Study of the Crystallization of Mandelic Acid in Water Using in Situ ATR-IR Spectroscopy

Different aspects related to the separation of enantiomers of mandelic acid by direct crystallization are explored. Solubility, nucleation, and solid-phase transformation in aqueous solutions are investigated. Results show that a metastable conglomerate can be formed for a substance known to appear as a racemic compound. After a time-lag, the conglomerate transforms into the stable racemic compound. The time-lag ranges from a few minutes to 8 h depending on the operation conditions. The time-lag decreases at increasing concentration/temperature and in the presence of micrometer-sized particles. In situ attenuated total reflectance infrared (ATR-IR) spectroscopy and a partial least squares (PLS) calibration model are used to record the concentration of dissolved mandelic acid. Since no sampling is required and the calibration set, which is used on both racemic and enantiomerically pure solutions, can be built using the cheaper racemate, the technique should be of particular interest in applications involving chiral substances.