Melting points experimental procedures

The data for our analysis were collected from production runs of about 10000 steps, corresponding to a total simulation time of approximately 2 ps. The temperature for each simulation was chosen as that value for which experimental data are available. In general, the temperature lies about 50 K above the corresponding melting point. A detailed description of the computational features and the simulation procedure -including systems and temperatures - is given in [7]. 

The enthalpies of solution were measured with a LKB 8700-1 precision calorimetry system. The experimental procedure and test of the instrument have been given before (6,7). EC (Fluka, purissimum) was distilled under reduced pressure and the middle fraction was stored over molecular sieves (4 A) for at least 48 hr. ACN (Merck, pro analysis) was dried over molecular sieves and used without further purification. The purity of both solvents (determined shortly before use), as deduced from GLC, was always better than 99.8%. The volume fraction of water, determined by K. Fischer titration (8) was always less than 3.10-4. The mixed solvents were prepared by weight as shortly as possible before the measurements. AH° of Bu4NBr in W-ACN mixtures have been measured at 25°C while those in W-EC are at 45°C, which is above the melting point of pure EC. 

Vaporization of the metal The metal must be melted in vacuum. Aside from this requirement, which applies in all cases, the experimenter can choose from a variety of options. The usual procedure involves resistance heating of a suitably shaped wire coll. This method can be used where the metal has a sufficiently high volatility below the melting point to produce the vapor at an appreciable rate. In other cases it may be necessary to vaporize the metal at or above its m.p. In these cases, it is... 

Certain types of test to measure short-term effects receive special attention. Thermal expansion, glass transition point, softening point, and melting point are thought of as separate properties although they are particular cases of the effect of temperature. Low-temperature resistance can be measured in many ways, but many of the procedures in common use have been developed specially for reasons of experimental convenience. 

Some areas of minimal hazard arise even in the EXPERIMENTAL PROCEDURES of this book. These are pointed out clearly by CAUTION followed by appropriate words of warning that are both boldfaced and underlined (for example, CAUTION Do not heat the tube so hot that it melts or sags.). 

The application of the procedure just outlined to lithium clusters leads to satisfactory values of the surface energy per atom b, in agreement with theoretical results obtained by more complicated methods and with experimental values for the liquid metal and the vapor pressure near the melting point (Tables 19 and 20). The values obtained for b satisfy the theorem of Wulff. For instance, for a cfc cuboctahedron bounded by (111) and (110) faces, one finds... 

The melting point of a solid organic substance is frequently adopted as a criterion of purity, but before any reliance can be placed on the test, it is necessary for the experimental procedure to be standardized. Several types of melting point apparatus are available commercially, but the most widely used method consists of heating a powdered sample of the material in a glass capillary tube located close to the bulb of a thermometer in an agitated bath of liquid. 

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