Experimental determination of the melting point

The filled melting point tube is now attached to the lower end of a thermometer in such a way that the substance is at the level of the middle 

Two convenient forms of bath are shown in Fig. II, 10, 2, a and b. The former consists of a long-necked, round-bottomed flask (a longnecked Kjeldahl flask of 100 ml. capacity is quite satisfactory) supported by means of a clamp near the upper part of the neck. The thermometer is fitted through a cork, a section of the cork being cut away (see inset) BO that the thermometer scale is visible and also to aUow free expansion of the air in the apparatus. The bulb is about three-quarters filled with 

Another convenient heating liquid is medicinal paraffin it lies a low specific heat, is non-inflammable and is non-corrosive, but it can only be safely heated to about 220° above this temperature it begins to decompose slightly. 

A mixture of equal parts of cottonseed oil and castor oil, containing about 1 per cent, of hydroquinone as an anti-oxidant, is a useful bath liquid which can be heated up to about 250.  

The highly stable and heat-resistant Silicone oils ore very satisfactory. Midland Silicone or Dow-Coming fluid 550 or 600 (or the equivalent I.C.I. product) is recommended. Their only drawback is their high cost. 

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The experimental determination of the melting point at high pressures is generally somewhat difiicult. In some cases it is easier to determine the pressure at which the fusion or solidification of a substance in a closed vessel comes to a stop at constant temperature. A very simple and ingenious mano-cryometer was invented by de Visser for this purpose (Fig. 24). The substance in A is separated by a column of mercury Digitized by Microsoft ... 

The melting point of a substance is the temperature at which the liquid and solid phases are in equilibrium. A good way of experimental determination of the melting point of a solid is to place a small amount of the perfectly dry substance in a thin-walled capillary melting point tube attached to the bulb of a thermometer suspended in some suitable liquid and to raise the temperature of the liquid bath until the solid melts. 

Schmidt M, Kusche R, KronmuUer W, von Issendorff B, Haberland H (1997) Experimental determination of the melting point and heat capacity for a free cluster of 139 sodium atoms. Phys Rev Lett 79 99-102... 

Three determinations of the melting point of technetium metal are in reasonably good agreement 2140 20 °C [13, 2200 50 °C [14], and 2162 40°C (15). The average melting point of 2167 °C is near those of neighboring elements in the same period —molybdenum (2610 °C) and ruthenium (2310 °C) —but almost 1000 °C lower than that of rhenium [14]. Ilic boiling point of technetium metal was estimated as 4900 K [16] no experimental value seems to be available. 

If the data required to construct a phase diagram are not available in the literature for the mixture of interest, the data will have to be obtained experimentally. In this case, the determination of the melting points of known compositions of the mixture has to be carried out. Several experimental techniques are described in detail by Sloan and McGhie 1988. 

We have seen that the experimental determination of the melting temperature of pol3nneric systems, suitable for use in thermodynamic analysis, possesses several inherent difficulties concerned with both concept and technique. Some of the main problems have been pointed out in this paper and the procedures by which they could be overcome has been indicated. Hence there are obvious difficulties in determining the equilibrium melting temperature from polymer data, even admitting the extrapolative procedure. A more detailed discussion of this aspect of the problem will be taken up in a subsequent publication (32). 

Experimental use of the melting-point lowering can be made to determine molar concentrations, of importance for the molar-mass determinations (see Sect. 1.4), and... 

There is a corresponding paucity of experimental determinations of the surface tension of solids, probably because no direct experimental method has been developed. A review of the work on the surface tension of solid metals has been given by Shaler 27). These values were obtained, in most cases, near the melting point of the metals and thermodynamic equilibrium was achieved. These experiments are thus quite different from those where the nonequilibrium state persists, with incomplete relief of surface stress. As this review is mainly concerned with high surface area adsorbents in a state of considerable surface stress in vacuo at least), the above results with metals will not concern us further. 

The first experimental determination of the activation energy Ea was done in 1949 by Robertson [24], His studies of liquid RDX yielded a value of 47.5 kcal/mol for Ea. Subsequent experiments seem to confirm that the Ea value is in the range 47 to 48 kcal/mol. Cosgrove and Owen [25] concluded that the decomposition occurs in the vapor phase based on experiments carried out near the melting point. In 1969 Rauch and Fanelli [26] reported that the liquid- and gas-phase reactions yield different products. They found that the gas-phase reaction produced N02 and the liquid-phase reaction N20 and C02. It has been widely accepted that N-N bond fission to yield N02 is the initial step in the decomposition. However, it has been difficult to firmly establish that as fact because of the difficulties in making kinetics measurements and the unreliability of ab initio predictions for molecules of this size. 

Mesomorphism of alkyl- and alkoxybenzene acids, which can be either smectic or nematic types, was explained [291] by aggregation of their molecules via intermolecular hydrogen bonds in cyclic dimers (hereafter we will use abbreviations wABA and nAOBA for alkyl- and alkoxybenzene acids, respectively, where n determines the number of C atoms in the alkyl radical). Experimental investigations of the dynamics of hydrogen bonds in the vicinity of the phase transitions have been performed in a series of works (see, e.g., Ref. 292). It was shown early [291,293] that in the infrared spectrum new bands appear with heating of the solid-state phase well in advance of the melt point. 

In Figures 2.7 - 2.9, the experimentally determined dependencies of the melting point depression of LiCl, NaCI, and KCI on composition, respectively, are compared with the theoretical course of liquidus curves for different values of the Storkenbeker s correction factor kst-... 

The behavior of K2Tip6 in the alkali metal halides LiP, NaP, LiCl, NaCl, KCl, and in the LiCl-KCl and LiP-LiCl eutectic mixtures was studied using the cryoscopic method by Danbk et al. (1975). The experimentally determined dependency of the melting point depression on molality and molar fraction of K2T1P6, in the proximity of the melting point of the above mentioned solvents were compared with the liquidus curves calculated according to the equation... 

One may consider, for instance, the determination of a melting point as a truly short-term study, because the whole experimental activity is terminated within a few minutes, and nobody would object to calling such a study short-term . But what about an environmental fate study in a lysimeter There the test item is applied to the test system (an activity taking a few minutes), then the whole system is left undisturbed for two years, after which the distribution of the test item in the column of soil is determined, again an activity of a few days at most. Is this a two year, long-term study, or has it to be labelled short-term , because for 99.9 % of the time the test system is just left alone with no experimental activity whatsoever occurring ... 

These three issues can become very cumbersome in the context of conducting short-term studies, because they place resource-intensive, administrative burdens on the test facility management, on the Study Director and on the Quality Assurance personnel which may indeed be out of proportion with the actual experimental work, e.g. for the determination of a melting point, or the investigation of the mutagenic activity in a bacterial test system. This means that a balance has to be achieved between... 

An experimental determination of the effect of ion size upon diffusion was performed by measuring the diffusivity of Rb in KCl. In the temperature region near to the melting point, the results obeyed an Arrhenius relationship ... 

The student should read Sections 1,10 to 1,16 carefully before commencing any experimental work. A supply of melting point capillaries is prepared as described in Section 11,10 (compare Fig. 77, R , I). The apparatus illustrated in Fig. 77. 10, 2, a is assembled with concentrated sulphuric acid as the bath liquid the thermometer selected should have a small bulb. The melting points of pure samples of the following compounds are determined in the manner detailed in Section 11,10 —... 

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