Calorimeter 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. 

Thermochemistry is concerned with the study of thermal effects associated with phase changes, formation of chemical compouncls or solutions, and chemical reactions in general. The amount of heat (Q) liberated (or absorbed) is usually measured either in a batch-type bomb calorimeter at fixed volume or in a steady-flow calorimeter at constant pressure. Under these operating conditions, Q= Q, = AU (net change in the internal energy of the system) for the bomb calorimeter, while Q Qp = AH (net change in the enthalpy of the system) for the flow calorimeter. For a pure substance. 

A cryogenic calorimeter measures Cp,m as a function of temperature. We have seen that with the aid of the Third Law, the Cp,m data (along with AHm for phase changes) can be integrated to give the absolute entropy... 

The output of a differential scanning calorimeter is a measure of the power (the rate of energy supply) supplied to the sample cell. The thermogram in the third illustration shows a peak that signals a phase change. The thermogram does not look much like a heating curve, but it contains all the necessary information and is easily transformed into the familiar shape. 

A thermogram from a differential scanning calorimeter. The peak indicates a phase change in the sample, and the difference in base line before and after the phase transition is due to the difference in heat capacities of the two phases. 

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. 

Because the area under the peaks in the scanning calorimeter is proportional to the heat of transition, the instrument can be calibrated by running a known amount of membrane lipid suspended in water. It is necessary to assume, of course, that all of the lipid is in the bilayer conformation in water. If the lipid content of the membranes is known, the fraction of the lipids contributing to the peak observed for the membranes can then be calculated by comparing peak areas for the membranes and the lipids in water. Our preliminary results using this approach indicate that at least 60% of the lipids in the membranes participate in the phase change. Work is in progress to obtain more precision. 

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)... 

As indicated in Equation 8.3, qtot is not generally simply equal to the reaction heat-flow rate qReact (see Equation 8.4) but is affected by other physical or chemical processes which have heat changes, e.g. mixing or phase changes. As will be shown in Section 8.3, even for a simple reaction such as the hydrolysis of acetic anhydride, a significant heat of mixing occurs which must be taken into account. Furthermore, it should always be kept in mind that the qtot values determined by a reaction calorimeter also contain measurement errors such as base line drifts, time distortions or ambient temperature influences. 

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. 

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. 

If there are no phase changes in this temperature range then the heat effect measured by the calorimeter is related to the heat capacity thus... 

HDU had an apparent change from 91-92 C. The fibrous nature of the sample made observation somewhat difficult. Measurement using a differential scanning calorimeter showed a slight change of curvature indicating the possibility of a second order phase change at 117 C. The "transition" does not appear to be reversible. No return to the LT color was observed after two weeks of observation. 

Isothermal which meant, for Swietoslawski, that the surrounding thermostat was kept at constant temperature. He included in this category the adiabatic systems in which the temperature of the thermostat, instead of being brought identical to that of the sample (to keep adiabatic conditions) was simply kept constant. He also included here phase-change calorimeters (the Bunsen ice calorimeter or the water vapour calorimeter successively described, in the same year 1887, by Joly and Bunsen). 

The "isothermal group is more heterogeneous,because, apart from the phase-change calorimeters, it curiously includes those "passive adiabatic calorimeters whose surrounding shield is kept isothermal "within 0.002 C. .. 

Isothermal calorimeters, which make use of a phase change, the best example being the Bunsen ice calorimeter. 

Comment after saying that isothermally jacketed calorimeters are an intermediate category, Skinner nevertheless ends with only two main groups, associating the isothermally jacketed calorimeters with the adiabatic ones (as is proposed in Sections 4.1 to 4.3 ). Also, he puts phase-change and conduction calorimeters in the same category. This idea was kept, under the name of passive diathermal , in section 4.4. The calorimeters forming, in section 4.5,... 

C) True Isothermal (i.e. both in space and in time) or extended isothermal (i.e. only isothermal in space) calorimeters Tq follows Ts these are proportional systems and include phase-change, power-compensation and heat-flowmeter calorimeters. 

Being in the scope of a review of modern trends, the above classification did not need to explicitly list the phase-change calorimeters. 

Among direct calorimetric methods there are two main modifications adiabatic [65-72] and conduction [73-78] calorimeters. Adiabatic calorimeters have been widely employed to determine enthalpies of phase changes for volatile compounds, whereas conduction calorimeters are among the most accurate methods for compounds with low vapour pressures. 

Several modifications of differential scanning calorimeters, DSC, have been developed during the last few years for the measurement of enthalpies of phase changes [194-197] and Knudsen cells have been used as calorimetric cells. Modulated temperature thermogravimetry has also been proposed for the measurement of enthalpies of sublimation [198]. 

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