A method to apply Zavitsas' aqueous electrolyte model to multicomponent solutions and its equivalence to Zdanovskii's rule

Zavitsas developed an aqueous electrolyte thermodynamic model and applied it to single electrolyte solutions previously. Here, a method to apply the model to multicomponent solutions is derived by setting the water activity to the mole fraction of free water in the mixture. The method is shown to work effectively for the CsBr-KNO3-H2O and CsCl-KNO3-H2O systems at 100.3 degrees C even at concentrations greater than 6 molal. The largest error in the prediction of water activities was just -0.00365 for the CsBr-KNO3-H2O system and -0.00631 for the CsCl-KNO3-H2O system. Zavitsas' model naturally obeys Zdanovskii's rule because when two electrolyte solutions with the same fraction of free water are mixed together, water is obviously going to have that same fraction in the mixture. This means that Zavitsas' model will likely work well for the many electrolytes that obey Zdanovskii's rule.