Please explain the units used in the graph which is uV in the DTA graph.
if we know the weight of the sample and from the area of the curve how can we know the enthalpy of a process
The change in the enthalpy is measured via DSC analysis as it is proportional to the integral area of the endothermic/exothermic peak, the sample mass and the calibration factor are required for such measurements. DTA analysis is NOT designed for this purpose, thus, it is NOT commonly used for the measurements of the change in the enthalpy.
However, as reported by Robert F. Speyer, the area under a DTA peak is proportional to the change in enthalpy of the sample as long as the following assumptions are valid:
1. The sample and reference heat capacities and container thermal conductivities are the same.
2. The thermal conductivity between the system and the medium is high.
3. The analysis is done under conditions of constant pressure.
Whether these assumptions are valid or not is dependent on a given DTA design.
Although DTA is not commonly used for this, as the previous answer indicates, you can calibrate the DTA signal by using the melting of a metal standard of well known enthalpy of fusion. It is very important in the case of DTA calibration that the metal standard melting tempreature is very close in temperature range to the process peak you would like to determine.
You can Change the units from V, Watts, J, J/gr any other one you need.
Dear Oscar Said Hernández-Daguer ,
It is NOT a matter of unit converting. The question is "does the area under the peak in the DTA thermograph proportional to the change in enthalpy?".
http://www.dartmouth.edu/~pchem/75/pdfs/DSC.pdf
https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node41.html
I am going to look for more information to give a better answer.
Aboothahir ... The previous answers are beautiful. To conclude the situation, there are several standard material that undergo phase changes (including crystal shape change and melting for others). Such material are characterized with a well defined enthalpy. You may choose the standard material which undergo phase change in the neighborhood of the temperature range of interest to you. You record the signal ( the whole peak) and measure its area as precise as you can for both sample and standard. Divide the weight of the standard material and by its molecular weight to get the number of moles. Multiply the result by the standard enthalpy value to get quantitatively the number of joules involved in this change. You will then get a calibration of the area units in terms of enthalpy units, i.e. Say 20 joules per squared mm area. Here the calibration is done Multiply this by the peak area of your sample. The result will be number of joues involved for the phase change of your sample. Once you know the M.Wt of your sample calculate the molar enthalpy of your change. DSC is more precise in such calculations regarding the tightness of the system for heat flow. Regards
I perfectly agree to the above statements and would like to add that some Instruments simply allow the enthalpy calibration of DTA (or even cDTA) signals the same way your are used to with DSC-Signals. Technically, it is very much the same but DTA-sensors, in comparison to real heat flow DSC, are less precise. Therefore, we do not support cp-evaluation (even though some peiple also try this). With a pronounced thermal effect you can ecpect reasonable results.
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