Calorimeter selection

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. 

The selection of the operating principle and the design of the calorimeter depends upon the nature of the process to be studied and on the experimental procedures required. Flowever, the type of calorimeter necessary to study a particular process is not unique and can depend upon subjective factors such as teclmical restrictions, resources, traditions of the laboratory and the inclinations of the researcher. 

Accelerating Rate Calorimetry. This is a heat-wait-search technique (see Fig. 5.4-62). A sample is heated by a pre-selected temperature step of, typically, 5 C, and then the temperature of the sample is recorded for some time. If the self-heating rate is less than the calorimeter detectability (typically 0.02 "C) the ARC will proceed automatically to the next step. If the change of the sample temj)erature is greater than 0.02 °C, the sample is no longer heated from outside and an adiabatic process starts. The adiabatic run is continued until the process has been completed. ARC is usually carried out at elevated pressure. 

Usually, isothermal calorimeters are used to measure heat flow in batch and semi-batch reactions. They can also measure the total heat generated by the reaction. With careful design, the calorimeter can simulate process variables such as addition rate, agitation, distillation and reflux. They are particularly useful for measuring the accumulation of unreacted materials in semi-batch reactions. Reaction conditions can be selected to minimize such accumulations. 

The radiation source for the twin calorimeter of figure 10.2 is a 100 W tungsten lamp. The wavelength is selected by a monochromator, and the light is split in two parts and led into the radiation-absorbing cells of each unit by three light cables. With a 2 mm slit, the band pass is about 13 nm, and for radiation with A = 436 nm the power delivered to each cell is about 60 p,W. The reference cells are simply steel rods and receive no light. 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

The RSST calorimeter (see Annex 2) is a pseudo-adiabatic, low thermal inertia calorimeter, intended for screening purposes. It can identify the system type and measure adiabatic rate of temperature-rise and rate of gas generation by the reacting mixture. It is therefore well-suited to the task of selecting the overall worst case scenario from a small number of candidates. Alternatively, a calorimeter designed to obtain relief system sizing data may be used for this purpose (see Annex 2). 

Heat is the most common product of biological reaction. Heat measurement can avoid the color and turbidity interferences that are the concerns in photometry. Measurements by a calorimeter are cumbersome, but thermistors are simple to use. However, selectivity and drift need to be overcome in biosensor development. Changes in the density and surface properties of the molecules during biological reactions can be detected by the surface acoustic wave propagation or piezoelectric crystal distortion. Both techniques operate over a wide temperature range. Piezoelectric technique provides fast response and stable output. However, mass loading in liquid is a limitation of this method. 

The situation becomes more complicated if experiments are carried out in non-isothermal conditions. First of all,many non-isothermal measurement procedures are possible. The selection of a particular method depends on the process characteristics and methods of interpretation. Scanning calorimeters, which measure the quantity of heat released as the ambient temperature is varied linearly. The rate of temperature change can be varied by the experimenter. 

There are numerous calorimeters available on the market Nevertheless, only a relatively restricted choice may be used for the determination of the data required for safety assessment. These are essentially selected for their robustness with... 

This is the most common mode of addition. For safety or selectivity critical reactions, it is important to guarantee the feed rate by a control system. Here instruments such as orifice, volumetric pumps, control valves, and more sophisticated systems based on weight (of the reactor and/or of the feed tank) are commonly used. The feed rate is an essential parameter in the design of a semi-batch reactor. It may affect the chemical selectivity, and certainly affects the temperature control, the safety, and of course the economy of the process. The effect of feed rate on heat release rate and accumulation is shown in the example of an irreversible second-order reaction in Figure 7.8. The measurements made in a reaction calorimeter show the effect of three different feed rates on the heat release rate and on the accumulation of non-converted reactant computed on the basis of the thermal conversion. For such a case, the feed rate may be adapted to both safety constraints the maximum heat release rate must be lower than the cooling capacity of the industrial reactor and the maximum accumulation should remain below the maximum allowed accumulation with respect to MTSR. Thus, reaction calorimetry is a powerful tool for optimizing the feed rate for scale-up purposes [3, 11]. 

These data contain a broad lambda type transition with a heat capacity peak at 227.5 K. Powers and Blalock (9) measured high temperature enthalpy data for KOH(cr) in both the a and B phases in a Bunsen ice calorimeter. Their enthalpy data are scattered and not precise enough to accurately define the heat capacities for the a phase. Therefore, the selected heat capacities between 298 and 516 K are estimated by graphical extrapolation of the low temperature heat capacity data. Heat capacities for the B phase are from Powers and Blalock (9). 

Using the temperature probe, measure the temperature of the water. Press TRIGGER on the CBL to collect the temperature reading. Record this temperature in your data table. Select STOP from the TRIGGER menu on the graphing calculator. Leave the probe in the calorimeter. 

To gather additional information on a reaction, reaction calorimeters are often coupled with other analytical devices e.g., on-line FTIR, particle-sizing probes, turbidity probes, pH or other ion selective probes, etc.). Therefore, we developed a reaction cell that allows stirring, different dosing profiles for one or two reactants and can accommodate a small optical probe coupled to a miniaturized spectrometer, Figure 2. 

This overview has attempted to show that calorimetry has found applications in all areas of the pharmaceutical industry from discovery through receptor site binding to characterisation, on through compatibility to formulation and stability. The nanocalorimeters should find their real application in HTS whilst the more macro (but still micro ) calorimeters will still be needed, because of their better long-term stability to define the physicochemical properties of the selected pharmaceutical systems. 

A 50 ml aliquot of the urea solution is pre-thermostated to the operational temperature of the calorimeter for these experiments the calorimeter is housed in a constant temperature environment and operated at 25 °C. The urea solution is then run in a continuous loop, at a known flow rate, until a stable baseline is achieved. This solution is then inoculated with 4.55 ml of a standard, fixed concentration, urease solution (also buffered to pH 7.0 and pre-thermostated) and the resulting calorimetric output recorded as a function of time. This is repeated for all concentrations of urea. Figure 5 shows a selection of typical calorimetric outputs for this enzyme system. 

You will be retrieving information on heats of formation from reference tables and data bases. The values in the tables have been reconciled from innumerable experiments. To determine the values of the standard heats (enthalpies) of forniation, the experimenter usually selects either a simple flow process without kinetic energy, potential energy, or work effects (a flow calorimeter), or a simple batch process (a bomb calorimeter), in which to conduct the reaction. Consider an experiment in a flow process under standard state conditions in which the experimental arrangement is such that the summation of sensible heat terms on the right-hand side of Eq. (4.33) is zero and no work is done. The steady-state (no accumulation term) version of Eq. (4.24a) for stoichiometric quantities of reactants and products reduces to... 

More recently, it was demonstrated that the thermistor approach could be used to monitor specific interactions of fluoride ions with silica-packed columns in the flow injection mode. A thermometric method for detection of fluoride [56] was developed that relies on the specific interaction of fluoride with hydroxyapatite. The detection principle is based on the measurement of the enthalpy change upon adsorption of fluoride onto ceramic hydroxyapatite, by temperature monitoring with a thermistor-based flow injection calorimeter. The detection limit for fluoride was 0.1 ppm, which is in the same range as that of a commercial ion-selective electrode. The method could be applied to fluoride in aqueous solution as well as in cosmetic preparations. The system yielded highly reproducible results over at least 6 months, without the need of replacing or regenerating the ceramic hydroxyapatite column. The ease of operation of thermal sensing and the ability to couple the system to flow injection analysis provided a versatile, low-cost, and rapid detection method for fluoride. 

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