Technique of sublimation

The theory of sublimation, t.e. the direct conversion from the vapour to the sohd state without the intermediate formation of the liquid state, has been discussed in Section 1,19. The number of compounds which can be purified by sublimation under normal pressure is comparatively small (these include naphthalene, anthracene, benzoic acid, hexachloroethane, camphor, and the quinones). The process does, in general, yield products of high purity, but considerable loss of product may occur. 

The simplest form of apparatus consists of a small porcelain evaporating dish covered with a filter paper which has been perforated with a number of small holes a watch glass of the same size, convex side uppermost, is placed on the filter paper. The substance is placed inside the dish, and the latter heated with a minute flame on a wire gauze or sand bath. The sublimate collects in the Fig. II, 45, 1. watch glass, and the filter paper below prevents the sublimate from falling into the residue. The watch glass may be kept cool by covering it with several pieces of damp filter 

A simple apparatus for sublimation in a stream of air or inert gas is shown in Fig. II, 45, 2.. 4 is a wide-necked conical flask provided with 

A simple apparatus for sublimation in a stream of air or of inert gas is shown in Fig. II, 45, 3.. d is a two-necked flask equipped with a narrow inlet tube B with stopcock and a wide tube C 12-15 inm. in diameter. The latter is fitted to a sintered glass crucible and the usual adapter and suction flask E. A well-fitting filter paper is placed on the sintered glass filter plate to collect any sublimate carried by the gas stream. 

Sublimation under reduced pressure. The so-called vacuum sublimation may be carried out in the apparatus of Fig. II, 45, 4. The cold finger is fitted into the larger tube by means of a rubber stopper 

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The combination of Knudsen s technique of sublimation (evaporation) of the mixtures of compoimds with mass spectrometric monitoring of the... 

M. J. Pikal, S. Shah, D. Senior, and J. E. Lang, Physical chemistry of freeze drying measurement of sublimation rates of frozen aqueous solutions by a microbalance technique, J. Pharm. Sci., 72, 635-650 (1983). 

Equally precise and meticulous as an experimentalist, he devoted time during his days at Pittsburgh to write comprehensive articles on such practical techniques as crystallization, vacuum distillation, and sublimation, which were published in the Weissberger Techniques of Organic Chemistry series. His preoccupation with careful experimental techniques and then-accurate recording in the literature remained with him always. He abhorred vague descriptions of procedures, speculative interpretations not based on... 

In addition, the sublimation of ice in freeze-drying, discussed in Chapter 16, has become an important operation particularly in the biological and food industries. The various industrial applications of sublimation techniques are discussed by several authors(3,40 95 96 97), and the principles underlying vaporisation and condensation and the techniques for growing crystals from the vapour phase 99-1 are also presented in the literature. 

Although the application of fluidisation techniques to sublimation-desublimation processes was first proposed by Matz" 11, the technique has not yet been widely adopted for large-scale commercial use, despite its obvious advantage of improving both heat and mass transfer rates. G aiko 112 1 has, however, reported on a fluidised-bed de-sublimation unit operating in the United States for the production of aluminum chloride at the rate of 3 kg/s (11 tonne/h). 

In Table 5.1 the surface structures of ordered metal monolayers adsorbed on metal surfaces are listed. For each substrate, the crystallographic structure, the distance between nearest neighbors, and the heat of sublimation (that is proportional to the surface free energy) are given. For each metal adsorbate the identical information is provided along with the technique of deposition and all the ordered surface structures that form with increasing coverage. 

Two papers reported powder pattern crystallographic results. The paper by Santos et al. (7) stood out from the rest because it presented a collection of more classical physical chemistry experiments. In this paper the authors described the use of micro-combustion calorimetry, Knudsen effusion to determine enthalpy of sublimation, differential scanning calorimetry, X-ray diffraction, and computed entropies. While this paper may provide some justification for including bomb calorimetry and Knudsen cell experiments in student laboratories, the use of differential scanning calorimetry and x-ray diffraction also are alternatives that would make for a crowded curriculum. Thus, how can we choose content for the first physical chemistiy course that shows the currency of the discipline while maintaining the goal to teach the fundamentals and standard techniques as well ... 

K proved that the ground state of Vs0 corresponds to S = 1 and that the ZFS parameter D is positive. An anisotropic spectrum with S = was shown to confirm the isolated C vacancy model. The properties of sublimation grown semi-insulating SiC for device applications have also been studied19 by a wide range of techniques. 

In most commercial applications, conditions are such that the rate of sublimation is controlled by heat transfer. The development of techniques for improving the heat-input rate is the objective of many investigations. 

The handling of sublimable materials is easier than with vacuum techniques. 

Experimental evidence of surface enrichment in Pt3Sn stems from AES (//) and selective chemisorption (14). Both techniques indicate surface enrichment of tin, the element with the lower heat of sublimation. Table I shows that AES yields lower values of surface enrichment than surface titration. This is not surprising because AES scans not only the atoms of the surface layer, but also those of lower-lying layers. However, a quantitative comparison of AES and chemisorption data (14) shows that the results can only be matched if enrichment occurs by inversion of the outer layers, i.e., if depletion of tin atoms occurs in layers next to the outer layer enriched in tin atoms. 

R.B. Cundall et al, "Vapor Pressure Measurements on Some Organic High Explosives , J-ChemSoc, Faraday Trans I, 74 (6), 1339—45 (1978) CA 89, 181933 (1978) [Equilibrium vap press were detd for various expls by the Knudson cell technique. The data for HMX follows the Clausius-Clapeyron eqtn. The values detd for the const A and B in the eqtn, log10p = A—(B/T), plus the std enthalpy, entropy and Gibbs energy of sublimation from the authors calcns are presented in Table 7 ... 

When the separation procedures described in detail above are unsatisfactory for the separation of a mixture of organic compounds, purely physical methods may be employed. Thus a mixture of volatile liquids may be fractionally distilled (compare Section 2.26) or a mixture of non-volatile solids may frequently be separated by making use of the differences in solubilities in inert solvents. The progress of such separations may be monitored by application of the various chromatographic techniques detailed in Section 2.31, or indeed these techniques may be employed on the preparative scale for effecting the separation itself (e.g. flash chromatography, p. 217). The techniques of counter current distribution, fractional crystallisation or fractional sublimation (Section 2.21) may also be employed where appropriate. 

Despite the kinetic lability of the Ln-X-cr-bonds (even the thermodynamically very stable Ln-OR bond is subject to rapid ligand exchange reactions [49]) organolanthanide compounds are thermally very robust over a wide range of temperature (Fig. 5) [114, 116, 139, 144-151]. Thermal stability is not only favorable in catalytic transformations at elevated temperatures [47], for the support of volatile molecular precursors is of fundamental importance in chemical vapor deposition techniques the sublimation behavior is a criterion of thermal stability and suitability for these processes (Fig. 5). 

AIN thermal conductivity was measured by Slack et al [12], using AIN single crystals obtained by the sublimation technique of Slack and McNelly [21]. The results of these measurements and the theoretical estimates of Slack are listed in TABLE 4. 

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