Quasi-isothermal/isobaric

The thermal decomposition of a number of double complexes containing the [Cr(CN)6]3-anion has been investigated and the results obtained under quasi-isothermal and -isobaric conditions are summarized in Scheme 31.336 The CN-flipping reactions involved in the scheme resemble similar reactions of Prussian blue analogues such as Co3[Cr(CN)6]2 337 and Fe3[Cr(CN)6]2-338 The unusually low magnetic moment of Cr(CN)6Cr indicates considerable metal-metal interaction via the CN bridges. 

The alkaline earth halide hydrates and related salts have been studied by TG and other thermal techniques under a variety of atmospheric and instrumental conditions. Using the quasi-isothermal and quasi-isobaric techniques in different types of sample holders, Paulik et al. (61) found that various hydrate stoichiometries could be obtained. As shown in Figure 4.8, the inflection points in curve (/) indicate the presence of CaBr2-2H20 and CaBr2 H20. In curves (2)-(4), the inflection points correspond 10 CaBr2-3H20 and... 

Paulik F, Paulik J, Erdey L (1958) The Derivatograph. I. An Automatic Recording Apparatus for Simultaneously Conducting Differential Thermal Analysis, Thermogravimetry, and Derivative Thermogravimetry. Z anal Chem 160 241-252. For standardization, quasi-isothermal and isobaric analyses and some example research with the Derivatograph see also (1966) Anal Chim Acta 34 419-426 Paulik F, Paulik J (1973) J Thermal Anal 5 253-270 (1975) 8 557-576. 

A more complex system of temperature and atmosphere control used in the study of decomposition rates is quasi-isothermal and quasi-isobaric thermogravimetry [6]. In this method a heating programme linearly increases the sample temperature until a mass change is detected. On detection of a mass... 

It has already been stated that SCTA envelopes the original CRTA techniques. Essentially, there are two forms of CRTA - the quasi-isothermal/quasi-isobaric methods (Paulik) and the constant rate method (Rouquerol). The Paulik method is based on thermogravimetry (TG) and is known as Q-TG . It depends on a control system that maintains a constant rate of mass loss and the measured parameter is temperature as a function of time. CRTA relies on using a transducer to monitor the pressure of evolved gas in a continuously evacuated chamber. The sample is heated in such a way as to maintain the monitored gas pressure constant. Since the pressure is maintained constant, the rate of gas pump-off is maintained constant and thus the rate of mass loss (when a single gas is evolved) is also maintained constant. Hence, CRTA is effectively vacuum thermogravimetry. The similarity between the Paulik and Rouquerol techniques is immediately apparent. Both maintain the reaction rate constant and both control the pressure of the evolved species in the reaction environ ment. Hence, the differences in the techniques are purely semantic. These techniques allow a precise... 

Paulik F and Paulik J (1986) Thermoanalytical examination under quasi-isothermal/quasi-isobaric conditions. Thermochimica Acta 100 23-59. 

A valuable approach for measuring thermal degradation kinetic parameters is controlled-transformation-rate thermal analysis (CRTA) - a stepwise isothermal analysis and quasi-isothermal and quasi-isobaric method. In this method, some parameters follow a predetermined programme as functions of time, this being achieved by adjusting the sample temperature. This technique maintains a constant reaction rate, and controls the pressure of the evolved species in the reaction environment. CRTA is, therefore, characterised by the fact that it does not reqnire the predetermined temperature programmes that are indispensable for TG. This method eliminates the nnderestimation and/or overestimation of kinetic effects, which may resnlt from an incomplete understanding of the kinetics of the solid-state reactions normally associated with non-isothermal methods. 

Isa and Oruno describe a method that enables identification of intermediates (in the gypsum dehydration process) more easily. The method involves the use of simultaneous TG-DTG-DTA under various sealed atmospheres corresponding to three systems—open completely, sealed, and quasi-sealed. Endothermic DTA peaks appear earlier (129 and 133°C) than the point of decreasing TG. This technique, resembling the quasi-isothermal and isobaric thermogravimetry (Q-TG), is superior to the latter in that it needs less of the sample. 

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