Abstract
The solution calorimetry method is based on the observation that amorphous forms are normally significantly higher in energy than are crystalline forms. The utility and validity of the calorimetric method were investigated for cephalothin sodium, cefazolin sodium, cefamandole nafate, and cefamandole sodium. Amorphous, partially crystalline, and crystalline forms were prepared and characterized by X-ray diffraction (powder), by solution calorimetry, and, for cephalothin sodium, by the thermal decomposition rate at 50°. Qualitatively, there was a good correlation between calorimetric crystaliinity and the (less precise) crystallinity derived from X-ray data. The energy and structure of the amorphous state depend on the history of the sample; even samples of the same crystalline polymorph, containing no amorphous phase, may differ in energy. Thus, the absolute value of the crystaliinity (X-ray or calorimetric) depends on the choice of amorphous and crystalline standards. The heat of solution is a precise (±1%) and unambiguous measure of the relative crystaliinity; and provided amorphous and crystalline standards are appropriately chosen, the calorimetric crystaliinity correlates well with chemical stability.
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