Melting-points

The purity of a crystallised organic substance is checked by the melting point. This easily determined constant also serves for the identification of substances and, in the case of new compounds, for their characterisation. The apparatus is a long-necked, round-bottomed flask into which a tested thermometer is fixed by means of a cork a sector is cut out of the cork with a sharp knife in order that the whole of the thermometer scale may be visible (Fig. 31). 

The heating liquid is pure concentrated sulphuric acid, with which the bulb of the flask is three-quarters filled. The substance, in powder form, is introduced into a small, thin-walled capillary tube. Such tubes are made as follows from test tubes (preferably from damaged tubes which must, however, be clean and dry ). The tubes are rotated in the flame of the blow-pipe till soft and then drawn out rapidly already after short practice the student can strike the correct diameter, which should be 1 0-1 -5 mm. internally. Suitable portions of the drawn-out material are cut off with scissors. It is convenient to cut double lengths (about 12 cm.), so that by sealing each length in the middle (micro-burner) two melting-point tubes are obtained ready for use. 

The melting-point tube is then attached to the lower end of the thermometer in such a position that the substance is at the level of 

When the melting point of a decomposing substance is to be determined the bath is heated rather rapidly to within 10°-20° of the temperature of decomposition, and from that point onwards the rate of heating is reduced to about 5° per minute. 

When stable substances sinter before melting it is a sign that they have not been completely purified, and should be recrystallised or redistilled. Some substances sinter, however, even when perfectly pure, i.e. they have no sharp melting point. In this connection the so-called liquid crystals may also be mentioned (Lehmann, Yorlander). 

Probably the most amazing feature of ionic liquids is that they are liquid. Intuitively, we expect salts to be solid at room temperature and to melt only at very high temperatures. Take sodium chloride (melting point 803°C) since its ions have similar size and shape, a solid, crystalline packing structure is obtained. In contrast, the ions forming ionic liquids do not pack well, which explains why they can remain liquid at low temperature.  

Three main factors have to be considered in the choice of the cation. 

GREEN REACTION MEDIA IN ORGANIC SYNTHESIS Table 2.2 

Among these anions, two are often used to form ionic liquids, namely, hexafluorophosphate PFg and bis(trifluoromethanesulfonyl)amide NTf2. In Table 2.3, the melting points of tetraalkylammonium and trialkylsulfonium salts formed with the NTf2 anion are indicated, clearly showing that a large variety of ionic liquids with low melting points can be prepared. 

it can be liquid at room temperature as it freezes only at 5 C. Similar behavior was observed for all imidazolium salts, with differences between the melting and the freezing points as high as 200°C. Once they have supercooled, ionic liquids can be kept in this state for a long period, sometimes up to weeks, but the addition of small crystals as seeds will lead to rapid crystallization.  

The mechanical and thermal behaviors depend partly on the degree of crystallinity. For example, highly disordered (dominantly amorphous) polymers make good elastomeric materials, while highly crystalline polymers, such as polyamides, have the rigidity needed for fibers. Crystallinity of polymers correlates with their melting points. 

The freezing point of a pure liquid is the temperature at which the liquid s molecules lose transitional freedom and the solid s molecules 

The viscosity of a substance measures its resistance to flow. The melt viscosity of a polymer increases as the molecular weight of the polymer rises. Polymers with high melt viscosities require higher temperatures for processing. 

The melt flow index describes the viscosity of a solid plastic. It is the weight in grams of a polymer extruded through a defined orifice at a specified time. The melt viscosity and the melt flow index can measure the extent of polymerization. A polymer with a high melt flow index has a low melt viscosity, a lower molecular weight, and usually a lower impact tensile strength. 

The number average molecular weight (M ) is related to the number of particles present in a sample. It is calculated by dividing the sum of the weights of all the species present (monomers, dimers, trimers, and so on) by the number of species present  

The preparation of the capillary tube requires a little practice. A piece 

The substance of which the melting point has to be taken should be perfectly dry. A sample is ground to a fine powder on a watch-glass with a clean glass rod, introduced into the capillary tube and shaken to the 

The melting-point apparatus consists of a small beaker or a large-sized test tube containing sulphuric acid up to a convenient level. The ther- 

The thermometer is first dipped in the sulphuric acid, and then the drop of acid which clings to the bulb is smeared on the side of the capillary tube containing the substance. The capillary tube is then made to adhere to the thermometer (Fig. 6) by capillary attraction, so that the substance in the tube is just opposite the bulb of the thermometer. This method is much better than using a rubber band, which is apt to perish in the sulphuric acid fumes, and gives rise generally to a speedy discoloration of the acid. 

