Theory Calvet calorimeter

A reaction should be stopped by flooding with a cold solvent. The amount of solvent needs to be sufficient to cool the reaction mass to a thermally stable level. To test this theory, flooding was tested in a Calvet calorimeter (Figure 10.4). The experiment showed that the dilution is endothermal with a heat release of—ffikjkg"1 of mixture (reaction mass and solvent). The reaction mass (2230 kg) has a specific heat capacity of 1.7kJ kg 1 K 1 and a temperature of 100 °C. The dilution is with 1000 kg of a solvent at 30 °C, with a specific heat capacity of 2.6kJ kg"1 K"1. The resulting mixing temperature (Tm) can be calculated from a heat balance ... 

A mathematical treatment of heat flow calorimeters can be made at almost any level of complexity. Owing to the intricacy of the non-steady states involved, the matter can hardly be covered in a comprehensive manner. Even with regard to a steady or quasi-steady state, a thorough treatment of the matter may turn out to be too complex, extensive, and in the final analysis of little practical value. A relatively simple theory can be formulated for the Calvet calorimeter, which is widely used in practice and allows slow scanning of the surroundings, too (see Section 7.9.2.3). 

An apparatus with high sensitivity is the heat-flow microcalorimeter originally developed by Calvet and Prat [139] based on the design of Tian [140]. Several Tian-Calvet type microcalorimeters have been designed [141-144]. In the Calvet microcalorimeter, heat flow is measured between the system and the heat block itself. The principles and theory of heat-flow microcalorimetry, the analysis of calorimetric data, as well as the merits and limitations of the various applications of adsorption calorimetry to the study of heterogeneous catalysis have been discussed in several reviews [61,118,134,135,141,145]. The Tian-Calvet type calorimeters are preferred because they have been shown to be reliable, can be used with a wide variety of solids, can follow both slow and fast processes, and can be operated over a reasonably broad temperature range [118,135]. The apparatus is composed by an experimental vessel, where the system is located, which is contained into a calorimetric block (Figure 13.3 [146]). 

The fluctuations of the apparatus function / result, of course, in a fluctuation Ah of the halfwidth (Figure 6.24). It can be shown that the haUwidth of the steeper arm of the apparatus function measures the resolution of the instmment along the abscissa (usually the time see Theory of Calvet s Calorimeter in Section 7.9.2.3). Accordingly, the fluctuation of the halfwidth provides a measure of the resolution of the desmeared measured curve along the abscissa. 

Most commonly used are heat-flow microcalorimeters of the Han-Calvet type [5, 8]. The detailed theory and operation of this calorimeter can be found elsewhere [11]. The apparatus is composed of an experimental vessel, where the studied system is located, which is placed into a calorimetric block (Fig. 3.1). The temperature of the block, which functions as heat sink, is controlled very precisely. The heat generated in the system flows to the heat sink and is accurately measured by means of detector. This is made of a large numbers of identical thermocouples (a thermopile) that surrounds the vessel and connected to the block (Fig. 3.2) in such a way that vessel and block temperature are always close to each other. A signal is generated by the detector that is proportional to the heat transfer per unit time. Undesired signals due to the external temperature fluctuations in the calorimetric block are minimized by connecting in opposition two heat flow detectors from two identical vessels, one of which is used to perform the experiment, the other being used as a reference. Heat related to the introduction of the probe and other parasitic phenomena are thus compensated. 

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