Laboratory calorimetry experiments

Evaluating the results of a laboratory calorimetry experiment to determine an enthalpy change... 

Calorimetric methods are infrequently used for routine quality control purposes because of their non-specific nature and relatively slow speed. However, data from calorimetry experiments are commonly presented in applications for new product licenses and in support of patent applications. To ensure the integrity of all calorimetry data, normal procedures for good laboratory practices, standard operating procedures, appropriate calibration methods, and regular instrument servicing are necessary. The use of DSC for the measurement of transition temperatures and sample purity is described in the United States Pharmacopoeia, and standard procedures for DSC analyses are also suggested by the ASTM (100 Barr Harbor Dr., West Conshohocken, Pennsylvania 19428). 

Safety. When transferring processes from the laboratory to the pilot plant, it is important to identify and address potential safety issues as early as possible in the transfer process. Typically, calorimetry studies and process hazard reviews are carried out to meet this requirement. Calorimetry experiments can assist in the identification and quantification of reaction exotherms associated with the process. This information can then be used to determine the capability of the pilot equipment to control the reaction. 

Heat transfer is even more serious for an exothermic reaction. Minor exotherms in the laboratory may not even be apparent or if they are noticed can be easily controlled with cooling. However, the same exotherm on a large scale can be difficult to control and even present a safety hazard. Therefore it is important to thoroughly understand the thermodynamics before increasing reaction scale. This is often done by calorimetry experiments. 

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. 

Two papers reported powder pattern crystallographic results. The paper by Santos et al. (7) stood out from the rest because it presented a collection of more classical physical chemistry experiments. In this paper the authors described the use of micro-combustion calorimetry, Knudsen effusion to determine enthalpy of sublimation, differential scanning calorimetry, X-ray diffraction, and computed entropies. While this paper may provide some justification for including bomb calorimetry and Knudsen cell experiments in student laboratories, the use of differential scanning calorimetry and x-ray diffraction also are alternatives that would make for a crowded curriculum. Thus, how can we choose content for the first physical chemistiy course that shows the currency of the discipline while maintaining the goal to teach the fundamentals and standard techniques as well ... 

This would not be problematic if standardized, reliable, reproducible, and inexpensive laboratory tests were available to estimate each of the required properties. Although several specialized laboratory tests are available to measure some properties (e.g., specific heat capacity can be determined by differential scanning calorimetry [DSC]), many of these tests are still research tools and standard procedures to develop material properties for fire modeling have not yet been developed. Even if standard procedures were available, it would likely be so expensive to conduct 5+ different specialized laboratory tests for each material so that practicing engineers would be unable to apply this approach to real-world projects in an economically viable way. Furthermore, there is no guarantee that properties measured independently from multiple laboratory tests will provide accurate predictions of pyrolysis behavior in a slab pyrolysis/combustion experiment such as the Cone Calorimeter or Fire Propagation Apparatus. 

Bomb" combustion calorimetry or constant-volume calorimetry is a technique that dates back to Lavoisier178 (Fig. 11.76), is now mostly relegated to undergraduate teaching laboratories and is in bad need of a renaissance. It measures the internal energy of combustion AEc, which is easily converted to AHc, and then converted to standard enthalpies of formation AHf s.is- In a typical "macro" experiment, with commercially available equipment, a very carefully measured mass m (-2.0 g) of a sample of molar mass M g/mol and... 

It is often difficult to compare the sonochemical results reported from different laboratories (the reproducibility problem in sonochemistry). The sonochemical power irradiated into the reaction system can be different for different instruments. Several methods are available to estimate the amount of ultrasonic power entered into a sonochemical reaction, the most common being calorimetry. This experiment involves measurement of the initial rate of a temperature rise produced when a system is irradiated by power ultrasound. It has been shown that calorimetric methods combined with the Weissler reaction can be used to standardize the ultrasonic power of individual ultrasonic devices. ... 

