Calorimetry measuring principles

Heat cannot be directly measured. In most cases heat measurement is made indirectly by using temperature measurement Nevertheless, there are some calorimeters able to measure directly the heat release rate or thermal power. Calorimetry is a very old technique, which was first established by Lavoisier in the 18th century. In the mean time, a huge choice of different calorimeters, using a broad variety of designs and measurement principles, were developed. 

Besides surface excesses, surface excess enthalpies can also be determined directly, especially since the advent of sensitive microcalorimeters. As stated, it is often advisable to study adsorptions and enthalpies together. In calorimetry, in principle two types of measurements can be distinguished. 

Relatedly, despite the synthesis and structural characterization of numerous arsine and stibine oxides, bomb calorimetry measurements have only been reported on triphenylarsine oxide . While corresponding measurements have been made on triphenylarsine, it is clearly premature to make general observations as to E—O bond enthalpies in the absence of additional data. In principle, reaction calorimetry should prove useful. Indeed, we note a solution phase (benzene) enthalpy of reaction study of triphenylarsine and -butyl hydroperoxide according to the reaction... 

A) Classification based on the measuring principle (pp 5-19). It lead fliem to distinguish two major methods of calorimetry ... 

After describing the principle of differential scanning calorimetry measurements, we briefly describe the most important concepts below. 

For most adsorption experiments the temperature at which the measurements are made is less than the triple point of the gas being used but above its freezing point. This being the case, one would normally expect that the adsorbate characteristics resemble the liquid phase rather than the solid phase of the adsorptive. This is the normal assumption used for most adsorption theories. The principle measurement performed as an adsorption experiment is the measurement of the adsorption isotherm. The adsorption isotherm is the measurement of amount adsorbed versus adsorptive pressure at constant temperature. This is the easiest measurement to make. Another type of measurement is calorimetry. One form of calorimetry measures the amount of heat evolved as the adsorptive is adsorbed. Another form measures the heat capacity of the adsorbate. There are various forms of calorimetry but the most accurate methods are very difficult to perform and only a few examples are available in the literature. Another form of calorimetry, which is easier to perform, is scanning calorimetry. This calorimetry form is a good tool to determine qualitative features of the adsorption and to yield a fair indication of the physical quantities. 

While in DTA (differential thermal analy ) the difference in temperature between sample and reference is detected for a given heat input, making it possible to determine phase transition temperatures very accurately, it is appropriate for thermodynamic analysis to work isothermally and to measure the energy input (e.g. electric current), necessary to keep the temperatures between sample and reference the same. This is the measurement principle of DSC (differential scanning calorimetry). 

Calorimetry is the basic experimental method employed in thennochemistry and thennal physics which enables the measurement of the difference in the energy U or enthalpy //of a system as a result of some process being done on the system. The instrument that is used to measure this energy or enthalpy difference (At/ or AH) is called a calorimeter. In the first section the relationships between the thennodynamic fiinctions and calorunetry are established. The second section gives a general classification of calorimeters in tenns of the principle of operation. The third section describes selected calorimeters used to measure thennodynamic properties such as heat capacity, enthalpies of phase change, reaction, solution and adsorption. 

In this chapter, we describe how time-resolved photoacoustic calorimetry (PAC) can be used to obtain both the energetics and kinetics of carbenes in solution.7-9 PAC measures the magnitude and temporal profile of volume changes in solution following deposition of energy. These time-resolved volume changes can be directly related to carbene reaction enthalpies. We will first discuss the principles of this photoacoustic technique and then how it has been... 

Heat capacities at high temperatures, T > 1000 K, are most accurately determined by drop calorimetry [23, 24], Here a sample is heated to a known temperature and is then dropped into a receiving calorimeter, which is usually operated around room temperature. The calorimeter measures the heat evolved in cooling the sample to the calorimeter temperature. The main sources of error relate to temperature measurement and the attainment of equilibrium in the furnace, to evaluation of heat losses during drop, to the measurements of the heat release in the calorimeter, and to the reproducibility of the initial and final states of the sample. This type of calorimeter is in principle unsurpassed for enthalpy increment determinations of substances with negligible intrinsic or extrinsic defect concentrations... 

The basic principle of solution calorimetry is simple. In one experiment the enthalpy of solution of, for example, LaA103(s) [32] is measured in a particular solvent. In order to convert this enthalpy of solution to an enthalpy of formation, a thermodynamic cycle, which gives the formation reaction... 

