Anyone, please help me to calculate the enthalpy change of thermal transitions like melting , crystallization and glass transition from DSC result.
I have the following DSC result...attached herewith
Dear Madhab Bera,
the specific heat at constant pressure is the change of Enthalpy with temperature Cp=(dH/dT) so you have to integrate Cp with respect to Temperature. I.e. the change in enthalpy is the area below the the peak inthe DSC measurement. (Be care full to have a stable and flat base line)
There are several free web-Sites dealing with this topic. You may refer to the following ones
http://www.perkinelmer.com/CMSResources/Images/44-74542GDE_DSCBeginnersGuide.pdf
http://pslc.ws/macrog/dsc.htm
http://www.jove.com/science-education/5559/using-differential-scanning-calorimetry-to-measure-changes-in-enthalpy
Hope this answer is of help for you.
Best regards
Jürgen Hartmann
Thank u sir ..............but I am not able to calculate delta H.
We know delta H = Cp.dT
Here, delta Cp value is given for a particular thermal transition say for melting it is 18.945 J/gK
Onset temperature , end temperature, and the peak is also given 54.2, 61.1 and 60.7 respectively.
So, with this data how can I calculate enthalpy change for melting?
I am not able to do this, please help
This is clearly described in the second reference I sent you (http://pslc.ws/macrog/dsc.htm)
You have to integrate your curve you have obtained with the DSC (the bmp you added to this question). Then you have the area under the curve (take care of the base line!). This area has the dimesion J x K/(g x s). You have to divide this result by the heating rate you used for the measurement (in K/s). Then you obtain J/g. Multiplying this with your sample mass should give you the enthalpy in Joule.
Best regards
Jürgen Hartmann
Hello,
are you sure that you encounter a glass transition? Your attached figurecontains some analysis but the shown tangents look very odd and overall the shown peak should be in my opinion related to melting/crystallization.
Concerning chemical reactions or melting/crystallization the procedure described by Jürgen Hartmann should work.
In general glass transition should not involve delta H in the typical sense. A change in heat capacity is observed which gives you a step in your DSC curve. For some Materials and depending on their thermal history the step is superimposed by a peak which is related to trapped stresses (roughly put). During glass transition the mobility increases and the material can relax.
I hope this is helpful
regards
Matthias Jaunich
A comment about your curve: you have two exothermic peaks and not only one. The first small bump is also a heat event. Therefore the integration may not be straightforward because if you integrate the whole peak, you will get the enthalpy sum for two distinct events.
Another strange thing on your curve is that the label says "glass transition". This is definetely not a glass transition but a normal, first order transition (most likely a crystallisation with an associated heat value). A glass transition gives only a step in the curve and not a peak.
A typical heat capacity vs. temperature plot for glass-forming substances can be found depicted as Fig. 1 from the following paper:
Article Aspects of the non-crystalline state
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