non-electrolyte solutions in the McMillan-Mayer standard state, an activity expansion due to Mayer has great advantages over the n o d concentration expansion (virial equation) for strongly associating species. For weakly interacting systems, both approaches are suitabIe. The activity expansion eliminates the need to differentiate between strong " chemical " interactions and weak " physical " interactions since the same equation is used in each situation. The equation has been modified to represent electrolyte solutions in the McMillan-Mayer standard state by requiring that it be consistent with the Debye-Huckel and higher order limiting laws for strong electrolytes and that it be equivalent to a chemical association model for weak electrolytes. The result is a compact equation which contains no arbitrary ion-size parameters and which does not require the classification of an electrolyte as strong or weak. For 2:2 electrolytes, the equation gives a very good fit to the anomalous low concentration region. For practicd thermodynamic calculations, similar equations for molal activity coefficients are proposed ; good fits of the data are obtained.