A thermodynamic approach to the description of hygroscopic properties of crystal-soluble solids is introduced. Isotherms of water vapor sorption by a pure substance and the products with a heterogeneous or homogeneous impurity are compared. Solubility diagrams of substances in water are used to evaluate the influence of impurities on the hygroscopic properties of the products. Evidence demonstrates that water vapor sorption by a chemical product below the hygroscopic point of a basic component is determined by heterogeneous impurities. The introduction of the term "a hygroscopically pure substance" removes inconsistencies between the experimental results related to water vapor sorption and abstract theorems. The paper discusses relevant publications, concerning hygroscopicity of soluble substances, where the impurities are ignored resulting in procedural and theoretical errors.
Keywords: absorption; adsorption; deliquescence; hygroscopicity; thermodynamics; water in solids; water sorption.
© 2015 Wiley Periodicals, Inc. and the American Pharmacists Association.