Other Calorimeters

Experimenters have used great ingenuity in designing calorimeters to measure reaction enthalpies and to improve their precision. In addition to the constant-pressure reaction calorimeter and bomb calorimeter described above, three additional types will be briefly mentioned. 

A phase-change calorimeter has two coexisting phases of a pure substance in thermal contact with the reaction vessel and an adiabatic outer jacket. The two coexisting phases 

Thermodynamics and Chemistry, second edition, version 3 2011 by Howard DeVoe. Latest version www.chem.vimd.edu/thermobook 

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It should be noted that, prior to Parr, other calorimeters existed which used oxygen under pressure for combustion in closed vessels, namely, those of Berthelot (1881) and its modifications and variations, Berthelot-Vieille, Moreau, Landrieu-Malsallez, and of the Commission des Substances Explosives . Later bombs were those of Mahler (1892), Attwater (1899) and Kast (constructed at Chemisch-Technische Reichsanstalt, New-Babelsberg, near Berlin, Ger)... 

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

As with any calorimeter, each part of the coffee-cup calori-meter has an associated heat capacity. Because these heat capacities are very small, however, and because a coffee-cup calorimeter is not as accurate as other calorimeters, the heat capacity of a coffee-cup calorimeter is usually assumed to be negligible. It is assumed to have a value of 0 J/ C. 

Other calorimeters include heat-leak calorimeters", such as of Thomas Parks (Ref 25,p 545), "automatic calorimeters such as of Andrews, Berl Stull (Ref 25,p 551) "vacum-walled calorimeter (Ref 3,p 153) "aneriod (unstirred) calorimeters" (Ref 3,pp 23,160-7), "rotating bomb calorimeters", such as of Popov, Shirokikh and of Hubbard (Ref 25,p 594) liquid-phase calorimeter" of Kistiakowsky (Ref 25,p 636), "gas calorimeter of Cutler- Hammer (Ref 18a), calorimeter for gaseous heat capacities of Waddington (Ref 15,p 802), "flow calorimeter of Junkers (Ref 15,p 805)," flow calorimeter of Osborne et al (Ref 25,p 565), "flow calorimeter of Pitzer (Ref 25,p 566), "flow calorimeter of Bennewitz Schulze (Ref 25,p 567) and "fiame calorimeter of Rossini" (Ref 25,pp 600--2). An apparatus for detn of heats of vaporization is described in Ref 25,p 615 and an "adsorption calorimeter in Ref 25,p 618... 

Reaction Calorimeters. The previous discussion focused on oxidation reactions (oxygen and fluorine bomb calorimeters), but many other calorimeters of specialized design are used to monitor chemical reactions phase change, solution, and so on. 

A number of other calorimeters which have been constructed for various special purposes will be described later on. 

As has been shown in this chapter, the differential scanning calorimeter and microcalorimeters have been shown to have wide applications in all aspects of pharmaceutical sciences, such as in preformulation and formulation development. However, the utility of these instruments is only possible with careful attention to following established calibration procedures. The DSC and other calorimeters are a critical part of a pharmaceutical scientists armamentarium. 

Figure 6.8 (on the next page) depicts the preweighed combustible sample in a metal-walled chamber (the bomb), which is filled with oxygen gas and immersed in an insulated water bath fitted with motorized stiirer and thermometer. A heating coil connected to an electrical source ignites the sample, and the heat evolved raises the temperature of the bomb, water, and other calorimeter parts. Because we know the mass of the sample and the heat capacity of the entire calorimeter, we can use the measured AT to calculate the heat released. 

Other calorimeters described include a microcalorimeter (111) similar to the Calvet instrument (112). high-temperature differential calorimeters (114-117X and others (118-121). 

Other calorimeters that have been used for melting determinations have been described by Clarke et al. (20), Aston and Fink (21), Pilcher (22), Mazee (23), and Ruehrwein and Huffman (24). 

A bomb calorimeter. The reaction is carried out inside a rigid steel bomb (photo of actual disassembled bomb shown on right), and the heat evolved is absorbed by the surrounding water and other calorimeter parts. The quantity of energy produced by the reaction can be calculated from the temperature increase. 

An apparatus suitable for endothermic systems was first described by Van Ness and co-workers in 1961. Several other calorimeters based on that original design have been reported in the literature. An apparatus which... 

Dilution calorimeters normally require about 50 cm of each component for the two runs necessary to cover the composition range, and is usually measured at 20 to 30 compositions. This type of calorimeter appears to have a higher precision than other calorimeters can be measured to about 0.2 J moI or 0.1 to 0.2 per cent of the maximum value of H, whichever is the greater in magnitude. 

By surrounding the DSC with low-temperature baths lower temperatures can be reached as indicated. For other calorimeters see Sect. 4.2. 

First, the amount of substance investigated in any calorimeter must be precisely known to obtain quantitative caloric data. The sample mass is normally determined with a balance. Section 1.1.1 describes calorimeters in which the exchanged heat serves for the phase transition of a substance (e.g., ice into water). Thus, the measurement of heat is reduced to the measurement of the amount of a substance. This quantity can be measured either directly by weighing or indirectly by determining the changes of volume or pressure associated with the phase transition. For other calorimeters, the volume and/or the pressure of a substance must be known, too. 

There are other calorimeters on the market that are called "isothermal calorimeters by the manufacturers but in reality belong to the category of isoperibol calorimeters (see Section 7.9.1 ff). Such calorimeters are presented below. 

Data of other calorimeters from TA Instruments are listed in the following table ... 

Pressure calorimeters allow the measurement at moderate elevated pressure during the experiment even controlled pressure change is possible to see its influence on the heat flow rate. Most of the reaction calorimeters (see Section 7.11.1) allow measurements at elevated and controlled pressure. Even combustion calorimeters (see Section 7.9.1.2) belong to this category. For other calorimeters, accessories for pressure control are available from the manufacturers. For extreme high-pressure calorimetry above 100 MPa (Ikbar), see Section 8.3.1. Only some special pressure calorimeters should be mentioned in what follows. Some DSCs that work at (controlled) elevated pressure exist ... 

For the investigation of chemical reactions, different calorimeters exist. Reaction calorimeters, mostly built of glass, are suitable for liquid reactions only. For the aforementioned curing reaction, these and many other calorimeters cannot be used because the produced resin is solid, and cleaning of the calorimeter vessel is not possible. The differential scanning calorimeter is advisable for such curing... 

For simplicity it is assumed that Cp is also the heat capacity of the empty reference calorimeter (sample holder). The equality of the heat capacities of the sample and reference holders is best adjusted experimentally otherwise a minor complication in the mathematics is necessary, requiring knowledge of the different masses and the heat capacity of aluminum or other calorimeter material. Checking the precision of several analyses with sample holders of different masses, it was found that matching sample and reference sample pans gives higher precision than calculating the effect of the different masses. ... 

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