Calvet-type calorimeter for the study of high-temperature processes II. New ballistic method for the enthalpy of vaporization of organic materials at high temperatures

Abstract A new method, ballistic Calvet calorimetry, is described for determining the enthalpy of vaporization of organic materials at high temperatures. A sample is dropped from room temperature into a Calvet-type calorimeter set near the normal boiling temperature of the sample, producing an endothermic peak in the calorimetric signal. Because sample heat-up and evaporation are rapid, the peak height (as distinct from the more traditional peak area) is proportional to the total heat transferred. Results on pure organic compounds from 525 to 1025 K are compared with literature values derived from calorimetric, spectroscopic, and vapor-pressure measurements. These comparisons indicate that the total enthalpy change { H m o ( T ,g) − H m o (298.15 K, cr or l)} is measured with an accuracy of ±1.0 to ±1.5 per cent when conditions are optimized. The method is rapid, convenient, and applicable to small samples (10 to 100 mg). Because of the short residence time of the organic sample at high temperatures, the danger of decomposition is reduced. A series of measurements on dibenzothiophene between 590 and 630 K is described, and the thermodynamic properties of this compound are reviewed. The method is also accurate for complex industrial mixtures.