Calculation of the vapor pressure–temperature relationship using thermogravimetry for the drug allopurinol

Allopurinol sublimates when subjected to thermogravimetry in a nitrogen atmosphere. The process is zero order and plots of ln k versus reciprocal temperature (K) give a value of 132.64 kJ mol−1 for the activation energy. On the basis that equilibrium is achieved at all temperatures the data is used to establish a plot of the vapor pressure curve versus temperature from which a Clausius–Clapeyron relationship can be used to establish the ΔH for sublimation. The method requires that the equipment is calibrated and data for p-aminobenzoic acid is used for this purpose.