Differential thermal analysis peak areas

Differential thermal analysis was performed with the DuPont 900 differential thermal analyzer the heating rate was usually 10°C. per minute. To determine heats of reaction, the calorimeter attachment to the Du Pont instrument was employed. Planimeter determinations of peak areas were converted to heat values by using standard calibration curves. For the infrared spectra either a Beckman IR5A instrument or a Perkin Elmer 521 spectrophotometer with a Barnes Engineering temperature-controlled chamber, maintained dry, was used. Specimens for infrared were examined, respectively, as Nujol mulls on a NaCl prism or as finely divided powders, sandwiched between two AgCl plates. For x-ray diffraction studies, the acid-soap samples were enclosed in a fine capillary. Exposures were 1.5 hours in standard Norelco equipment with Cu Ko radiation. For powder patterns the specimen-to-film distance was 57.3 mm. and, for long-spacing determinations, 156 mm. 

Quantitative estimation of the sulphate content by the fusion method was found to be difficult because of the low percentage of the impurity. The anatase, thus prepared, was amorphous. The surface area of this anatase sample (B.E.T.) was 54 m2/g and the differential thermal analysis curve of the anatase sample is shown in fig. 2a. Although no exothermic peak due to crystallization was observed, the endothermic peak shows a definite splitting around... 

In the classical differential thermal analysis (DTA) system both sample and reference are heated by a single heat source. The two temperatures are measured by sensors embedded in the sample and reference. In the so-called Boersma system, the temperature sensors are attached to the sample pans. The data are recorded as the temperature difference between sample and reference as a function of time (or temperature). The object of these measurements is generally the determination of enthalpies of changes, and these in principle can be obtained from the area under a peak together with a knowledge of the heat capacity of the material, the total thermal resistance to heat flow of the sample and a number of other experimental factors. Many of these parameters are often difficult to determine hence, DTA methods have some inherent limitations regarding the determination of precise calorimetric values. 

Previous articles in this Series dealt with etherifications of cellulose, and an atlas on infrared analysis includes spectral data for various cellulose ethers. The preparation and industrial importance of starch ethers have been reviewed. The degree of substitution of cellulose ethers may be determined by differential thermal analysis. Where an endothermic or exothermic peak that is characteristic of the cellulose derivative occurs in the analysis curve, the peak height and area have been shown to correlate with the degree of substitution. 

Another technique related to DSC is DTA, differential thermal analysis. In this method sample and reference are heated by a single source and temperatures are measured by thermocouples embedded in the sample and reference or attached to their pans. Because heat is now supplied to the two holders at the same rate, a difference in temperature between the sample and the reference develops, which is recorded by the instrument. The difference in temperature depends, among other things, on the value of K, which needs to be low to obtain large enough differences in temperature to measure accurately. The area under a transition peak now depends on k and it is difficult to determine this accurately or to maintain it at a constant... 

For many colloidal and precipitated silicas, Tiwari et al. (114) found that differential thermal analysis (DTA) peaks, which are proportional to the number of SiOH groups that are dehydrated, can be closely correlated to the BET surface areas. The OH coverage per unit surface (BET) area is independent of particle size or pore size as long as micropores are absent. 

The calorimetric or differential thermal analysis methods indicate the amount of crystallinity by the size of the area associated with the peak that occurs in the scans (see Fig. 1-34 and 1-35). These areas can be compared to those for a polymer with known crystallinity. The technique is rapid and quite precise. It does, however, require a first-class analytical device, which is not inexpensive. 

Heats of solid-solid transitions must be determined direcdy, either by calorimetry or by measurement of the area under the endothermic peak on a differential-thermal analysis trace. The heats of transition from one polymorphic crystal form to another are listed in order of decreasing temperature, each value having the designation of the form which is stable below the transition temperature. 

Fig. 17.3 Differential thermal analysis (DTA) of concrete sample ground to <80 mesh. Area of cement peak is proportional to amount of cement in sample...

Information on the free energy, heat, and entropy of water adsorption on clays during the subsequent stages of the adsorption and desorption process can be calculated from water-vapor sorption isotherms obtained at different temperatures. Alternatively, these quantities can be determined by combining the data of a single isotherm with data for the heats of adsorption obtained directly. In appropriate calorimeters, one can measure the heat of adsorption of increments of vapor admitted to the sample, or one can measure the heats of immersion of samples that are previously equilibrated with water vapor at various relative vapor pressures. The heat of desorption can also be obtained from the peak areas of differential thermal analysis curves of partially and completely hydrated samples (Barshad [1952]). 

Despite the delay and difficulties in derivation of a suitable theoretical treatment for differential thermal analysis, it is clear that such derivations have not only justified certain aspects that had been empirically established—such as the relationship between peak area and amount of reactant and the necessity for dilution of the specimen with reference material— but have also revealed aspects that were not fully appreciated—such as the fact that the proportionality relationship holds strictly only for a ATf curve and the importance of heat conduction along thermocouple wires. Further developments in quantitative application obviously depend on designing experimental conditions compatible with those demanded by theory Wilburn [1972]. 

Sewell, E. C., 1955a. Effects of thermocouple wires on peak areas in differential thermal analysis. Research Note, Building Research Station, D.S.I.R. 

The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30°C (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transdueer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Q eas, is measured to determine the pseudo-differential... 

DSC plots are obtainetl as the differential rate of heating (in units of watts per second, calories per second, or Joules per second) against temperature, and thus they represent direct measures of the heat capacity of the sample. The area under a DSC peak is directly proportional to the heat absorbed or evolved by the thermal event, and integration of these peak areas yields the heat of reaction (in units of calories per second per gram or Joules per second per gram). Owing to the ability to facilitate quantitative data interpretation, the use of DSC analysis has virtually supplanted the use of DTA analysis. 

Differential scanning calorimetry A Perkin-Elmer DSC-2 calorimeter with Thermal Analysis Data Station was used. The calorimeter was calibrated according to manufacturer s specifications. Heats of reaction were calculated from the peak areas using indium as a standard (AH=6.80 cal/g). Tg was taken as the onset of the endothermic deflection. The heating rate was set to 20 /min. For DSC analysis, samples were prepared by two techniques a) vacuum drying of varnish and b) by flaking off resin from prepreg. 

All heat evolutions which occur simultaneously, in a similar manner, in both twin calorimetric elements connected differentially, are evidently not recorded. This particularity of twin or differential systems is particularly useful to eliminate, at least partially, from the thermograms, secondary thermal phenomena which would otherwise complicate the analysis of the calorimetric data. The introduction of a dose of gas into a single adsorption cell, containing no adsorbent, appears, for instance, on the calorimetric record as a sharp peak because it is not possible to preheat the gas at the exact temperature of the calorimeter. However, when the dose of gas is introduced simultaneously in both adsorption cells, containing no adsorbent, the corresponding calorimetric curve is considerably reduced. Its area (0.5-3 mm2, at 200°C) is then much smaller than the area of most thermograms of adsorption ( 300 mm2), and no correction for the gas-temperature effect is usually needed (65). 

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