Microcalorimeters technique

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

These examples draw attention to two important aspects of the microcalorimet-ric technique, namely accuracy and product identification. 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

Standard heat capacities of transfer can be derived from the temperature dependence of standard enthalpies of solution (8). While this technique can give general trends in the transfer functions from water to mixed solvents (9), it is not always sufficiently precise to detect the differences between similar cosolvents, and the technique is rather laborious. Direct measurements of the difference between heat capacities per unit volume of a solution and of the solvent a — gq can be obtained with a flow microcalorimeter (10) to 7 X 10 5 JK 1 cm-3 on samples of the order of 10 cm3. A commercial version of this instrument (Picker dynamic flow calorimeter, Techneurop Inc.) has a sensitivity improved by a factor oi about two. 

We have recently developed a microcalorimetric technique for quantifying the energetic changes of microorganisms colonizing a sea-water-sediment interface in experimental microcosms. The inherent low specificity of direct microcalorimetry and, moreover, the high sensitivity and reliability of modern microcalorimeters proved advantageous, since unknown and subtle events, not shown by more specific methods, may be detected. 

The calorimetric technique used in the titration experiment illustrated in Figure 9 allows short time intervals between the injections due to a comparatively low time constant for the instrument in combination with the electrical compensation technique. Rather, slow heat conduction microcalorimeters can be used in fast titration experiments if a dynamic correction, based on the Tian equation (equation (17)), is employed (Bastos et al., 1991 Backman et al., 1994). 

In recent years, a new fire-test instrument was developed the pyrolysis combustion flow calorimeter (PCFC) or microcalorimeter.209 210 This instrument (Figure 21.17) was developed by Richard Lyon and his coworkers at the FAA laboratories. It enables the determination of parameters such as specific heat release rate (W/g), heat of combustion (J/g), and ignition temperature (°K), to be quickly determined from very small (1-50 mg) test specimens. The technique has been standardized by ASTM as ASTM D 7309. Data from the PCFC has been shown to be capable of being correlated... 

In principle, to carry out immersion microcalorimetry, one simply needs a powder, a liquid and a microcalorimeter. Nevertheless, it was early realized that the heat effects involved are small and the sources of errors and uncertainties numerous. Many attempts have been made to improve immersion microcalorimetric techniques. Before commenting on this type of experiment, we describe the equipment and procedure which has been found by Rouquerol and co-workers to be of particular value for energy of immersion measurements (Partyka et al., 1979). 

Immersion calorimetry has much to offer for the characterization of powders and porous solids or for the study of adsorption phenomena. The technique can provide both fundamental and technologically useful information, but for both purposes it is essential to undertake carefully designed experiments. Thus, it is no longer acceptable to make ill-defined heat of immersion measurements. To obtain thermodynamically valid energy, or enthalpy, or immersion data, it is necessary to employ a sensitive microcalorimeter (preferably of the heat-flow isothermal type) and adopt a technique which involves the use of sealed glass sample bulbs and allows ample time (usually one day) for outgassing and the subsequent temperature equilibration. 

Regular pulses of pure ethane on bulk V2O5 maintained at 823 K in a microcalorimeter linked to a gas chromatograph provided kinetic data of theoretical significance, as weU as an insight into the mechanism of the reduction process. The results of this work carried out using mainly calorimetric techniques led to the conclusion that diffusion of oxygen from the bulk is predominant in the selective oxidation of ethane and that the redox process plays a more important role than the acidic sites in the case of unsupported vanadium pentoxide. 

In the former case, the solid remains suspended in the liquid in the microcalorimeter cell. Then a mother solution is added, either in one step (to obtain an integral heat. A ffUnt)) or in several steps, leading to differential heats, A H(dlff)l). In the latter case one could also speak of titration calorimetry. some commercial microcalorimeters are especially constructed for such titrations. Since, with these techniques, part of the added adsorptive remains in solution, the enthalpy of dilution A yH must be subtracted it is dependent on composition and can be determined in a blank without adsorbent. The difference between A y H(int) and A y H(dlff) has been discussed before, see sec. 1.3c. 

