Thermal event, endothermic

The only thermal event in the differential thermal analysis curve of (/))-penicillamine is the melting endotherm at 185 °C. Either polymorph of (z>)-penicillamine gives the same endotherm [2]. 

Differential thermal analysis (DTA) is the monitoring of the difference in temperature between a sample and a reference as a function of temperature [42], Differences in temperature between the sample and reference are observed when changes occur that require a finite heat of reaction. If AH is positive (endothermic reaction), the temperature of the sample will lag behind that of the reference. If the AH is negative (exothermic reaction), the temperature of the sample will exceed that of the reference. Differential thermal analysis is not normally used for quantitative work instead it is used to deduce temperatures associated with thermal events. 

In the DTA measurement, an exothermic reaction is plotted as a positive thermal event, while an endothermic reaction is usually displayed as a negative event. Unfortunately, the use of power-compensation DSC results in endothermic reactions being displayed as positive events, a situation which is counter to IUPAC recommendations [38]. When the heat-flux method is used to detect the thermal phenomena, the signs of the DSC events concur with those obtained using DTA, and also agree with the IUPAC recommendations. 

The DSC thermogram for Form I was obtained at a heating rate of 20 C/minute under nitrogen, and is shown in Figure 3. The sole thermal event noted was the melting endotherm which began with an onset temperature of 259°C, and exhibited a peak maximum at 275"C. The endothermic heat of fusion was determined to be 248 J/g. 

Closely related to DSC is the much older technique of differential thermal analysis (DTA). DTA works on the simpler principle of measurement (via thermocouple) of temperature difference between a sample and reference as the same heating power is supplied to both. The DTA trace therefore represents a temperature effect, which is related only semiquantitatively to AH. A combined DTA-TGA trace for the Werner clathrate (Section 9.4) [Ni(NCS)2(4-phenylpyridine)4] CgHg is shown in Figure 9.22. The DTA trace shows that all three thermal events observed are endothermic. The first is associated with loss of the benzene guest, while the second and third relate to loss of the coordinated 4-Phpy ligands. Note the high temperature (about 350 °C) required to remove the enclathrated benzene. This is a clear indication of the thermal stability of the Werner clathrate family.18... 

The differential scanning calorimetry (DSC) thermogram of ciprofloxacin was obtained using a DuPont TA-9900 thermal analyzer interfaced with a DuPont Data Unit. The thermogram shown in Figure 2 was obtained at a heating rate of 10°C/min, and was run from 40 to 400°C. The sole observed thermal event was the melting endotherm which was observed at 322° C. 

The purpose of this section is to define the various parameters that are measured by DSC. The types of thermal events, exothermic or endothermic, that can be measured by DSC are reported in Table 1. The following sections will describe some of the more fundamental thermal events. Examples from the pharmaceutical field will be given to illustrate the techniques. The examples will be based on either single components such as drug substance and bulk excipients or on a mixture of components such as physical blends of drugs and excipients, solid dispersions, formulated drugs after granulation, and/or compression. 

When the thermal event lakes place (in this case melting) the power output will change to reflect the endothermic event as described earlier. One might rea.sonably expect the melting process to be effectively instantaneous, as when one reaches the melting point all the intermolecular bonds holding the molecules in the crystal lattice should break... 

Differential thermal analysis (DTA) data for talc exhibit an endotherm at approximately 950-975°C. This thermal event has been attributed to decomposition of the talc to MgSi03, Si02, and water [25, 26]. DTA has also been used to determine mineral impurities in talc batches [27]. 

If an endothermic event occurs in the sample, the temperature of the sample lags that of the reference, which follows the heating program. A typical DTA curve for such a thermal event is shown in Fig. 2C. If an exothermic process occurs in the sample, the response will be in the opposite direction. The negative peak shown in Fig. 2C is termed an endotherm and is characterized by its onset temperature. The area under a DTA peak is proportional to the enthalpy associated with the thermal event. Because such proportionality is not linear with temperature, only approximate enthalpy data can be obtained from DTA. The nature of the reference material is important. It must undergo no thermal events over the DTA operational temperature range and it must not chemically react with its container. Alumina has been... 

Thermal events in the sample thus appear as deviations from the baseline, in either an endothermic or exothermic direction, depending upon whether more or less energy is supplied to the sample compared with the reference material. The operational temperature range of DSC is less than that of DTA—being typically subambient to 750°C. Temperature and energy calibration of DSC instruments is achieved using the ICTAC certified reference materials. ... 

A number of parameters can be measured from the various thermal events detected by DSC. For example, for a melting endotherm, the onset, peak temperatures, and enthalpy of fusion can be derived. The onset temperature is obtained by extrapolation from the leading edge of the endotherm to the baseline. The peak temperature is the temperature corresponding to the maximum of the endotherm, and the enthalpy of fusion is derived from the area of the thermogram. It is an accepted custom that the extrapolated onset temperature is taken as the melting point however, some users report the peak temperature in this respect. We tend to report both for completeness. 

Solution calorimetry can also be used to evaluate amorphous/crystalline content in a binary mixture. The enthalpy of solution for the amorphous compound is an exothermic event, whereas that of the crystalline hydrate is endothermic. Enthalpy of solution is a sum of several thermal events, that is, heat of wetting (incorporating sorption process, such as surface sorption and complexation), disruption of the crystal lattice, and solvation. The order of magnitude of solution enthalpy for the crystalline compound suggests that the disruption of the crystal lattice predominates over the heat of solvation. In addition, the ready solubility of the compotmd in aqueous media is probably governed by entropy considerations. 

It is clear from Figure 1.2 that the technique may easily and rapidly detect the temperature at which a particular thermal event is occurring. Indeed, current uses of the technique are based largely on the ability of the method to detect the initial temperatures of thermal processes and to qualitatively characterize them as endothermic or exothermic, reversible or irreversible, first order or higher order, etc. (2). The ability to run experiments in a range of atmospheres has also rendered the approach particularly useful for the construction of phase diagrams. 

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