A New Method for Classification of Salts and Cocrystals Using Solid-State UV Spectrophotometry.

Salt and cocrystal formulations are widely used as techniques to improve physicochemical properties of compounds. Some spectrometric techniques to distinguish cocrystals from salts have been reported; however, it has not been possible to adapt these formulations with many compounds, because of limitations, high difficulty, and exceptions. Therefore, we focused on the possibility of UV spectrometry, which had not been reported as a classification technique for salts and cocrystals. The integration values of solid-state UV/visible (Vis) spectra of indomethacin salts were larger than those of physical mixtures of indomethacin and counter molecules, while that of indomethacin cocrystal was not large compared with that of the physical mixture. From these results, differences between a salt and a cocrystal were observed in their solid-state UV/Vis absorption spectra for indomethacin complexes. Therefore, it is suggested that solid-state UV/Vis absorption spectra can be used as a new technique to classify salts and cocrystals.