Polymorphism of anhydrous oxalic acid unravelled

Highlights: • Previous studies were found to give contradictory outcomes considering preparation of the oxalic acid polymorphs. • Phase behavior of oxalic acid cannot be reliably described using a single experimental technique. • Combination of DSC, vapor pressure measurements and X-ray powder diffraction was used to resolve the phase behavior. • Thermodynamically consistent vapor pressure equations are developed using the simultaneous correlation procedure. Despite being a very common compound in the industry as well as in the nature, the thermodynamic description of oxalic acid is far from being complete and reliable. The literature is contradictory considering both preparation of the two known polymorphs and the triple point temperature above which β-form should be more stable. The published vapour pressure data for α-phase show also a considerable disagreement. In this work, the polymorphs of anhydrous oxalic acid are subjected to a combined thermodynamic and X-ray diffraction study since a single technique cannot give an exhaustive answer on the phase behaviour of the compound. The simultaneous correlation procedure that combines vapour pressure data, calorimetric phase-transition properties, and heat capacities of the crystalline and ideal-gas phase is used to develop a consistent thermodynamic description. Finally, the triple point temperature and transition enthalpy are derived.