Sublimation micro

For an example of sublimation, see the preparation of anthra-quinone, p. 259 for semi-micro sublimation, see p. 69. 

The advantages of this method are twofold (i) It is possible to observe minute changes in colour and structure before and during the process of melting. (2) It is possible to use a single crystal which, e.., is often obtained from a semi-micro sublimation. 

Dissolve 1 g. of anthracene in 10 ml. of glacial acetic acid and place in 50 ml. bolt head flask fitted with a reflux water-condenser. Dissolve 2 g. of chromium trioxide in 2 ml. of water and add 5 ml. of glacial acetic acid. Pour this solution down the condenser, shake the contents of the flask and boil gently for 10 minutes. Cool and pour the contents of the flask into about 20 ml. of cold water. Filter off the crude anthraquinone at the pump, wash with water, drain well and dry. Yield, 1 g. Purify by re crystallisation from glacial acetic acid or by sublimation using the semi-micro sublimation apparatus (Fig. 35, p. 62, or Fig. 50, p. 70). 

Currently, there is continuing work on an iadustry standard method for the direct determination of monomer, dimer, and trimer acids. Urea adduction (of the methyl esters) has been suggested as a means of determining monomer ia distilled dimer (74). The method is tedious and the nonadductiag branched-chain monomer is recovered with the polymeric fraction. A micro sublimation procedure was developed as an improvement on urea adduction for estimation of the polymer fraction. Incomplete removal of monomer esters or loss of dimer duriag distillation can lead to error (75). 

As regards general methods for distinguishing between the alkaloids of opium, mention may be made of the following Kofler and Kofler s study of the micro-sublimation of these alkaloids and the characters of the micro-sublimates the comparison by Maplethorpe and Evers of the picrates of a series of opium bases, comparison of the colour reactions of a series of opium alkaloids, and their behaviour with specific reagents and precipitants. ... 

Rhoeadine crystallises from a mixture of chloroform and ether, sublimes at 215-25" /0-02 mm., and then melts at 256-57-5° (vac.), and has [a] / ° -f 232° (CHClj). According to Awe it has m.p. 272-4° (Kofler s micro-apparatus), and [a]] ° -f 243° (CHClj). A crystalline liydriodide, B. HI. 2H2O, was prepared by the addition of potassium iodide to a solution of the alkaloid in acetic acid. Rhoeadine contains one methoxyl group and a dioxymethylene group, but no reactive hydrogen (Zerewitinoff). 

Figure 8.3 Apparatus used for sample preparation involving distillation or sublimation. A, distillation apparatus for organic volatiles in water B, micro apparatus for continuous steam distillation with continuous llquid-llguid extraction of the distillate C, micro sublimation appeuratus.

Milnes and Mostaghaci [5.5] compared the consequences of different drying methods on the density, the sinter rate and micro structures of sublimated TiO-, suspensions. Evaporation of water in a micro-oven and by radiation heating leds to strongly bound agglomerates, while freeze drying resulted in softly bound secondary clusters. The freeze dried powder reached in 2 h of sintering 98 % of the theoretical density, while differently dried powders needed twice as much time and had a less fine microstructure. 

Small (micro) scale vacuum sublimator from readily available parts... 

Kofler Micro Hot State Apparatus. Electrically heated apparatus which might be considered a modification and improvement of the Fisher-Johns apparatus. It permits microscopic studies of fusion, sublimation, crystallization and explosion phenomena Refs 1) L. Kofler, Mikrochemie 15, 242 (1934) 2) N.D. Cheronis J.P. Entrikin,... 

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 ... 

Early devices fbr thermogravimetric analysis were limited in precision and convenience when compared with DTA or DSC equipment. Now devices for simultaneous DSC and TG are on the market which can operate at high temps in reactive atms permitting the simulation of high temp reactions on a micro scale. The use of TG for the study of reaction kinetics was described in Sect 5.3.3. An exptl study of the sublimation of ammonium perchlorate was published by Jacobs and Jones (Ref 25). Similar techniques should find application in the study of other propint systems. The product gases have been collected for further analysis using gas chromatography and mass spectrometry... 

In a typical experiment 200 mg of Zr(03P0K)2 were added to a solution of piperonal 1 (75 mg, 5 mmol) and Meldrum s acid 2a (72 mg, 5 mmol) in dichloro-methane (2 mL) placed in a 100-mL round-bottomed flask. The solvent was removed under reduced pressure. The round-bottomed flask was placed in a micro-wave oven (Whirlpool, M430, Jet 900) and irradiated (450 W) for a total of 14 min and in 2-min intervals. After cooling, the crude product was purified by sublimation (10 mTorr, isomantle at a temperature between 180 and 220 °C) to give 5-(3,4-methylenedioxyphenyl)metliylene-2,2-dimetliyl[l,3]dioxane-4,6-dione 3a (0.14 g, 100%), mp 179 °C. 

Another design is illustrated in Fig. 2.88 and may be purchased from appropriate suppliers or made to specification the sizes of the pot and of the cold finger are appropriate to the quantity of material to be sublimed, which may be as low as 20 mg. Frequently these assemblies, sometimes with slight modification, may be used for the high vacuum micro-distillation of viscous liquids (Section 2.28). 

When a product has been isolated from a reaction the next step is to purify it. The degree of purity required will depend on the use for which the sample is intended, a synthetic intermediate might only require rough purification, whereas a product for elemental analysis would require rigorous purification. This section describes the most important purification techniques, crystallization, distillation, sublimation, and chromatography. It is assumed that the reader is familiar with the basic principles of these methods, so the emphasis is on more demanding applications such as the purification of air-sensitive materials, and purifications on a micro-scale. 

Colorless crystals were formed upon sublimation of the micro-crystalline solid at 40-50 C in 0.7-mm-diameter quartz X-ray capillaries. These had been sealed under an atmosphere of nitrogen. Precession photographs indicated a primidve orthorhombic cell, space group Pnma or Pna2i. 

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

Sublimation plays an important role in the purification of organic compounds on a micro scale since it involves little loss of material, is convenient and fast, and provides very efficient purification. An essential condition for its application is, however, that the substance has sufficient vapor pressure at the temperature to be used. 

Attention must also be paid to the distance between the condenser and the evaporator when subliming substances that have low vapor pressures. Apparatus that has a variable condenser position is thus extremely useful because this position can then be varied to suit the substance and the pressure to be used. The distance from the substance to the condenser is usually 7-25 mm in micro apparatus. 

Kurtikyan, Ford, and coworkers have used infrared and UV spectroscopy to probe the species formed in the interaction of NO, molecules with micro-porous solvent-free layers of (por)M complexes (M=Mn (138—140), Fe (141—150), Co (151-155)) at varying temperatures. Sample preparation of such (por)M layers and their interactions with NO, have been described in detail elsewhere (156,157). Importantly, in this sample preparation technique, the (por)M species are sublimed onto KBr, Csl, or Cap2 substrates at low temperature to form porphyrin sponges which permit diffusion of volatile Hgands into the bulk layer (158). Using this method, clean IR spectra... 

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