ABSTRACT
The physical, chemical, and mechanical properties of pharmaceutical drugs and dosage forms are critically dependent on the presence of moisture. The water sorption-desorption isotherm describes the relationship between the equilibrium amount of water vapor sorbed to a solid and the thermodynamic quantity, water activity, at constant temperature and pressure. Many drugs and excipients utilize water as an integral part of their crystal structure. Very different mechanisms of water sorption/desorption occur for the different samples. The most fundamental manner of demonstrating the relationship between sorbed water vapor and a solid is the water sorption-desorption isotherm. Generation of water sorption-desorption isotherms for a particular solid can lend considerable insight into the nature of the water-solid interaction, as well as the surface characteristics of the solid. Saturated salt solutions at controlled temperature maintain a constant relative humidity as long as excess salt and bulk solution are present.
