Calorimetry phase change

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. 

From the discussion presented of reactions in solids, it should be apparent that it is not practical in most cases to determine the concentration of some species during a kinetic study. In fact, it may be necessary to perform the analysis in a continuous way as the sample reacts with no separation necessary or even possible. Experimental methods that allow measurement of the progress of the reaction, especially as the temperature is increased, are particularly valuable. Two such techniques are thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). These techniques have become widely used to characterize solids, determine thermal stability, study phase changes, and so forth. Because they are so versatile in studies on solids, these techniques will be described briefly. 

Measurements based on the law of conservation of energy are of two main types. In phase change calorimetry the enthalpy of the reaction is exactly balanced by the enthalpy of a phase change of a contained compound surrounded by a larger reservoir of the same compound used to maintain isothermal conditions in the calorimeter. The latter enthalpy, the measurand, is often displayed indirectly through the change in the volumetric properties of the heat reservoir compound, e.g. ice/water. 

We are being somewhat disingenuous here. If performed and interpreted correctly and with the appropriate ancillary phase-change enthalpy information, the enthalpy of formation of an arbitrary species by ion-molecule reaction chemistry and by combustion calorimetry must be the same. That the ionization potential of quinuclidine is higher than l,4-diazabicyclo[2.2.2]octane simply says that there is a stabilizing effect in the radical cation of the latter not found in the former. This information does not say that there is a stabilizing effect in the neutral molecular form of the latter not found in the former. After all, we trust the reader is not bothered by the fact that the ionization potential order of the cyclohexenes increases in the order 1,3-diene < 1,4-diene < 1-ene < 1,3,5-triene (benzene). 

Thus, chiral discrimination may be observed that differentiates the force-area curves of the enantiomers of some surfactants from their racemic modifications. Apparent phase changes in the monolayer can be related to parallel behavior in the crystalline state through X-ray diffraction and differential scanning calorimetry. Formation of racemic compounds and quasi-racemates can be observed in some cases. 

Differential scanning calorimetry (DSC) is a common technique for the classification of individual phase transitions in liquid-crystalline materials and has been applied for the phase characterization of alkyl-modified chromatographic surfaces. Hansen and Callis [187] applied DSC to investigate phase changes in Cig and C22... 

Further developments in calorimetry include the invention of the twin- calorimeter" by Joule (1845) and its modification by Pfaundler (1869XRef 25,p 543) "phase- change calorimer (isothermal) of Bunsen(Ref 15,p 796 Ref 25,p 547) "labirinth flow calorimeter (Ref 25,p 549) "adiabatic calorimeter (nonisothermal), first used by Richards in 1905 (Ref 15,p 797) and modified by Yost, Osborne others (Ref 25,p 550)(See also Ref 3,p ll6)(Parr adiabatic calorimeter is described in Refs 16 29) "constant- temperature- enviroment calorimeter", first used by Nemst in 1907, was modified by Giauque in 1923(Ref 15>p 797)... 

One of the seemingly few interrelations between classical and buried enamines arises from the near-constant 30 kJ mol-1 for the difference of enthalpies of formation found for most gas phase phenyl and vinyl-X derivatives33. For the most relevant case of X = NH2, the difference for the experimental enthalpies of formation of aniline and vinylamine5 is —87.1 ( 1.0) —31 ( 17) 56 ( 17) kJ mol-1. Something seems seriously awry. The enthalpy of formation of aniline in its gas phase was derived from classical combustion calorimetry and phase change techniques34 and is expected to be trustworthy. The enthalpy of formation of gaseous vinylamine, however, comes from a composite of measurements and assumptions ... 

Phase-change adsorption calorimetry. This was the earliest type of diathermal-conduction calorimetry and was originally developed in the form of ice calorimetry by Lavoisier and Laplace (1783), who weighed the liquid water, and by Bunsen (1870), who measured the change of volume. Dewar (1904) devised an elegant adsorption calorimeter at liquid air temperature the heat was evaluated from the volume of air vaporized. Of course, the temperature of the calorimeter is imposed by the temperature of the phase change. Because these calorimeters lack adaptability and cannot be readily automated, they are mainly of historical interest. 

The thermogravimetric analyses (TGA) of microcrystalline powders of this complex trace revealed two discrete mass losses at 110 °C and 145 °C corresponding to a mass percent of four (4.5%) and twelve benzene molecules (13.5%) respectively. It was rationalized that the initial mass loss corresponded to the loss of four disordered benzene units. The mass loss at 145 °C was attributed to the loss of the ordered twelve benzenes. Differential scanning calorimetry (DSC), optical microscopy and X-ray powder analysis showed that there was no phase change associated with the first mass loss. 

Differential thermal analysis (DTA) measures the amount of heat released or absorbed by a sample as it is heated at a known rate." When the enthalpy change is determined, the method is called differential scanning calorimetry (DSC). The presence of exothermic or endothermic processes at certain temperatnres provides information about the nature of phase changes and chemical reactions occurring in the material as it is heated. DTA can often be used as a sensitive method for establishing the presence or absence of secondary phases in samples if these phases undergo phase transformations at known temperatures. ... 

Behme, R.J. Brooke, D. Heat of fusion measurement of a low melting polymorph of carbamazepine that undergoes multiple-phase changes during differential scanning calorimetry analysis. J. Pharm. Sci. 1991, 80 (10), 986-990. 

The orthorhomic-monoclinic (o-B) phase change has been studied by many workers. An excellent review by Meyer (10) summarizes these studies. We adopt the results obtained by Montgomery (6) via adiabatic calorimetry = 368.3 0.3 K and j.gH =... 

Behme ly and Brooke D. Heat of Fusion Measurement of a Low Melting Polymorph of Carbamazepine that Undergoes Multiple-Phase Changes During Differential Scanning Calorimetry Analysis./PAarro Sci 1991 80 986—990. 

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