Hi Fransisco, DTA as shown in Wikipedia (the material under study and an inert reference are made to undergo identical thermal cycles, while recording any temperature difference between sample and reference). So, if there is no interaction to sample (matter) which is no change followed by endothermic or exothermic process in advance,  it means the difference between sample and reference is zero and we got the straight line in graphic (dT vs time or temperature). 

It is simply that the material has reached a stable state with no change in phase or no chemical changes are observed. You can ensure that if you mage a TG (thermal gravimetric) test. you should observe a mass loss that continues till certain temperature after which no endothermic or exothermic DT or DSC peaks are observed 

Dear Rudy and Adel, thanks for your answers, but I still have doubts. As I understand the same Q is applied to bot sample and reference. If part of that Q, say DQ, is absorbed by the sample as latent heat, a difference in temperature say DT is to be produced, after that, both sample and reference continue to translate the added heat into temperature change. What is wrong in my reasoning?

I attach a diagram with the expected curve according to my reasoning, which shows no abrupt return no zero value after change of phase.

Best regards.

Francisco,

You are plotting delta T versus temperature.  Endothermic and exothermic events will cause the temperature of the sample to be less than or greater than the reference respectively.  Once the event has taken place, a melt for example, then the temperature difference between the sample and reference will tend to move back to the baseline value as the sample and reference are heated to higher temperatures.  You observe this for the PETN sample.  The exothermic peak results from the decomposition of the PETN sample and heat is released.  The delta T for this event is positive and then tends to move back to the baseline value as the sample and reference are heated to higher temperatures.

Fransisco... It is really one of our best moments when the DTA or DSC curves return to the zero line. It is an indication of complete change  and the production of a stable product or phase. Once you have the zero level or baseline, the heat supplied to both the reference and the sample results in only the raising the temperature and nothing else. It often happens that the newly formed material or say phase  reached the temperature  of  the next change (a thermal decomposition or a change in the phase), the behavior of the material will be different from the reference and the heat supplied to the sample will be used to activate the change. For exothermic changes, there will be two sources of heating: the furnace and the reaction or the phase change. ... We  can go for thorougher discussion if needed ..  Good luck.. Mahmood

Dear Mahmood:

But, this "bell" shape, its a common behaviour in DTA results?.

Can you the pictures attached in my second post and share your thoughts please?

Best regards.

Dear Francisco:

Can you tell me what the heating rate used in that assay? Sometimes the heating rate causes peak broadening, or some impurity (at least in analysis by DSC). Thanks!

Dear Iguatina, the heating rate was 10°C/min, the paper is "thermal decomposition of mixtures containing nitrocellulose and pentaerythritol tetranitrate" thermochimica acta 208 (1992) 147-160. Author Chen-Chia Huang.

Best regards.

Thank you Francisco!

I've read the article, and I think that peaks of decomposition (generally exothermic events) are broad due to the nature of decomposition process, in most cases heterogeneous reactions. It's very hard perform calculations in such processes, in certain circumstances we can hardly see one good peak to obtain thermodynamic parameters.

As for the question about signal back to zero, I agree with the answers given earlier by our colleagues.

Greetings from Brazil

The restoration of the base line in DTA after the decomposition reaction  is more of an exception than a rule and  can occur only in the cases where the total heat capacity of the products matches  those of the reactants.This happens  very rarely and one  more often ends up with significant deviation in the base line due to mismatch of the heat capacity of the reactants and the products.The base line can however be restored only if the amount of the sample is much less than the mass of the sample container so that even after the decomposition reaction the contribution of heat capacity of the product is relatively much less than that of the sample container.Since the two sample containers used in DTA are nearly equal in  mass prior to the experiment. the small heat  capacity difference between the reactants and the products becomes  insignificant  if size of the sample and the product formed is much much less than the mass of the container.

Thanks to all for your answers.

The baseline of a DTA (or DSC) curve returns to zero only then, if the thermoanalytic setup has total balance between sample and reference. This is seldom the case: Even if you are measuring 2 empty crucibles (sample & reference), one might have slightly more mass, is closer to the heater, or something like that. To get rid of this, it is a good idea to make a "correction measurement" in advance. This means you use the same setup and the same temperature program you intend to use later with your sample, but perform in this "correction measurement" with empty crucibles. You will see that the DTA curve is not always exactly at zero, but moves with T slowly to the endothermal or exothermal direction. Afterwards, optimum with the same crucibles, you measure your sample. You can then compute the difference of both curves, which gives the effect of the sample alone. Have a look on the attached Fig., where the correction measurement is blue and has a small upward bend. The sample (CaF2) has some minor effects around 200-500°C, and then the melting above 1400°C.

DSC is more accurate than DTA because the heat flow is calibrated for DSC: in DTA the signal is micro volts instead energy units in DSC. To remove the slope of HF curve in DTA measurement you must preform a blank test to correct the HF signal.

The answer given by  Detlef   Klemm is identical to the one I gave previously. The issue here is about the deviation of the base line of  DTA  after the completion of the decomposition reaction. If the base line is completely restored then the total heat capacity of the Sample  + Container   =   Total heat capacity of the Products   +  Container.  =  total heat capacity of the Reference material  + Container. The total heat capacity of Sample   +  Container  is already mached with that of the Reference material and Container  prior to the start of the DTA experiment.

For quantitative application, both DTA as well as DSC instruments have to be suitably calibrated for heat capacity and enthalpy.

It is true that the instruments commercially available in recent years are provided with the software for base line correction.But the basic issue is why the base line is restored completely in the experiment reported by  Francisco Angel. I think that the explanation given by me and Prof. Klemm is adequate to understand this observation..  .

I think this question can be answered using a DSC curve simulated using “DSC Curve Solutions,” for which I am the developer. 

When a DSC run starts, a tail appears; this is followed up by a horizontal curve segment until a thermal event occurs at higher temperatures.  After the thermal event, it returns to nearly the same horizontal line.  Most people chop off the tail segment in presentation of their DSC curves, so it appears returning to “zero.” 

The attached is for the DSC curve of an iPP sample, heating at 10°C/min to 190°C, isothermal for 10 min then cools down at 15°C/min.  The black curve is the actual experimental curve, the red the simulation with the “DSC Curve solutions”.  Regards, Jinan