MELTING POINT-DETERMINATION OF PHYSICAL PROPERTIES

7. MELTING POINT


The importance of the determination of the melting point of a solid, crystalline material is obvious.
A brief but comprehensive discussion of the determination of melting points has been given by Shriner and Fuson,42 from which much of the following is taken.
Procedure: Heat a piece of 15 mm. glass tubing43 in a flame until the glass is soft; then draw out into a thin walled capillary tube about 1 mm. in diameter. Cut into lengths of about 6 cm., and seal one end in a name. Powder a small amount of the compound in a polished agate mortar and introduce some of the powder into the capillary tube. Hold the capillary tube vertically and gently rub with a file, which causes the powder to settle to the bottom ; pack the material by tapping the tube on the desk. Fasten the tube to the thermometer by means of a rubber band (cut from a piece of rubber tubing) so that the sample is close to the mercury bulb (see Diagram 4.3). Place a heavy white mineral oil in the beaker and heat with a low flame. Clamp a cylindrical metal shield, open at the top and bottom, in the position as shown in Diagram 4.3 in order to protect the flame from drafts. Heat at a rate to cause a rise in temperature of about 1o or 2o per min. Stir the oil bath continuously. Note the temperature at which the compound starts to melt44 and that at which it is entirely liquid; record these values as the melting point range. Note also the temperature recorded by the auxiliary thermometer (t2) ', the bulb of this thermometer should be placed midway between the surface of the oil and the top of the mercury thread in t1. Calculate the stem correction by means of the following formula :
Correction = 0.0001 54N(t1 - t2)
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41 "The Systematic Identification of Organic Compounds," John Wiley & Sons, Inc., New York (1940), 85.
49 Soft glass test tubes are very satisfactory since the walls are relatively thin.
44 According to "The United States Pharmacopoeia," Thirteenth Rev., 668, the temperature at which the column of the sample is observed to collapse definitely against th side of the tube at any point is defined as the beginning of the melting. The temperatur at which the material becomes liquid throughout is defined as the end of the melting.
Apparatus for the detormination of melting point
It is important to record the melting point range of a compound since this is a valuable index of purity. A large majority of pure organic compounds melt within a range of 0.5o or melt with decomposition over a narrow range of temperature (about 1o).
When determining the melting point of a solid that readily sublimes e.g., borneol certain precautions become necessary. The rate of heating of the oil bath should be increased considerably. The capillary should not be introduced into the hot oil until the temperature is within 10o to 20o of the expected melting point. The use of a sealed capillary may be necessary, i.e., a capillary that has both ends fused. The use of a Fisher-Johns or similar type apparatus is not recommended for materials that sublime readily.
Other types of melting point apparatus have proven satisfactory e.g., the Fisher-Johns, Thiele, and Thiele-Dennis.
If a compound has a high melting point a Maquenne block may conveniently be used. It is claimed that the Dennis melting point apparatus is very satisfactory for compounds melting up to 300o.
Special types have been developed for determination of the melting point of waxes,45 and the softening point of amorphous material.46
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45 A.S.T.M. Melting Point Apparatus, Designation D87.

46 A.S.T.M. Softening Point Apparatus (Ring and Ball Method), Designation D36.

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