As mentioned above, titration methods have also been adapted to calorimeters whose working principle relies on the detection of a heat flow to or from the calorimetric vessel, as a result of the phenomenon under study [195-196,206], Heat flow calorimetry was discussed in chapter 9, where two general modes of operation were presented. In some instruments, the heat flow rate between the calorimetric vessel and a heat sink is measured by use of thermopiles. Others, such as the calorimeter in figure 11.1, are based on a power compensation mechanism that enables operation under isothermal conditions. 

Isotopic methods Indirect calorimetry is limited by the need to measure the gases, which means that it cannot be used for free-living studies. Consequently, methods involving the administration of isotopically labelled compounds have been devised. These are based on the principle of isotopic dilution, a widely employed method for estimating concentrations, particularly of hormones and other proteins in the blood (Appendix 2.6). The requirements for these methods are ... 

With a somewhat stiffer monomer, 1,6-hexanediol diacrylate, (HDDA) we have previously observed that the ultimate conversion as measured with differential scanning calorimetry (DSC) also depends on light intensity. This has been attributed to the experimentally observed delay of shrinkage with respect to chemical conversion (7). In principle, such a dependence of conversion on intensity should show up in the mechanical properties as well. However, these are difficult to measure with thin samples of HDDA. 

Another method to obtain enthalpies of formation of compounds is by solute-solvent drop calorimetry. This method was pioneered by Tickner and Bever (1952) where the heat formation of a compound could be measured by dissolving it in liquid Sn. The principle of the method is as follows. If the heat evolved in the dissolution of compound AB is measured, and the equivalent heat evolved in the dissolution of the equivalent amount of pure A and B is known or measured, the difference provides the enthalpy of formation of the compound AB. Kleppa (1962) used this method for determining enthalpies of formation of a number of Cu-, Ag- and Au-based binaries and further extended the use of the method to high-melting-point materials with a more generalised method. 

An apparatus with high sensitivity is the heat-flow microcalorimeter originally developed by Calvet and Prat [139] based on the design of Tian [140]. Several Tian-Calvet type microcalorimeters have been designed [141-144]. In the Calvet microcalorimeter, heat flow is measured between the system and the heat block itself. The principles and theory of heat-flow microcalorimetry, the analysis of calorimetric data, as well as the merits and limitations of the various applications of adsorption calorimetry to the study of heterogeneous catalysis have been discussed in several reviews [61,118,134,135,141,145]. The Tian-Calvet type calorimeters are preferred because they have been shown to be reliable, can be used with a wide variety of solids, can follow both slow and fast processes, and can be operated over a reasonably broad temperature range [118,135]. The apparatus is composed by an experimental vessel, where the system is located, which is contained into a calorimetric block (Figure 13.3 [146]). 

The amount of heat that is absorbed or liberated in a physical or chemical change can be measured in a well-insulated vessel called a calonmeter (Figure 14-1) Calorimetry is based on the principle that the observed temperature change resulting from a chemical reaction can be simulated with an electrical heater The electrical measurements of current (/), heater resistance (R), and duration ( ) of heating make it possible to calculate how much heat is equivalent... 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

Reaction characterisation by calorimetry generally involves construction of a model complete with kinetic and thermodynamic parameters (e.g. rate constants and reaction enthalpies) for the steps which together comprise the overall process. Experimental calorimetric measurements are then compared with those simulated on the basis of the reaction model and particular values for the various parameters. The measurements could be of heat evolution measured as a function of time for the reaction carried out isothermally under specified conditions. Congruence between the experimental measurements and simulated values is taken as the support for the model and the reliability of the parameters, which may then be used for the design of a manufacturing process, for example. A reaction modelin this sense should not be confused with a mechanism in the sense used by most organic chemists-they are different but equally valid descriptions of the reaction. The model is empirical and comprises a set of chemical equations and associated kinetic and thermodynamic parameters. The mechanism comprises a description of how at the molecular level reactants become products. Whilst there is no necessary connection between a useful model and the mechanism (known or otherwise), the application of sound mechanistic principles is likely to provide the most effective route to a good model. 

As pointed out in Section 8.2, most physical and chemical processes, not just the chemical transformation of reactants into products, are accompanied by heat effects. Thus, if calorimetry is used as an analytical tool and such additional processes take place before, during, or after a chemical reaction, it is necessary to separate their effects from that of the chemical reaction in the measured heat-flow signals. In the following, we illustrate the basic principles involved in applying calorimetry combined with IR-ATR spectroscopy to the determination of kinetic and thermodynamic parameters of chemical reactions. We shall show how the combination of the two techniques provides extra information that helps in identifying processes additional to the chemical reaction which is the primary focus of the investigation. The hydrolysis of acetic anhydride is shown in Scheme 8.1, and the postulated pseudo-first-order kinetic model for the reaction carried out in 0.1 M aqueous hydrochloric acid is shown in Equation 8.22 ... 

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