The heat evolved when a reactive molecule contacts the surface of the solid is related to the energy of the bonds formed between the adsorbed species and the adsorbent and hence to the nature of the bonds and to the chemical reactivity of the surface. Although diverse techniques have been used to study this interaction, only a few provide information about the strength of chemisorption itself. The measurement of the heat of adsorption by a suitable microcalorimeter is the most reliable method for this purpose. The key to the... 

Microcalorimetry is a growing technique complementary to DSC for the characterization of pharmaceuticals. Larger sample volume and high sensitivity means that phenomena of very low energy (unmeasurable by DSC) may be studied. The output of the instrument is measured by the rate of heat change dq/dt) as a function of time with a high sensitivity better than 0.1 pW. Microcalorimery can be applied to isolated systems in specific atmospheres or for batch mode where reactants are mixed in the calorimeter. Solution calorimetry can be used in adiabatic or isoperibol modes in microcalorimeters at constant temperature. (See the corresponding article about calorimetry of this edition.)... 

Charlu and Kleppa (2) reported a enthalpy of formation value of -291.0 0.9 kcal mol for VgOg based on oxidation studies to VgOgCcr) in a high temperature microcalorimeter. An advantage of this technique is that complete oxidation to VgOgCcr) was achieved whereas in the study by Mah and Kelley ( ) a mixture of the two oxides and V Og was obtained. Other combustion... 

Kelley (1 ). The adopted value is -341.1 kcal mol" which was obtained by rounding the reported value of Mah and Kelley (1.). For more details refer to the V O Ccr) table (2). Charlu and Kleppa (3 ) reported a enthalpy of formation value at A H (298.15 K) -342.4 0.78 kcal mol" based on oxidation studies to V OgCcr) in a high temperature microcalorimeter. The combustion by this technique yielded complete oxidation to VgO Ccr) as opposed to the method of Mah and Kelley ( ), whereby A H for V20 (cr) was determined simultaneously with that for VgO Ccr) due to incomplete combustion of V(cr). A combustion study by Siemonsen and Ulich (4) led to the reported value of -342 2 kcal mol" for A H (V20, cr, 293 K). Additional thermodynamic data which relate 2 3 ° with VgO Ccr) or V20g(cr) is contained in the sodium oxide fusion studies by Mixter (5), the H2O-H2 equilibrium study by Muller (6), the CO-CO2 equilibrium study by Spencer and Justice (7), and the reduction of V O Ccr) with SO2 by Flood and Kleppa ( ). See V20g(cr) table for some additional information (2 ). 

Analytical Techniques. The cloud point of the blends was determined with a light-transmission device (21). Once the blend was cloudy, the test tube was taken out and chilled in ice, so that the time and conversion at the cloud point, tc and a could be obtained. The Tg value and conversion were measured by DSC (Mettfer TA3000 microcalorimeter) (22). The gel time, fge], of rubber-cyanate blends was determined as the time at which insolubles appeared in tetrahydrofu-ran (THF). That of PES-cyanate was determined by dynamic mechanical analysis (Rheometrics RDA700). 

The reduction behaviour of the catalysts was studied in an indigenously designed TPR unit. The metal dispersion was measured by oxygen titrations using dynamic pulse flow technique (Pulse chemisorb 2700, Micromeritics, USA). Acidity and acid strength distribution were determined through heats of adsorption of ammonia by Calvet C-80 microcalorimeter (Setaram, France) and by TPD of ammonia using Catalyst Data System, Baroda (India), TPD unit. 

A Setaram C80 differential microcalorimeter coupled to an evacuable glass gas-handling system was used to monitor ammonia adsorption and associated enthalpies of adsorption. Catalyst samples (ca. 150 mg dry weight) were conditioned in the calorimeter at 100 °C under vacuum for two hours, with an empty reference cell. Successive pulses of ammonia (ca. 0.06 mmol) were introduced to the sample at 100 °C. Enthalpy changes associated with each dose were converted to molar enthalpies of adsorption and are expressed as functions of resin coverage. Further details of the technique have been reported previously. ... 

It should be noted that calorimetry is one of the oldest physicochemical experimental methods with a history of more than a century of scientific appUcation for which numerous experimental devices and techniques have been designed, tested, and applied. Since the first extensive review of the use of microcalorimetry in the fields of biochemistry, biotechnology, and biology by Calvet and Prat in 1956 [2], a wide variety of different microcalorimeters have been developed and employed in various branches of the life sciences. 

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