Calorimeter proper

All calorimeters consist of the calorimeter proper and its surround. This surround, which may be a jacket or a batii, is used to control tlie temperature of the calorimeter and the rate of heat leak to the environment. For temperatures not too far removed from room temperature, the jacket or bath usually contains a stirred liquid at a controlled temperature. For measurements at extreme temperatures, the jacket usually consists of a metal block containing a heater to control the temperature. With non-isothemial calorimeters (calorimeters where the temperature either increases or decreases as the reaction proceeds), if the jacket is kept at a constant temperature there will be some heat leak to the jacket when the temperature of the calorimeter changes. 

Figure 10.4 Adiabatic high-temperature calorimeter [15], 1 Calorimeter proper 2 Silver guard 3 Silver shield 4 Shield heater 5 Thermocouple sleeve 6 Silica glass container 7 Sample 8 Calorimeter heater 9 Pt resistance thermometer 10 Silica ring spacer 11 Type S thermocouple 12 Guardheater 13 Removable bottom. Reproduced by permission of F. Grpnvold.

Figure 6.1 Examples of calorimeters in which the calorimeter proper (a) contains the reaction vessel and (b) coincides with the reaction vessel.

The calorimetry lexicon also includes other frequently used designations of calorimeters. When the calorimeter proper contains a stirred liquid, the calorimeter is called stirred-liquid. When the calorimeter proper is a solid block (usually made of metal, such as copper), the calorimeter is said to be aneroid. For example, both instruments represented in figure 6.1 are stirred-liquid isoperibol calorimeters. The term scanning calorimeter is used to designate an instrument where the temperatures of the calorimeter proper and/or the jacket vary at a programmed rate. 

The energy change associated with the process under study induces an energy change of the calorimeter proper, which can be determined by monitoring a corresponding temperature change or heat flux. In some calorimeters the reaction occurs in a closed vessel whose volume does not vary in the course of the experiment. This happens, for example, in bomb combustion calorimetry, where the reaction takes place inside a pressure vessel called the bomb, and in... 

Figure 6.2 Schematic representation of (a) an adiabatic calorimeter, (b) an isoperibol calorimeter, and (c) a heat conduction (or heat flow) calorimeter. fc and 7] are the temperatures of the calorimeter proper and the external jacket, respectively, and is the heat flow rate between the calorimeter proper and the external jacket.

The basic output from a combustion experiment made with an isoperibol calorimeter is a temperature-time curve, such as the one represented in figure 7.2. In the initial or fore period (between ta and tf and in the final or after period (between tf and tf), the observed temperature change is governed by the heat of stirring, the heat dissipated by the temperature sensor, and the heat transfer between the calorimeter proper and the jacket. The reaction or main period begins at tu when, on ignition, a rapidtemperature rise results from the exothermic... 

The observed temperature change of the calorimeter proper during the main period, 7> - 7j, is not exclusively determined by the amount of heat released in the bomb process. It is also due to the heat exchanged with the surroundings, the heat of stirring, and the heat dissipated by the temperature sensor. The observed temperature change must therefore be corrected for these contributions by an amount represented by A7COrr in equation 7.2 to obtain the adiabatic temperature rise ... 

In well-designed isoperibol calorimeters, the heat transfer between the calorimeter proper and the jacket takes place according to Newton s law, with conduction being the dominant mechanism [3,21,35-38]. In this case, the rate of temperature change during the initial and final periods, g, is given by... 

In the case of an electrical calibration, at the beginning of the main period a potential V is applied to a resistance inside the calorimeter proper, causing a current of intensity / to flow over a period t. As a result, an amount of heat Q = Vlt is dissipated in the calorimeter proper, causing the observed temperature rise. If the calibration is carried out on the reference calorimeter proper (without contents ), then eci = ecf = 0 and the internal energy change of the calorimetric system during the main period is... 

