Instruments 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. 

Instrumentation. H and NMR spectra were recorded on a Bruker AV 400 spectrometer (400.2 MHz for proton and 100.6 MHz for carbon) at 310 K. Chemical shifts (< are expressed in ppm coupling constants (J) in Hz. Deuterated DMSO and/or water were used as solvent chemical shift values are reported relative to residual signals (DMSO 5 = 2.50 for H and 5 = 39.5 for C). ESl-MS data were obtained on a VG Trio-2000 Fisons Instruments Mass Spectrometer with VG MassLynx software. Vers. 2.00 in CH3CN/H2O at 60°C. Isothermal titration calorimetry (ITC) experiments were conducted on a VP isothermal titration calorimeter from Microcal at 30°C. 

Figure 2. Comparison of the Differential Scanning Calorimetry (DSC) thermograms of the homopolymer HB and various block copolymers to that of the LDPE. Weight of each polymer sample is indicated in the parentheses. The instrument range is 2 mcal/s for all the runs.

Measurements of differential scanning calorimetry (DSC) were obtained on a TA Instruments 2910 thermal analysis system (Fig. 2). Samples of approximately 1-2 mg were accurately weighed into an aluminum DSC pan, and covered with an aluminum lid that was crimped in place. The samples were then heated over the range of 20-140 °C, at a heating rate of 10 °C/min. Valproic acid was found to boil at 227 °C. 

The development of the theory of heat-flow calorimetry (Section VI) has demonstrated that the response of a calorimeter of this type is, because of the thermal inertia of the instrument, a distorted representation of the time-dependence of the evolution of heat produced, in the calorimeter cell, by the phenomenon under investigation. This is evidently the basic feature of heat-flow calorimetry. It is therefore particularly important to profit from this characteristic and to correct the calorimetric data in order to gain information on the thermokinetics of the process taking place in a heat-flow calorimeter. 

X-ray diffraction studies are usually carried out at room temperature under ambient conditions. It is possible, however, to perform variable-temperature XPD, wherein powder patterns are obtained while the sample is heated or cooled. Such studies are invaluable for identifying thermally induced or subambient phase transitions. Variable-temperature XPD was used to study the solid state properties of lactose [20], Fawcett et al. have developed an instrument that permits simultaneous XPD and differential scanning calorimetry on the same sample [21], The instrument was used to characterize a compound that was capable of existing in two polymorphic forms, whose melting points were 146°C (form II) and 150°C (form I). Form II was heated, and x-ray powder patterns were obtained at room temperature, at 145°C (form II had just started to melt), and at 148°C (Fig. 2 one characteristic peak each of form I and form II are identified). The x-ray pattern obtained at 148°C revealed melting of form II but partial recrystallization of form I. When the sample was cooled to 110°C and reheated to 146°C, only crystalline form I was observed. Through these experiments, the authors established that melting of form II was accompanied by recrystallization of form I. 

If the observed AH is positive (endothermic reaction), the temperature of the sample will lag behind that of the reference. If the AH is negative (exothermic reaction), the temperature of the sample will exceed that of the reference. Owing to a variety of factors, DTA analysis is not normally used for quantitative work instead, it is used to deduce temperatures associated with thermal events. It can be a very useful adjunct to differential scanning calorimetry, since with most instrumentation DTA analysis can be performed in such a manner that corrosive... 

The solution experiments may be made in aqueous media at around ambient temperatures, or in metallic or inorganic melts at high temperatures. Two main types of ambient temperature solution calorimeter are used adiabatic and isoperibol. While the adiabatic ones tend to be more accurate, they are quite complex instruments. Thus most solution calorimeters are of the isoperibol type [33]. The choice of solvent is obviously crucial and aqueous hydrofluoric acid or mixtures of HF and HC1 are often-used solvents in materials applications. Very precise enthalpies of solution, with uncertainties approaching 0.1% are obtained. The effect of dilution and of changes in solvent composition must be considered. Whereas low temperature solution calorimetry is well suited for hydrous phases, its ability to handle refractory oxides like A1203 and MgO is limited. 

