Enthalpy

The amount of heat that is supplied or withdrawn from a system is expressed in isobaric processes by the enthalpy change of the system. Outside of a phase transition, the enthalpy change depends on the heat capacity and the temperature change of the system. It is also called sensitive heat. The phase change itself is indicated by the phase change enthalpy or heat of fusion. Since the change in temperature is relatively small compared to the enthalpy change, it is also called latent heat. The enthalpy change measured in the DSC during a phase change is therefore the sum of sensitive and latent heat. The enthalpy change is the integral form (over time) of the heat flow signal in the DSC. Phase change enthalpy is determined by the integral between the heat flow signal and the virtual baseline. Since the "real" shape of the baseline can only be interpreted during a continuous measurement, the phase change should occur in a narrow temperature range in order to be able to determine the phase change enthalpy as precisely as possible. Therefore, the heating rate should be chosen as low as possible. The sensitive component is determined by the absolute integral of the heat flow signal. In conventional DSC devices, the heat flow signal is calculated by the difference of the heat capacities to a reference system. Therefore, possible heat flow signals of the measuring system without sample must be subtracted from the sample measurement to determine the sensitive part. Compared to the phase change, the heat flow signal in the sensitive area is relatively low, which is why high heating rates are recommended for the determination of the sensitive enthalpy.