Fluorine bomb calorimetry is a development from the early 1960s. Before that time, reliable enthalpy data concerning fluorides were very scarce, principally because fluorine gas is so very reactive. Fluorine bomb calorimetry was extended to high-pressure (up to 15 atm of fluorine) metal combustion bombs by Hubbard and co-workers [2] at the Argonne National Laboratory (ANL) in the United States in 1961. The technique has been developed over the past 30 years, and is now comparable in precision and accuracy to the other types of calorimetry. Enthalpies of formation have been determined from direct fluorination experiments. 

Stoichiometric saturation measurements in carefully controlled laboratory experiments offer perhaps the most promising technique for the estimation of thermodynamic mixing parameters (3 Glynn and Reardon, Am. J. ScL, in press). Unfortunately, the results obtained can usually not be verified by a second independent and accurate method, such as reaction calorimetry or measurement of thermodynamic equilibrium solubilities (4). The conditions necessary in obtaining good stoichiometric saturation data (as opposed to thermodynamic equilibrium data) were discussed earlier. 

In the last few paragraphs the describing adjectives were selected with care suitable, reliable, but never correct, exact or true. The reaction calorimetry as applied in safety engineering must not be mistaken for the classical calorimetry known from physical chemistry. Most of the measurements are conducted with comparably highly concentrated solutions and consciously using material directly obtained fix>m the plant or development laboratory. Many theoretical approaches of physical chemistry, however, are valid only if the experiments are conducted in very diluted systems. Especially the heat of reaction, which, as has already been mentioned several times, must be looked upon as a gross value in those safety related experiments, is a property extremely sensitive to the presence of impurities or similar influences. 

Constant-pressure calorimetry can be used to determine A rxn—the heat of reaction for the reactant quantities specified by the balanced equation. However, when we carry out calorimetiy experiments in the laboratory, we typically use much smaller quantities of reactants than those represented in a chemical equation. By measuring the temperature change of the surroundings (a known quantity of water in which the reactants are dissolved), we can determine rxn for the reactant quantities in the experiment. We can then divide rxn by the number of moles of reactant to determine AH xn ... 

In practice, the enthalpy of gasification is rarely calculated because detailed and reliable thermodynamic data for the polymer and its decomposition products are generally unavailable. Direct laboratory measurement of Lg using differential thermal analysis and differential scanning calorimetry has been reported, but Lg is usually measured in a constant heat flux gasification device or fire calorimeter (see under Steady Burning). In these experiments a plot of mass loss rate per unit surface area (mass flux) versus external heat flux has slope 1/Lg where... 

The synthesis of 1,2, and 3 have been reported previously. All reactions were carried out in an inert atmosphere unless otherwise noted. Solvents were purified by established procedures. 1,3-Dichlorotetramethyldisiloxane and 1,5-dichlorohexamethyltrisiloxane were obtained from Silar Laboratories and used as received. -Butyllithium (2.5 M in hexane) was obtained from Aldrich and used as received. l,7-Bis(chlorotetra-methyldisiloxyl)-/w-carborane 1 was purchased from Dexsil Corporation. Hexachlorobutadiene was obtained from Aldrich and distilled before use. Cure and thermal analysis studies were performed on various mixtures of 1 and 3a in milligram quantities. Thermogravimetric analyses (TGA) were performed on a DuPont SDT 2960 Simultaneous DTA-TGA analyzer. Differential scanning calorimetry analyses (DSC) were performed on a DuPont 910 instrument. Unless otherwise noted, all thermal experiments were carried out at a heating rate of 10 °C/min and a nitrogen flow rate of 50 cc/min. 

The treatise represents the fusion of literature and experience gained in the domain of bench scale calorimetry, which the author started while working in the applied physics laboratory at Bayer AG Leverkusen in the 1970s. 

In this contribution, there will be described the principal type of calorimetric experiments which can be perfonned to elucidate the behavior of lipid model membranes. Most of the examples come from work performed in the author s laboratory. Numerous other groups are involved with calorimetric studies on tliese model membrane systems and there exists a vast amount of literature, because calorimetry has become a standard analytical method in the last ten to twenty years. It is our goal to present the principles of the method and not an extensive review over the current literature. Therefore, this chapter is necessarily biased towards work from the author s laboratory and apologies are extended to all those whose work is not adequately represented here. 

---