Isoperibol measured curve

For the identification of the onset temperature of the exotherm, the steady-state temperature difference may be plotted against the sample temperature. After calibration, the evolved heat can be estimated. A typical plot of an isoperibolic measurement is illustrated in Figure 2.16. The sample is heated by step-wise adjustment of the jacket (or oven) temperature. The actual sample temperature results from the heat accumulation as net difference between the heat generated by the chemical reaction and the heat transferred to the jacket (or oven). The resulting mean temperature difference is relatively small and not easy to detect accurately. Thus, a range of step changes in temperature is used to define a curve, which enables a more accurate determination of the start of the exothermic event and of To to be made. 

Figure 6.13 Measured curve of an isoperibol calorimeter for a constant heat flow of finite...

Because of the medium or relatively large sample quantities used and the instrumental sensitivity, isoperibolic calorimetry is a useful tool in determining the onset temperature of an exotherm. In fact, in its simplest construction, this is really the only measurement. Digital data acquisition does allow computer analysis of the peak or area under the curve, which indicates the order of magnitude of the exotherm. Generally, the detected onset temperatures are similar to those found in the ARC (see later in Section 2.3.23) and are significantly lower than in the DSC (Section 2.3.1.1) [79]. 

Low-temperature heat capacities of the solid coordination compounds Zn(Leu)S04 l/2H20(s) and Zn(His)S04T/2H20(s) (Leu = Leucine and His = Histidine) were measured by a precision automated adiabatic calorimeter over the temperature range between T = 78 K and T = 371 K. Di and coworkers [228,229] determined the initial dehydration temperature of the coordination compounds by analysis of the heat-capacity curve. The experimental values of molar heat capacities were fitted to a polynomial equation with the reduced temperatures (x), [x = f (T)], by a least-squares method. Enthalpies of dissolution of both the complexes were determined by isoperibolic solution-reaction calorimetry. 

In an isoperibol calorimeter the heat, q, released in the combustion reaction produces an increase of the temperature of the water inside it, which is measured by means of a suitable thermometer (usually a platinum resistance thermometer, a quartz thermometer or a thermistor). The stirrer ensures that the energy released in the combustion reaction is transferred to the calorimeter so that the combustion bomb, the water and the can are quickly brought to the same temperature. The heater is used to take the temperature of the calorimeter to the starting point of the experiment. It is desirable to have an experimental temperature increment of at least 1 K. Figure 2 shows a typical temperature-time curve obtained in an isoperibol combustion calorimeter. 

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