The aqueous solubility product of solid solutions: 2. Extension to binary solutions with stoichiometric coefficients greater than unity; analogy with vapor pressure of a binary liquid solution

Abstract

For simple binary solid solutions with stoichiometric coefficients of unity, the analogy of total aqueous solubility product with total vapor pressure is straightforward. For either case, ideal systems show linearity of mole fraction with either total solubility product or total vapor pressure. For binary solid solutions with stoichiometric coefficients greater than unity, there is curvature in the plot of mole fraction against total solubility product, even for an ideal system. The curvature is similar to positive deviation from Raoult's law in a liquid-vapor system. The analogy of the total solubility product with the total vapor pressure is fully examined. Conceptual similarities between the two systems are explored. It is shown that Lippmann's solutus curve, which he developed by analogy with the isothermal condensation curve of a liquid-vapor system, coincides with solubility calculations by classical thermodynamics. A hypothetical liquid-vapor model system is constructed to show that the curvature in total solubility diagrams, introduced by stoichiometric coefficients greater than unity, would have an analog in the liquid-vapor system if the liquid-vapor system contained the stoichiometric constraints inherent in a solid-aqueous system.