Calorimetry, static

The calorimetric method may be used in two ways immersion of the bare outgassed solid in pure liquid, and immersion of the solid precoated with the vapor phase. The former approach is not widely used because of problems due to the state of the surface (impurities and defects), although the heats of immersion per unit area of a number of liquid/solid systems are known. In order to reduce surface effects Chessick et at. [105] and Taylor [106] used liquid nitrogen the 

If the solid is first equilibrated with saturated vapor, then immersed in pure liquid adsorbate, the solid-vapor interface is destroyed and the heat liberated should correspond to /,. die surface energy of the pure liquid. The above assumption is made in what is termed the absolute method of Harkins and Jura (HJa) [107] who obtained a heat of immersion of 1.705 kJ kg-1 for titanium dioxide which, when divided by the surface energy of the adsorbent, water (11.8 kJ kg-1) gave a surface area of 14.4 m2 g-1 in agreement with the BET value. For a comprehensive bibliography and description of the calorimeter used, readers are referred to Adamson [30]. The validity of the HJa method may be questioned because exposure to a saturating vapor causes capillary condensation which reduces the available surface. A correction is also required for the thickness of the adsorbed film. 

The same technique was used by Clint et al. [108] for the determination of the surface of carbon black by the adsorption of n-alkanes. Equation (5.21) was used with the following correction for small particles where the thickness of the adsorbed layer t was not negligible in comparison with the particle radius r  

For low surface areas this method gave reasonable agreement with other techniques, but the measured surface areas for particles with high surface areas were too low. The method is essentially comparative since the entropy is obtained using a reference sample. The method was considered to be unsuitable for powders having a specific surface smaller than 20 m2 g-1. 

A review of calorimetric methods of surface area determination has been presented by Zettlemoyer et al. [111-1121, who also discuss some of the difficulties encountered in the measurement and interpretation of heats of immersion. They also developed a static microcalorimeter which was capable of determining surface areas as low as 1 m2 g-1 [1131. 

---

A comparison between pulsed flow and conventional pulsed static calorimetry techniques for characterizing surface acidity using base probe molecule adsorption has been performed by Brown and coworkers [20, 21]. In a flow experiment, both reversible and irreversible probe adsorption occurring for each dose can be measured, and the composition of the gas flow gas can be easily modified. The AHads versus coverage profiles obtained from the two techniques were found to be comparable. The results were interpreted in terms of the extent to which NH3 adsorption on the catalyst surface is under thermodynamic control in the two methods. 

One category of calorimetric measurements is batch or static calorimetry, where, in the simplest implementation, the sample is contained in a vessel, a measured amount of energy is... 

In the direct calorimetric determination (-id/f rta)r), the amount adsorbed (%) is calculated either from the variations of the gas pressure in a known volume (volumetric determination) or from variations of the mass of the catalyst sample in a static or continuous-flow apparatus (gravimetric determination). In a static adsorption system, the gas is brought into contact with the catalyst sample in successive doses, whereas in a dynamic apparatus the catalyst is swept by a continuous flow. Comparative calorimetric studies of the acidity of zeolites by measuring ammonia adsorption and desorption using static (calorimetry linked to volumetry) and temperature-programmed (DSC linked to TG) methods can be found in the literature [17],... 

Combustion or bomb calorimetry is used primary to derive enthalpy of fonuation values and measurements are usually made at 298.15 K. Bomb calorimeters can be subdivided into tluee types (1) static, where the bomb or entire calorimeter (together with the bomb) remains motionless during the experiment (2) rotating-... 

The worst hazard scenarios (excessive temperature and pressure rise accompanied by emission of toxic substances) must be worked out based upon calorimetric measurements (e.g. means to reduce hazards by using the inherent safety concept or Differential Scanning Calorimetry, DSC) and protection measures must be considered. If handling hazardous materials is considered too risky, procedures for generation of the hazardous reactants in situ in the reactor might be developed. Micro-reactor technology could also be an option. Completeness of the data on flammability, explosivity, (auto)ignition, static electricity, safe levels of exposure, environmental protection, transportation, etc. must be checked. Incompatibility of materials to be treated in a plant must be determined. 

IMR = ion-molecule reactions RB = rotating-bomb combustion calorimetry RC = reaction calorimetry SB = static-bomb combustion calorimetry. 

Because of the controversy surrounding the use of static-bomb calorimetry for determining enthalpies of formation of organogermanium compounds1,2, the reliability of most data in Table 1 cannot be fully assessed. It is, however, possible to discuss generally some of the results. 

There is general agreement that static-bomb combustion calorimetry is inherently unsatisfactory to determine enthalpies of formation of organolead compounds2,3. Unfortunately, as shown in Table 6 only three substances have been studied by the rotating-bomb method. The experimentally measured enthalpies of formation of the remaining compounds in Table 6 were determined by reaction-solution calorimetry and all rely on AH/(PbPh4, c). 

Thermochemical parameters of some unstable nitrile oxides were evaluated using corresponding data for stable molecules. Thus, for 2,4,6-trimethylbenzo-nitrile N-oxide and 2,4,6-trimethoxybenzonitrile N-oxide, the standard molar enthalpies of combustion and sublimation at 298.15 K were measured by static-bomb calorimetry and by microcalorimetry, respectively, this made it possible to derive the molar dissociation enthalpies of the N—O bonds, D(N—O) (17). 

Static-bomb combustion calorimetry is particularly suited to obtaining enthalpies of combustion and formation of solid and liquid compounds containing only the elements C, H, O, and N. The origins of the method can be traced back to the work of Berthelot in the late nineteenth century [18,19]. 

Flame combustion calorimetry in oxygen is used to measure the enthalpies of combustion of gases and volatile liquids at constant pressure [54,90]. Some highly volatile liquids (e.g., n-pentane [91]) have also been successfully studied by static-bomb combustion calorimetry. In general, however, the latter technique is much more difficult to apply to these substances than flame combustion calorimetry. In bomb combustion calorimetry, the sample is burned in the liquid state and must be enclosed in a container prior to combustion. Encapsulation may be difficult, because it is necessary to minimize the amount of vaporized compound inside the container as much as possible. In addition, volatile liquids tend to burn violently under a pressure of 3.04 MPa of oxygen, which leads to incomplete combustion. These problems are avoided in flame combustion calorimetry, where the sample is carried to the combustion zone as a vapor and burned under controlled conditions at atmospheric pressure. 

A gas flow techique was successfully used by Hacker et al. [66] in 1961, who studied the recombination of O atoms on quartz and platinum using ESR spectroscopy and isothermal calorimetry with mutually consistent results. However, only in the last few years has the technique been developed for the study of recombination under conditions far removed from those associated with static side arm systems. 

---