Solubility of Form α and Form γ of Glycine in Aqueous Solutions

The solubilities of two polymorphs of glycine (α-form and γ-form) in different aqueous solvent mixtures were measured by using the analytical gravimetric method. Based on the measured solubility data in pure water, the relative stability and the enthalpy and entropy of dissolution of the two forms were determined using van’t Hoff plots. The solubilities of forms α and γ were also investigated in aqueous solutions containing methanol or polyethylene glycol (PEG200) at 20 °C. It was shown that the solubilities of both forms decreased with increasing concentration of methanol and PEG200. The pH dependence of solubility of the γ-form was measured in the pH range of (0.35 to 13.7) at temperatures of (20 and 25) °C. The solubility profiles show a U-shape with pronounced changes at pH below 3.0 or above 10.0.