Photo-Stability of Several Crystal Forms of Cianidanol

The photo-stabilities of five crystal forms of cianidanol, monohydrate·I, tetrahydrate·I, monohydrate·II, anhydrate·II and anhydrate·IV, on exposure to ultraviolet (UV) irradiation were investigated. Monohydrate·II was quite stable to UV irradiation at 20°C and 0-96% relative humidity, while the photo-stabilities of the other four crystal forms were influenced by humidity. Lower permeability of oxygen into the crystals as compared to the other crystal forms was assumed to be one of the reasons for the high photo-stability of monohydrate·II.