In the thermal gravity-differential thermal analysis (TG-DTA), it is said that the area under a DTA peak is the enthalpy change, which is not affected by the heat capacity of the sample. I am not clear about this. So, can anyone suggest the reason behind? Thank you!
To my knowledge, the area under a DTA peak is proportional to the enthalpy change; for a given compound, the area depends of the mass inside the experimental ampoule (if the mass of the reference is always the same).
If we compare the area of the DTA peak at the melting (for example) for two compounds with the same mass but different heat capacity, these areas must be different since at the transition dH=TdS+Vdp
Best regards
Pedro
For a real DTA intrument (not a heat-flux DSC) the peak area is only approximately proportional to enthalpy change (if sample masses are different, then you should divide the peak area by mass).
For DTA, especially at high temperatures, there are heat transfer effects such as radiation losses. These effects depend on sample geometry and emissivity coefficient. Therefore, the enthalpies obtained from classical DTA have much larger uncertainty compared do DSC (whether heat-flux or power-compensating).
Both heat capacity and latent heat of transformation (enthalpy) are materials constant for given substance and transformation. If the transformation is melting for example or first order face transition, at melting point we have discontinuity in heat capacity, it is sad that it diverges to infinity, while instruments (DSC, DTA) shows peak at this temperature. In power compensated DSC the signal is directly proportional to the energy, while heat flux DSC and in DTA needs calibration.
Just before the melting the sample has a heat capacity of the solid sample Cps and after a melting peak in DSC (DTA) it has a heat capacity of liquid sample Cpl. If the difference between Cps and Cpl is appreciable, in order to obtain precise value of enthalpy of melting one should integrate the peak using sigmoidal baseline. Hope to be useful.
Even in power compensated DSC a calibration is needed as well, to exactly consider:
a) how much of the compensate energy is actually being consumed by the sample, in case of endothermic events or
b) how much of the generated energy by the sample is actually being transferred to the energy controlling system, in case of exothermic events;
This is why in any type of DSC, it must be calibrated before use.
Of course any DSC needs calibration. My point was to mark ,that while signal in power compensated DSC is directly proportional to delta E , in heat flux DSC and in DTA signals are proportional to delta T . So that there exist delicate difference in principles of calibration of both type of instruments.
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