Isoperibolic principle

The isoperibolic principle is an intermediate mode where the temperature of the thermostated jacket Tj is maintained at a constant temperature. The sample... 

A liquid serves as the calorimetric medium in which the reaction vessel is placed and facilitates the transfer of energy from the reaction. The liquid is part of the calorimeter (vessel) proper. The vessel may be isolated from the jacket (isoperibole or adiabatic), or may be in good themial contact (lieat-flow type) depending upon the principle of operation used in the calorimeter design. 

Isoperibolic instruments have been developed to estimate enthalpies of reaction and to obtain kinetic data for decomposition by using an isothermal, scanning, or quasi-adiabatic mode with compensation for thermal inertia of the sample vessel. The principles of these measuring techniques are discussed in other sections. 

Because of the operating principles of the equipment, especially in the isoperibolic mode, complex calculation and calibration procedures are required for the determination of quantitative kinetic parameters and the energy release during decomposition. Also, for a reaction with a heterogeneous mixture such as a two-phase system, there may be mass transfer limitations which could lead to an incorrect T0 determination. 

The principles of titration calorimetry will now be introduced using isoperibol continuous titration calorimetry as an example. These principles, with slight modifications, can be adapted to the incremental method and to techniques based on other types of calorimeters, such as heat flow isothermal titration calorimetry. This method, which has gained increasing importance, is covered in section 11.2. 

An important variation of the adiabatic principle is isoperibol calorimetry. Well-defined heat leaks, minimized by efficient calorimeter construction and experiment design, are compensated for by calculation and/or extrapolation. The isoperibol design holds the temperature of the immediate environment surrounding the calorimeter constant. The word isoperibol literally means "constant temperature environment. ... 

Another measurement principle is the DSC, after Boersma [8]. In this case, no compensation heating is used and a temperature difference is allowed between sample crucible and reference crucible (Figure 4.5). This temperature difference is recorded and plotted as a function of time or temperature. The instrument must be calibrated in order to identify the relation between heat release rate and temperature difference. Usually this calibration is by using the melting enthalpy of reference substances. This allows both a temperature calibration and a calorimetric calibration. In fact, the DSC after Boersma works following the isoperibolic operating mode (see Section 4.2.2). Nevertheless, the sample size is so small (3 to 20 mg) that it is close to ideal flux. 

This nomenclature is close to that proposed by Hemminger and Hohne in 1984. It makes use of the same three primary criteria the principle of measurement, the mode of operation and the construction principle. Each criterion leads to its own classification, as shown hereafter. The main difference from the 1984 classification is that, instead of only proposing two major methods of calorimetry (compensation of the thermal effects and measurement of the temperature differences, respectively) there are now three. This is obtained by splitting the second one into calorimeters that measure a heat-accumulation (including the adiabatic and the isoperibol calorimeters) and calorimeters that measure a heat-flow. 

Isoperibol (quasi-isothermal) calorimeters are used in medicine and biology for determinations of the metabolic heats of organisms under various conditions (Dauncey, 1991). Here the calorimeter system (container or chamber) is large enough to accommodate one animal or person in relative comfort. The measurement principle is similar to the upper examples the container for the organism, positioned in thermostatized surroundings, is enclosed in a uniform layer or wall of a heat conductive material, and the temperature difference between the two... 

Adiabatic and Isoperibol Calorimeters.—Most calorimeters used in combustion and reaction calorimetry undergo a change of temperature when reaction takes place. If the calorimeter is surrounded by a jacket, the temperature of which is controlled to be the same as that of the calorimeter, no heat-exchange occurs between the siuroundings and the calorimeter, which is then described as adiabatic. However, if the temperature of the environment is maintained constant (in a type of calorimeter conveniently described as isoperibol and sometimes, incorrectly, as isothermal) some heat-exchange occurs between the calorimeter and its surroundings, but may be accurately determined by analysis of the temperature-time curves before and after reaction takes place, provided the reaction is of short duration (say not exceeding 15 min). With slower processes, isoperibol calorimeters are less useful, and the adiabatic principle is easier to effect and yields more accurate results. 

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