Differential heat absorption

In the water absorption study, a Setaran thermobalance B 60 and a Richard Eyraud isotherm differential microcalorimeter were used. The same experiment permits one to obtain both the sorption isotherm and the differential heat absorption values ( ). The accuracies were better than O.I mg and 10 cal/sec, respectively. The relative humidity was obtained by changing the temperature of a water/ice bath (accuracy 0.1°C) which was connected to the sample. We measured both the amount of absorbed water and the heat of absorption after a change in the relative humidity level. We can therefore define the average heat of absorption per water molecule corresponding to the molecules which have been absorbed after this change in water relative humidity. 

Cross-linking constrains the conformational flexibility of biopolymers and, as a rule, stabilizes their secondary, tertiary, and quaternary structures against the denaturing effects of high temperatures.29 We used differential scanning calorimetry (DSC) to compare the heat-induced conformational transitions of selected RNase A samples that were characterized in Figure 15.2. A brief introduction to DSC is provided in Section 15.15.1 for those readers unfamiliar with this biophysical method. Trace 1 in Figure 15.3a is the heat absorption... 

Experimental Methods In Differential thermal analysis (DTA) the sample and an inert reference substance, undergoing no thermal transition in the temperature range under study are heated at the same rate. The temperature difference between sample and reference is measured and plotted as a function of sample temperature. The temperature difference is finite only when heat is being evolved or absorbed because of exothermic or endothermic activity in the sample, or when the heat capacity of the sample changes abruptly. As the temperature difference is directly proportional to the heat capacity so the curves are similar to specific heat curves, but are inverted because, by convention, heat evolution is registered as an upward peak and heat absorption as a downward peak. 

A thermochemical method that simultaneously measures differences in heat flow into a test substance and a reference substance (whose thermochemical properties are already well characterized) as both are subjected to programmed temperature ramping of the otherwise thermally isolated sample holder. The advantage of differential scanning calorimetry is a kinetic technique that allows one to record differences in heat absorption directly rather than measuring the total heat evolved/... 

In this connection, it is interesting to consider questions of the heat of adsorption on a non-uniform surface, of the fraction of the specific heat of the adsorbed gas related to redistribution of the gas on the surface for a change in temperature, and so on—both for the general case and for a distribution of the form (17). The elaboration of these problems, however, at present would necessarily be nothing more than a mathematical exercise. We note only that for the distribution equation (17), the smaller the value of q, the sharper is the maximum of the specific heat near T = 7, while the differential heat of absorption at T = 0 obeys the equation... 

Fig. 17. Differential heat of NH3 absorption as a function of the ammonia uptake for heteropolyacids. (From Ref. 132.)...

Fig. 22. Differential heats of absorption of NH3 on (1) H3PW 204o and (2) 20 wt% H3PW 2O40/ SiOi. (From Ref. 150.)...

The differential scanning calorimetry (DSC) thermograms for a-, (3- and -cyclo-dextrins are identical. Two heat absorption peaks are present the first occurs at 100°C as water is evaporated from the crystals the second, occurring at 250°C, is a result of crystal melting and thermal decomposition. 

The acid-base properties of decationated ZSM-5 zeolite have been studied in some detail using adsorption microcalorimetry, as shown in Table VIII (169-173). As the calcination temperature for HZSM-5 zeolites was increased from room temperature to 1073 K, a maximum in acidity was observed while the initial differential heat of ammonia adsorption increased continuously. Vedrine et al. (92) also found a maximum in the intensity of the IR hydroxyl bands (169) of HZSM-5 at 673 K. The IR absorption band of pyridine adsorbed on Brpnsted sites followed the same trend as that found for the hydroxyl stretching bands, confirming that above 673 K the Bronsted acidity decreased as the dehydration temperature increased. 

The absorption coefficient (of solubility) a of a gas is the volume of gas reduced to 0 C and 1 atm. pressure which will be dissolved by unit volume of solvent at the experimental temperature under a partial pressure of the gas of 1 atm. Show that for a dilute solution, Ns in equation (36.23) may be replaced by a. The absorption coefficient of nitrogen is 0.01685 at 15" and 0.01256 at 35 C. Determine the mean differential heat of solution per mole of nitrogen in the saturated solution in this temperature range. 

Speil et al. (12) first pointed out that the actual peak temperature is the point at which the differential heat input equals the rate of heat absorption and therefore... 

The conclusion that may be drawn from these computations is that the presence of water cooling tubes will reduce k by 1.3 percent if only the effect of differential heating is considered. The added absorption of the aluminium tubes is the most important residual factor which reduces k and so should be added to get an overall estimate of the water effect if the aluminium reduces k by 0.4 percent, an upper limit to the adverse effect on k of water cooling is therefore 1.7 percent. 

Figure 5.28. Differential scanning calorimetry of model protein, plotted (with heat absorption peak down) as function of increasing degree of ionization of carboxyl function. Ionization to form less than 2 carboxylates per 100 residues reduces the heat of the transition to about one-fourth. Charge destroys hydrophobic hydration, raises T and gives rise to solubility. (Reproduced with permission from Urry et al. )...

The heat absorbed upon dissolution of salts with univalent ions, not capable of hydrate formation, will increase according as the ionic radii of the ions become greater, since the energy of hydration decreases in the same sense Upon dissolution of KI and NaCl more heat per gram equivalent is absorbed in the former case. Sometimes the differential heat of dissolution of the anhydrous salt is positive in concentrated solutions and becomes negative upon further dilution. This will occur if the salt forms a crystalline hydrate, which then dissolves with absorption of heat... 

In contrast MEA absorption-regeneration operates on a temperature differential with absorption at a relatively low temperature, 27°-60°C, and regeneration at 100 -140 C. Thus, it is necessary to cool the incoming gas to the absorber and to heat the MEA solution in the regenerator with steam. The MEA process has been improved by the addition of promoters and corrosion inhibitors, thereby lowering heat requirements. 

Fig. 19.9. Differential thermal scan showing discontinuities in heat absorption (AH/g) for the semi-crystalline (A) and amorphous (B) polymers In Fig. 19.8.

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