Electrical calibration has the advantage of being more flexible. It can afford s0 through equation 7.23 ifitisdone on the reference calorimeter proper. Flowever, it can also be performed on the initial or final state of the actual experiment leading to (e0 + ecl) or (e0 + ecf), respectively. Twenty or 30 years ago the electrical calibration required very expensive instrumentation that was not readily available except in very specialized places, such as the national standards laboratories. Although the very accurate electronic instrumentation that is available today at moderate prices may change the situation, most users of combustion calorimetry still prefer to calibrate their apparatus with benzoic acid. 

In equations 7.27 and 7.28 m(BA), m(cot), m(crbl), and m(wr) are the masses of benzoic acid sample, cotton thread fuse, platinum crucible, and platinum fuse wire initially placed inside the bomb, respectively n(02) is the amount of substance of oxygen inside the bomb n(C02) is the amount of substance of carbon dioxide formed in the reaction Am(H20) is the difference between the mass of water initially present inside the calorimeter proper and that of the standard initial calorimetric system and cy (BA), cy(Pt),cy (cot), Cy(02), and Cy(C02)are the heat capacities at constant volume of benzoic acid, platinum, cotton, oxygen, and carbon dioxide, respectively. The terms e (H20) and f(sin) represent the effective heat capacities of the two-phase systems present inside the bomb in the initial state (liquid water+water vapor) and in the final state (final bomb solution + water vapor), respectively. In the case of the combustion of compounds containing the elements C, H, O, and N, at 298.15 K, these terms are given by [44]... 

The energy produced in the calorimeter proper as a result of friction in the rotating mechanism and stirring of the calorimetric liquid by the rotation of the bomb may be substantial. Yet provided that this effect is constant, its contribution to the energy of the calorimetric process can be accurately subtracted. If the bomb is rotated during the calibration and the sample runs, and if the rotation is started and ended at the same instants of the respective main periods, then the energy... 

Figure 7.7 Scheme of an isoperibol macro rotating-bomb combustion calorimeter. A calorimeter proper B bomb C thermostatic bath D motors for rotation of the bomb E drive shaft F stirrer of the calorimeter proper G motor that drives the stirrer F H motor that drives the stirrer of the thermostatic bath I miter gear J gas outlet valve K gas inlet valve L crucible. 

Therefore, by using equation 7.64 it is possible to automatically include the energy of rotation in the calculation of Arcorr. As indicated in section 7.1, it is frequently observed that the temperature of the calorimeter proper during the initial and final periods is, to a good approximation, a linear function of time. In this case, the values of gj and gf in equation 7.64 are derived from a linear least squares fit to the experimental temperature-time data obtained during those periods. When significant departures from linearity are observed, T0Cj and Tx,f... 

The corrections due to the bomb rotation can also be eliminated by using a dynamic calorimeter, in which the whole calorimeter is rotated and the rotation mechanism is outside the calorimeter proper. An example of such instrument is the aneroid calorimeter developed by Adams, Carson, and Laye [77], shown in figure 7.9. 

Figure 7.9 Scheme of the aneroid dynamic combustion calorimeter designed by Adams, Carson, and Laye [77], A jacket B jacket lid C motor that drives the rotation of calorimetric system D rotation system E bomb (which is also the calorimeter proper) F channels to accommodate the temperature sensor, which is a copper wire resistance wound around the bomb G crucible H electrode I gas valve. Adapted from [77]. 

The rate of temperature change of the calorimeter proper during the initial and final periods and at any point during the titration period (denoted by the subscripts i, f, and r, respectively) are given by... 

It seems that immersion calorimetry into liquid nitrogen or liquid argon allows to go one step further in the determination of the internal surface area of micropores. These experiments requested a specially designed calorimeter operating at 77 or 87 K, with the special feature that the brittle end broken to start the immersion is located out of the calorimeter proper and therefore has no effeet on the calorimetric measurement. 

In Fig. 4, K is the calorimeter proper, which is hung from the two lead-in wires it is contained in a pear-shaped vessel which is evacuated as completely as possible by means of a Gaede pump, and usually also by means of cocoanut charcoal which, having been previously strongly ignited in vacuo, was cooled in liquid air. 

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