Temperature- and moisture-dependent phase changes in crystal forms of barbituric acid

The dihydrate of barbituric acid (BAc) and its dehydration product, form II were investigated by means of moisture sorption analysis, hot-stage microscopy, differential scanning calorimetry, thermogravimetry, solution calorimetry, IR- and Raman-spectroscopy as well as powder X-ray diffraction. The dihydrate desolvates already at and below 50% relative humidity (RH) at 25 degrees C whereas form II is stable up to 80% RH, where it transforms back to the dihydrate. The thermal dehydration of barbituric acid dihydrate (BAc-H2) is a single step, nucleation controlled process. The peritectic reaction of the hydrate was measured at 77 degrees C and a transformation enthalpy of Delta(trs)H(H2-II) = 17.3 kJ mol(-1) was calculated for the interconversion between the hydrate and form II. An almost identical value of 17.0 kJ mol(-1) was obtained from solution calorimetry in water as solvent (Delta(sol)H(H2) = 41.5. Delta(sol)H(II) = 24.5 kJ mol(-1)). Additionally a high-temperature form (HT-form) of BAc, which is enantiotropically related to form II and unstable at ambient conditions has been characterized. Furthermore, we observed that grinding of BAc with potassium bromide (KBr) induces a tautomeric change. Therefore. IR-spectra recorded with KBr-discs usually display a mixture of tautomers, whereas the IR-spectra of the pure trioxo-form of BAc are obtained if alternative preparation techniques are used. (C) 2008 Elsevier B.V. All rights reserved.