Air humidity solubility assay

Measurement of drug solubility is one of the key elements of compound characterization during the drug discovery and development process. A broad variety of solubility assay methods have been developed, including equilibrium method which requires analysis of the equilibrium composition and kinetic method which monitors the concentration of a compound dynamically at the time when a precipitate first appears or disappears in the solution. Despite the numerous experimental methods, precise drug solubility values are hard to obtain for time-consuming, sample size and manual work. In this article, we reported a new method, namely air humidity solubility assay, which measures the relative humidity of the air in equilibrium with the solution at a given temperature, and then calculates solubility from the relative humidity according to extended-non random two liquid (NRTL) model. NaCl was used as a model drug, and the solubility was measured at the temperature of 20-50℃. The results indicate that the solubility of NaCl determined with the new method is generally comparable to that determined by gravimetry that is reported in literature. The new method has a relative error of less than 2%. Although the accuracy is lower than that of gravimetry, air humidity solubility assay is more convenient, practical, operational and universal. This method provides a supplement to the existing methods.