Properties of aqueous solutions. A treatise against osmotic and activity coefficients

Abstract

The current common and almost universal belief is that most water solutions do not behave ideally is incorrect. It is our understanding of what is happening that is not ideal. The culprit for this is the ill-conceived invention of osmotic and activity coefficients, which allegedly measure deviations from ideality. There are currently two different definitions of what constitutes an “ideal” solution. Solutions adhering to Raoult’s law are considered as behaving ideally. Osmotic coefficients, φ, measure deviations of the solvent from ideality, with φ = 1.000 indicating ideality. Activity coefficients, γ, measure deviations of solutes, with γ = 1.000 indicating ideality. Hardly any of the many aqueous solutions whose vapor pressures have been measured have φ or γ values of unity. The definitions of φ and of γ assume that strong electrolytes dissociate completely and that the amount of solvent available is constant over all solute concentrations. Raoult’s law was not based on any assumptions, but on the observation that the relative diminution of vapor pressure is proportional to the amount of solute particles. These two generally accepted, but different, definitions of ideality are in conflict with each other. They have generated an undeserved aura of mystique and complexity regarding the behavior of aqueous solutions. The present work shows that one needs to know only one thing for demonstrating ideal behavior by adherence to Raoult’s law. Each solute’s affinity for water, i.e., its unique hydrodesmic (water-binding) number, Hd. We compare results expressed in terms of osmotic and activity coefficients to those obtained by use of Hd. We examine vapor pressure measurements of the following typical and well-studied solutes: glycerol, glucose, KCl, NaCl, CaCl2 and H2SO4. Their vapor pressures have been measured many times and solutions of the four electrolytes are commonly used as a standard against which properties of other solutes are determined.