Slow heat evolution

Twin differential microcalorimeters have been described by Berghausen el al. (S), by Hackerman (8), and by Whalen and Johnson (9). Hacker-man employs thermistors, whereas the other two are based on thermocouples and in addition are run adiabatically. These calorimeters appear to have about 10 times the sensitivity of simpler designs, but for many purposes the large additional diflSculties in design, construction, and operation do not seem to be warranted. Berghausen and coworkers, however, have shown that they can estimate slow heat evolutions, after the first few minutes, due to surface reactions. 

There is, in addition, a readily recognizable rehydration rate dependence on the surface siloxane content. This is most apparent in the immersion heat data for silica FS, for which the time of slow heat evolution on immersion becomes appreciably longer for sample pretreatment temperatures above 225° C. Consideration of the precision of the calorimetric measurement with regard to slow heat input does not seem to have been afforded adequate attention. Immersion heat calorimeters constructed for the essentially instantaneous wetting heat measurement often do not perform adequately when heat is evolved slowly. 

Thermal Drift or Slow Heat Evolution. Evidence for Very Narrow Pores. For the initial increments of curve a for nitrogen on the bare surface of bone mineral at —195° (V/Vm < 0.05), the observed time-temperature curves were normal, exhibiting no evidence for any slow heat evolution, and the same was true for increments at V/Vm > 0.4, as well as for all points represented in curves b and c. However, for increments of curve a in the region V/Vm = 0.05 to 0.4, heat continued to be liberated for some time after the initial rapid thermal process. For different nitrogen increments within this range of coverage the heat produced in this slow process was from 5 to 12% in excess of that instantaneously evolved. The slow process was observed over a period of 20 to 30 minutes. In one extreme case it was still not complete after 45 minutes, which was about the maximum practicable period for observation. 

Specific Volume of Gases Formed on Explosion. 723ml/g (NG 712ml) (Ref 46) Stabilization. Chromatographically pure Mannitol Hexanitrate was mixed with varying percentages of 22 stabilizers and the mixts tested for stability in the 100° heat test best results were obtained with a mixt of 96% MHN, 2% Amm oxalate, and 2% dicyandiamide (4.07% wt loss after 48 hours, 5.74% after 96 hours) (Ref 56). The use of ethylene oxide as a stabilizer is reported in Ref 27 Thermal Decomposition. Slow heating causes decompn at 150° with evolution of red fumes (Ref 20, p 249)... 

As the reduction is a slow process, it is not necessarily complete when the blackpowder has done its work. The reduction reactions are endothermic and lower the total heat evolution. On the other hand they increase the amount of gas evolved. Hofmann considers that these equations represent... 

As already indicated, Tian s equation supposes (1) that the temperature of the external boundary of the thermoelectric element 8e, and consequently of the heat sink, remains constant and (2) that the temperature Oi of the inner cell is uniform at all times. The first condition is reasonably well satisfied when the heat capacity of the heat sink is large and when the rate of the heat flux is small enough to avoid the accumulation of heat at the external boundary. The second condition, however, is physically impossible to satisfy since any heat evolution necessarily produces heat flows and temperature gradients. It is only in the case of slow thermal phenomena that the second condition underlying Tian s equation is approximately valid, i.e., that temperature gradients within the inner cell are low enough to be neglected. The evolution of many thermal phenomena is indeed slow with respect to the time constant of heat-flow calorimeters (Table II) and, in numerous cases, it has been shown that the Tian equation is valid (16). 

Pour 50 cc. of concentrated sulfuric acid slowly into about 1 liter of water in an evaporating dish and add 50 g. of iron nails or turnings. When the action becomes slow heat the dish until the acid is practically all neutralized, as indicated by the fact that evolution of hydrogen ceases. Filter from the undissolved iron, carbon, silicon, and other residues, using a folded filter, and evaporate to crystallization. If the solution oxidizes appreciably in the operation additional iron and acid must be added to effect reduction. Oxidation will be indicated by a change in color from bottle green to a yellowish shade of green, or by the formation of a rusty precipitate. 

The heat evolution of the reaction is strongly influenced by the amount of water in the mixed acid (Fig. 65). The bottom curve (mixed acid with 20% H20) has practically no slope. In these experiments no glycol dinitrate was obtained. The initial increase of temperature here is probably due to the heat of solution of glycol in mixed acid followed by slow nitration. 

Slow heating is desirable to prevent too rapid evolution of hydrogen chloride. 

Accepting, however, the extended use of the word slow m tins connection, the reaction may be described as a good example of slow combustion. The term slow combustion is usually limited to cases of oxidation. Thus, for example, the slaking of lime is accompanied by considerable heat evolution ... 

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