Temperature calibration of a high-pressure DSC for measurements in ammonia

In this paper the temperature calibration of a Mettler Toledo DSC27HP high-pressure DSC for measurements in an ammonia atmosphere is described. Measurements were performed on the melting process of three reference substances in golden crucibles: indium, tin and lead. The procedure described by the German Society for Thermal Analysis and Calorimetry, GEFTA, was applied: to correct for temperature gradients in the DSC-cel extrapolated onset temperatures of the endothermic melting peaks were measured as a function of heating rate to find by extrapolation the extrapolated onset temperature at zero heating rate, which should be a temperature corrected for temperature gradients. However, measurements performed at different pressures (between 1.5 and 8.4 bar) showed that the evaluated extrapolated onset temperatures at zero heating rate decreased with increasing pressure. This observation cannot be explained by the known dependence of melting temperatures on pressure. Therefore pressure dependence of the extrapolated onset temperatures must be caused by experimental issues. It is assumed that, although the results were extrapolated to zero heating rate, results are still influenced by temperature gradients in the DSC-cell. As a mean value, the extrapolated onset temperatures at zero heating rate decreased by 0.053°C when the pressure was raised by 1 bar. Since the software package does not allow for the pressure dependence of calibration parameters, measurement results must be corrected manually for this effect.