Because of the medium or relatively large sample quantities used and the instrumental sensitivity, isoperibolic calorimetry is a useful tool in determining the onset temperature of an exotherm. In fact, in its simplest construction, this is really the only measurement. Digital data acquisition does allow computer analysis of the peak or area under the curve, which indicates the order of magnitude of the exotherm. Generally, the detected onset temperatures are similar to those found in the ARC (see later in Section 2.3.23) and are significantly lower than in the DSC (Section 2.3.1.1) [79]. 

The equipment is quite adequate for screening purposes. In its simplest form (i.e., a glass tube in an oven), it is a relatively low cost technique that can be assembled with standard laboratory equipment. However, the simple test set-up provides no quantitative thermal data for scale-up purposes, but only T0 values. The more advanced instruments like the SEDEX and SIKAREX, which are also isoperibolic calorimetry equipment, acquire specific thermal stability data that can be used for scale-up. Furthermore, the small autoclave tests provide gas evolution data. 

Though DSC is less accurate than a good adiabatic calorimeter (1-2 per cent versus 0.1 per cent), but its advantages of speed and low cost makes it outstanding instrument for most modern calorimetry. 

Calorimetry involves the use of a laboratory instrument called a calorimeter. Two types of calorimeters are commonly used, a simple coffee-cup calorimeter and a more sophisticated bomb calorimeter. In both, we carry out a reaction with known amounts of reactants and the change in temperature is measured. Check your textbook for pictures of one or both of these. 

Some scientists describe DSC techniques as a subset of DTA. DTA can be considered a more global term, covering all differential thermal techniques, while DSC is a DTA technique that gives calorimetric (heat transfer) information. This is the reason that DSC has calorimetry as part of its name. Most thermal analysis work is DSC, and Sections 15.3.3 and 15.3.4 provide information about the instrumentation and applications of this technique. 

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. 

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. 

Fluorine bomb calorimetry is in many aspects similar to oxygen bomb calorimetry. The experiments are carried out in isoperibol instruments, which, except for the bomb, are basically identical to those described in sections 7.1 and 7.2. The procedure used to calculate Acf/°(298.15 K) from the experimental results is also analogous to that discussed for oxygen bomb calorimetry in section 7.1. Thus, a temperature-time curve, such as the one in figure 7.2, is first acquired, and the corresponding adiabatic temperature rise, A Tad, is derived. 

The historical development of titration calorimetry has been addressed by Grime [197]. The technique is credited to have been born in 1913, when Bell and Cowell used an apparatus consisting of a 200 cm3 Dewar vessel, a platinum stirrer, a thermometer graduated to tenths of degrees, and a volumetric burette to determine the end point of the titration of citric acid with ammonia lfom a plot of the observed temperature change against the volume of ammonia added [208]. The capabilities of titration calorimetry have enormously evolved since then, and the accuracy limits of modern titration calorimeters are comparable to those obtained in conventional isoperibol (chapter 8) or heat-flow instruments (chapter 9) [195,198],... 

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. 

To illustrate an application of PAC, we chose reaction 13.22, which has been suggested as a test reaction for photoacoustic calorimetry, that is, as a procedure to assess the reliability of the instrument [285]. Wayner et al. [293] made a very careful PAC study of this reaction in several solvents. 

Wilhoit, R.C. "Recent Developments in Calorimetry", pp 200-225 in "Topics in Chemical Instrumentation", G.W.Ewing, Ed., Chemical Education Publishing Co., Easton, Pa., 1971... 

The 22 papers of the symposium [1] were presented under the headings Theory, Laboratory Studies, Calorimetry (2 sessions), Applications. Several papers are devoted to individual instrumental methods of measuring and assessing potential for exothermic runaway reactions to develop. An issue of the Journal of Loss Prevention in the Process Industries is devoted to a variety of, mostly calorimetric, studies of runaway reactions [2], The proceedings of a European Union Seminar in 1994 appear as a book, including hard data as well as debate about such matters as operator training [3],... 

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