DSC technique:
DSC technique is not only sensitive for the determination of ? H, but it is also very sensitive for the determination of heat capacities (Cp). When a sample is subjected to a heating programme is DSC, a rate of heat flow within the sample is proportional to its heat capacity. This might be detected through the displacement of the base line as described in Figure. The value of Cp might be determined at a particular temperature by measuring this displacement (d). :
Cp =d/heating rate × m
= (dH /dt )/ (dT / dt) × 1/ m
Using Eq. 11.9, we can deduce the unit of Cp. In DSC curve displacement (d) will be measured in mJ s-1. If heating rate is in °C s-1- and m is expressed in g, then,
C p = mJ s -1/g × 1/o C s -1 = mJ g -1C -1
Cp can also be expressed in term of mcal. As mCal g -1C -1 [conversion factor for J and Cal. is: 1 calorie = 4.2 J].
In practice we generally measure the base line shift through reference to a base line obtained for empty sample and reference pans. To additionally minimize experimental error we usually determine heat capacity of the sample by comparing with the known heat capacity of the standard.
K ′(H 2 - H 1) = mCp (dT/dt)
or K ′d = mCp(dT/dt)
where, H1 and H2 are differential heat generated when the instruments is first run without any sample at all and then with the test sample in position (in DSC curve (H2 - H1) is expressed as displacement, d). K ′ is calibration factor; it can be determined by calibration against standard substance. Therefore, K ′ from the Eq. 11.10 can be eliminated, if a material with a known heat capacity is used to calibrate the instrument.
Once of the commonly used standard is α -aluminium oxide (Al2O3) or synthesized sapphire for which specific heat has been determined to five significant figures in the temperature range 0 to 1200 K. After the base line and sample program, a third program is run with a weighed sapphire structure. At any temperature T, following equation applies:
K ′ d = m C p dT/dt
K ′d ′ = m ′ C p′ dT/dt
where d and d ′ are ordinate deflections (displacements) due to the sample and the standard respectively, m′ CP′ we get are mass and heat capacity of the standard.
d/d' = m/m' C p/ C' p = dm'/d'm
Thus the calibration requires only the comparison of the two displacement values at the same temperature. We can easily calculate value of Cp on putting the rest values in the Eq. The primary components of DSC are quite same except the differential energy measuring system. In DSC, two principle works: one based on power compensation and other heat flow method. In power compensation method smaller secondary heater are attached two equalizing the generated energy difference between sample and reference materials? Although in heat flow method heat flux passing through sample and reference are evaluated and their difference is associated energy consumed or released in the thermal reactions.