Correction.—Melting points are usually given as uncorrected for correction the following formula is employed — 

Vapor density data, another form of P-V-T measurement, can be used to calculate apparent molecular weight (1272), association factors (1011), or degree of association (1305). Again, this is a means of connecting equilibrium constants to temperature. 

These methods require accurate P-V-T data over a fair range of temperature for molecules whose critical constants arc known or for which some thermodynamic values have been measured. It is not surprising that only a few of the more common materials have been studied. 

A related method is typified by Pitzer and Weltner s paper on methanol (2156). They used heat capacity expressions corresponding to the virial equation to calculate virial coefficients. They use the calculated fourth virial coefficient to argue that H bonded tetramers arc present. A corresponding interpretation of B is not given. 

An example of the use of H bonding to vary melting point is given by Champetier s discussion of polyamides (345, 362). He compared the melting point with the number of H bonds per 100 chain atoms. His results can be summarized in a schematic fashion, as in Fig. 2-10. The H bonded materials have higher melting points than the polymethylene, and the melting point increases with the relative number of 

TABLE 2-VIII Comparison of Melting and Boiling Points of Intermolecular H Bonded Materials with Similar Sized Compounds 

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They are formed by treatinga-diketones, a-hyd-roxyaldehydes, hydroxyketones, aminoalde-hydes or aminoketones with arylhydrazines. Sugars can be identified by their osazones which have characteristic melting-points, formation times or crystal appearance. 

Its charge transfer complexes with aromatic hydrocarbons have characteristic melting points and may be used for the identification and purification of the hydrocarbons. 

During the production of mineral oils from vacuum distillates, one of the process steps, dewaxing , removes the high melting point materials in order to improve the oil s pour point. Dewaixing produces paraffins and waxes, the first coming from light distillates, and the second from medium or heavy distillates. 

Melting point for waxes NFT 60-114 ISO 3841 ASTM D 87 Observations under standardized cooling conditions... 

Molten naphthalene at its melting point of 82°C has the same density as does water at this temperature. Suggest two methods that might be used to determine the naphthalene-water interfacial tension. Discuss your suggestions sufficiently to show that the methods will be reasonably easy to cany out and should give results good to 1% or better. 

Molecular dynamics calculations have been made on atomic crystals using a Lennard-Jones potential. These have to be done near the melting point in order for the iterations not to be too lengthy and have yielded density functioi). as one passes through the solid-vapor interface (see Ref. 45). The calculations showed considerable mobility in the surface region, amounting to the presence of a... 

We noted in Section VII-2B that, given the set of surface tension values for various crystal planes, the Wulff theorem allowed the construction of fhe equilibrium or minimum firee energy shape. This concept may be applied in reverse small crystals will gradually take on their equilibrium shape upon annealing near their melting point and likewise, small air pockets in a crystal will form equilibrium-shaped voids. The latter phenomenon offers the possible advantage that adventitious contamination of the solid-air interface is less likely. 

For liquids that are reasonably fluid around their melting points, the kinetic factors in Eq. DC-15 come out about 10 /cm sec, so that Eq. IX-15 becomes... 

Another indication of the probable incorrectness of the pressure melting explanation is that the variation of the coefficient of friction with temperature for ice is much the same for other solids, such as solid krypton and carbon dioxide [16] and benzophenone and nitrobenzene [4]. In these cases the density of the solid is greater than that of the liquid, so the drop in as the melting point is approached cannot be due to pressure melting. 

While pressure melting may be important for snow and ice near 0°C, it is possible that even here an alternative explanation will prove important. Ice is a substance of unusual structural complexity, and it has been speculated that a liquidlike surface layer is present near the melting point [17,18] if this is correct, the low /t values observed at low sliding speeds near 0°C may be due to a peculiarity of the surface nature of ice rather than to pressure melting. 

Substances in this category include Krypton, sodium chloride, and diamond, as examples, and it is not surprising that differences in detail as to frictional behavior do occur. The softer solids tend to obey Amontons law with /i values in the normal range of 0.5-1.0, provided they are not too near their melting points. Ionic crystals, such as sodium chloride, tend to show irreversible surface damage, in the form of cracks, owing to their brittleness, but still tend to obey Amontons law. This suggests that the area of contact is mainly determined by plastic flow rather than by elastic deformation. 

It is evident that boundary lubrication is considerably dependent on the state of the monolayer. Frewing [48] found that, on heating, the value of fi rose sharply near the melting point sometimes accompanied by a change from smooth to stick-slip sliding. Very likely these points of change correspond to the transition between an expanded film and a condensed film in analogy with... 

Referring to Fig. XVII-17, use handbook data to calculate the vapor pressure of O2 ordinary liquid at the melting point of the 6 phase. Comment on the result. Locate the 2D S-L-V triple point. 

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