Vacuum melting under

Gases can be separated from solids by desorption under vacuum, melting under vacuum, and melting in an atmosphere of inert gas. " ... 

In addition to the above, purification of /V-methylacetamide can be achieved by fractional freezing, including zone melting, repeated many times, or by chemical treatment with vacuum distn under reduced pressures. For details of zone melting techniques, see Knecht in Recommended Methods for Purification of Solvents and Tests for Impurities, Coetzee Ed. Pergamon Press 1982. 

Hydrogen iodide (anhydrous) [10034-85-2] M 127.9, b -35.5°. After removal of free iodine from aqueous HI, the solution is frozen, then covered with P2O5 and allowed to melt under vacuum. The gas evolved is dried by passage through P2O5 on glass wool. It can be freed from iodine contamination by repeated fractional distillation at low temperatures. Fumes in moist air. HARMFUL VAPOURS. 

Lithium nitrate [7790-69-4] M 68.9, m 253", d 2.38. Crystd from water or EtOH. Dried at 180° for several days by repeated melting under vacuum. If it is crystallised from water keeping the temperature above 70°, formation of trihydrate is avoided. The anhydrous salt is dried at 120° and stored in a vac desiccator over CaS04. After 99% salt was recrystd 3 times it contained metal (ppm) Ca (1.6), K (1.1), Mo (0.4), Na (2.2). 

Phosphorus (white) [7723-14-0] M 31.0, m 590, d 1.82. Purified by melting under dilute H2S04" dichromate (possible carcinogen) mixture and allowed to stand for several days in the dark at room temperature. It remains liquid, and the initial milky appearance due to insoluble, oxidisable material gradually disappears. The phosporus can then be distilled under vacuum in the dark [Holmes Trans Faraday Soc 58 1916... 

Potassium (metal) [7440-09-7] M 39.1, m 62.3 , d 0.89. Oil was removed from the surface of the metal by immersion in n-hexane and pure Et20 for long periods. The surface oxide was next removed by scraping under ether, and the potassium was melted under vacuum. It was then allowed to flow through metal constrictions into tubes that could be sealed, followed by distillation under vacuum in the absence of mercury vapour (see Sodium). EXPLOSIVE IN WATER. 

Two other factors are noteworthy the deleterious effects on chemical and mechanical properties of small amounts of impurities residual from extraction of the metal, and its toxicity. The first of these factors is obviated by vacuum melting the raw metal (for purification) as an essential prerequisite to further processing. The toxicity of beryllium is essentially a pulmonary problem and great care must be taken in handling the finely divided metal or its compounds. In practice, this type of activity is usually carried out under well-ventilated conditions. Certain tolerance levels for atmospheric beryllium are now internationally accepted and merit careful study before work on beryllium is embarked upon. 

Dianilinoethane, containing water of crystallization, is best dried by melting under vacuum. 

The synthesis of high-molar-mass PLA and PGA by two-step polycondensations of lactic and glycolic acids, respectively, has recently been reported.374,375 It involves the formation of a low-molar-mass oligomer followed by a polycondensation step either in the solid state374 or in the melt under vacuum.375 The procedures are detailed in Section 2.4.1.5.2. 

Typically, Be-containing alloys and intermetallic phases have been prepared in beryllia or alumina crucibles Mg-containing products have been synthesized in graphite, magnesia or alumina crucibles. Alloys and compounds containing Ca, Sr and Ba have been synthesized in alumina , boron nitride, zircon, molybdenum, iron , or steel crucibles. Both zircon and molybdenum are satisfactory only for alloys with low group-IIA metal content and are replaced by boron nitride and iron, respectively, for group-IIA metal-rich systems . Crucibles are sealed in silica, quartz, iron or steel vessels, usually under either vacuum or purified inert cover gas in a few cases, the samples were melted under a halide flux . 

Vacuum atomization is a method conceptually similar to effervescent atomization. As schematically depicted in Fig. 2.19, a vacuum atomization facility consists of two chambers, one above the other. The overall dimension is about 18 x 4 m. In the lower chamber, metal is first induction-melted under vacuum and subsequently... 

Solid-state polycondensation (SSP) is thus a technique applied to thermoplastic polyesters to raise their molecular weight or IV. During solid-state polycondensation, the polymer is heated above the glass transition temperature and below the melt temperature of the polymer either under an inert gas or under vacuum. Increasing the intrinsic viscosity requires a residence time of up to 12 h under vacuum or under inert gas, at temperatures from 180 to 240 °C. 

Reduction of the fluoride Generally, Ca with RF3 is heated up to 1450°C in a Ta crucible. After cooling, the slag and the reduced metal are easily separated. The metal obtained contains Ca, some fluoride and Ta, which can be removed by vacuum melting (for La, Ce, Pr, Nd) or by vacuum melting plus distillation (Gd, Tb, Dy, Ho, Er, Lu, Sc, Y). Especially pure Ca must be used, such as triple-distilled Ca further re-distilled under low He pressure and handled in He-filled glove boxes. 

The phase diagram also illustrates why some substances which melt at normal pressure, will sublime at a lower pressure the line p = Pa intersects at Tg the locus OR of the points defining the solid-vapour equilibrium, i.e. at the pressure pj, the substance will sublime at the temperature T. Sometimes the opposite behaviour is observed, namely that a substance which sublimes at normal pressure will melt in a vacuum system under its own vapour pressure This is a non-equilibrium phenomenon and occurs if the substance is heated so rapidly that its vapour pressure rises above that of the triple point this happens quite frequently with aluminium bromide and with iodine. 

Fig. 9.36. Preparation of weighed samples of pure alkali metal. On the left the sodium is being melted under vacuum by means of a cool flame. On the right the bulb has been sealed off under vacuum.

Since commercial 3,5-dinitrobenzoyl chloride is of low purity because of contamination with 3,5-dinitrobenzoic acid, it was treated with thionyl chloride in boiling benzene under dry nitrogen. The product obtained after evaporation under vacuum melted at 68-70° and was stored over phosphorus pentoxide and potassium hydroxide in a vacuum desiccator. 

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