DSC Error & Interpretation [PDF]

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completed) do not have fundamental significance, but they can still be a useful characteristic of a DSC curve. The term procedural thermogram, often used for the temperature at which temperature change appears to commence. This indicates that a start of thermal reaction, temperature does not have a fixed value, but depends on the experimental procedure employed to get it. Similar to this there are many factors which influence a DSC curve. These factors may be due to instrumentation or nature of sample. We have listed the main factors which affect the shape, precision and accuracy of the experimental results: 1.

2.

Differential Thermal Analysis, Scanning Calorimetry and Thermometric Titrations

Instrumental factors: a)

Furnace heating rate.

b)

Recording or chart speed

c)

furnace atmosphere

d)

Geometry of sample holder/ location of sensors

e)

Sensitivity of recording mechanism.

f)

Composition of sample container.

Sample Characteristics: a)

Amount of sample

b)

Solubility of evolved gases in sample.

c)

Particle size

d)

Heat of reaction

e)

Sample packing

f)

Nature of sample

g)

Thermal conductivity.

Some of these factors we have are already described in sec.11.2.4 in detail.

11.3.4

Sources of Error

There are a number of sources of error in DSC, and they can lead to inaccuracies in the recorded data of heat. Some of the errors may be corrected by placing the thermo balance at proper place and handing it with the care. For understanding we are discussing some common source of errors during operation or common as discussed in DTA except the in accuracy caused by secondary heaters and thermostats. Errors can be avoided by proper placing of instrument in the laboratory, maintaining operating temperature, and constant power supply. By avoiding excessive heating rate and proper gas flow rate other errors can be also avoided. To further minimize the errors during experiments, similar to DTA, DSC instruments are also be calibrated for the temperature and peak area measurements with suitable standards. The only difference is that calibration constant in DTA situation is temperature dependent to a significant degree. Therefore in DTA measurement, we should calibrate peak areas using a standard which provide a reference peak in a same temperature range as the test sample. In DSC situation, K is independent of temperature. Therefore, it requires simple steps for the calibration of the instrument. For peak area calibration we require standard of high purity and accurately known enthalpy of fusion ( ∆ H f ) are required. Few examples of calibration standards are indium (In), benzoic acid, tin, lead, silver, gold, etc.

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Thermal Methods

SAQ 6 DTA and DSC, which method you will prefer for quantitative purposes and why? …………………………………………………………………………………………... …………………………………………………………………………………………... …………………………………………………………………………………………... …………………………………………………………………………………………...

11.3.5

Interpretation of DSC Curve

DSC curve of a pure compound is a fingerprint of that compound in the context of transition temperature as well as heat required for that transition. Therefore, DSC curve can be used to infer about the presence of a particular compounds and its thermal behaviour. The peaks observed shifting of base line either up or down. A typical DSC curve is shown in Fig 11.11. The peak above the base line is exothermic while down the base line is endothermic. We have seen above how area under DSC Curves is related to the amount of energy released or absorbed in a physico-chemical change. It has been shown that under certain conditions the area under the peak is proportional to the amount of heat evolved in a reaction. So this area under the curve is used for stochiometric ratio of analyzed compounds (quantitative interpretation). Now we see in next example how it can be used to compare thermal stability of a material for physical state and chemical states .This can be used for chemical identification of a material (qualitative interpretation). Such information can be used to select material for certain end-use application, predict product performance and improve product quality. DSC Curves of a polymeric mixture and probable transitions are shown in Fig. 11.14 for illustration about probable change in behaviour of a polymer sample.

Fig. 11.14: Change in Behavior of Polymeric Materials in DSC

The DSC technique is more sensitive than DTA and it provides clear presence of a thermal events occurring during course of heating of time ageing of material . Thus, the information acquired by DSC is more realistic. The technique is used for the presence of polymorphism, degree of crystallinity, curing fraction etc. Curves clearly indicate that Fig. 11.13is showing the peaks for the glass transition, ordering, melting and decomposition of individual polymers. The ratio of areasunder the curve by dividing the enthalpy of heat of decomposition, provides the ratio of individual monomers in a analysed copolymer sample . The heat of reaction (∆Hr) observed in DSC can be further used to calculate molar enthalpy of reactions by using following formula:

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∆Hm = ∆Hr × Mr /m Where, (∆Hm = molar enthalpy of reaction , ∆Mr = relative molar mass of analysed compound, m = Mass of substance used for analysis.

11.3.6

Differential Thermal Analysis, Scanning Calorimetry and Thermometric Titrations

Applications

Differential scanning Calorimetry (DSC) used to measure energy changes as a function of temperature or time. A typical graph is shown in Fig. 11.13. Using this technique it is possible to observe a number of characteristic properties of a sample like fusion, crystallization, glass transition temperatures (Tg) as well as other thermo chemical reactions. DSC can also be used to study oxidation, as well as other chemical reactions. Glass transitions may occur as the temperature of an amorphous solid is increased. These transitions appear as a step in the baseline of the recorded DSC signal. This is due to the sample undergoing a change in heat capacity; no formal phase change occurs. As the temperature increases, an amorphous solid will become less viscous. At some point the molecules may obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form. This is known as the crystallization temperature (Tc). This transition from amorphous solid to crystalline solid is an exothermic process and results in a peak in the DSC signal. As the temperature increases the sample eventually reaches its melting temperature (T m). The melting process results in an endothermic peak in the DSC curve. The ability to determine transition temperatures and enthalpies makes DSC an invaluable tool in producing phase diagrams for various chemical systems. The technique is widely used across a range of applications, both as a routine quality test and as a research tool. The equipment is easy to calibrate, using low melting indium for example, and is a rapid and reliable method of thermal analysis. The few notable specific applications of DSC are: The result of a DSC experiment is a curve of heat flux versus temperature or time. There are two different conventions: exothermic reactions in the sample shown with a positive or negative peak. This curve can be used to calculate enthalpies of transitions. This is done by integrating the peak corresponding to a given transition. It can be shown that the enthalpy of transition can be expressed using the following equation:

∆H = KA where ∆H is the enthalpy of transition, K is the calorimetric constant, and A is the area under the curve. The calorimetric constant will vary with the instrument and can be determined by analyzing a well-characterized sample with known enthalpies of transition. Most of well known spectroscopic methods of great value in the qualitative and quantitative chemical analysis are based on our ability to measure energy absorption or emission caused by transition from one energy state to another. The great potential of thermal spectroscopy for quantitative analysis was not realized in the past because of the absence of a suitable, fast scanning, the calibration run for the synthetic compounds and the base line technique used in the area measurement and can be used for the quantitative analysis of constituents present in the fiber blend. Many materials can exist in two or more different crystal line forms. The chemical reactivity and physical properties of different forms vary frequently one to another. Technological handling requires one of the perfect suitable form, hence the phenomenon is of great importance in chemistry and conveniently studied by DSC.

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