A process analytical technology based investigation of the polymorphic transformations during the anti-solvent crystallization of sodium benzoate from IPA/water mixture

Manufacturing of medicines often requires a crystallization stage. The crystallization method must be designed to produce the desired crystal form with specific shape, size distribution and polymorphic form. This paper presents the use of process analytical technology (PAT) tools, including focused beam reflectance measurement (FBRM) probe, turbidity meter and attenuated total reflectance (ATR) UV-Vis spectroscopy, for the monitoring and detection of polymorphic transformation during the anti-solvent crystallization of sodium benzoate from a propan-2-ol (IPA)/water mixture. A range of solid state analytical (SSA) techniques including optical microscopy, scanning electron microscope (SEM), X-ray powder diffraction (XRPD), high temperature X-ray powder diffraction (HT-XRPD), solid and solution state nuclear magnetic resonance (SSNMR), infra-red (IR) spectroscopy, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) confirmed the formation of a new, previously unreported channel solvate, which transforms into a channel hydrate during the anti-solvent crystallization experiments. A combination of DSC and hot stage microscopy (HSM) analyses discovered the formation of a higher melting polymorphic form of sodium benzoate, which also has not yet been reported in the literature. This paper provides an illustrative example how the combination of PAT tools and SSA techniques can lead to the discovery of new polymorphic forms and can provide the conditions under which different forms of the compound sodium benzoate exist so the system can be designed to produce the desired form.