CONGEALING POINT-DETERMINATION OF PHYSICAL PROPERTIES

6. CONGEALING POINT

The congealing point³⁸ offers a distinct advantage over the melting point and the titer, in the case of mixtures, such as essential oils.³⁹ In determining the congealing point, the oil is supercooled so that, upon congelation, immediate crystallization with liberation of heat occurs. This results in a rapid rise of temperature, which soon approaches a constant value and remains at this temperature for a period of time. This point is known as the"congealing point." With increasing percentage of crystalline material in an oil, the congealing point will approach a maximum.⁴⁰ Hence, this physical property is a good criterion of the percentage of such material. The congealing point is important in the evaluation of anise, sassafras and fennel oils.

Procedure: Place about 10 cc. of the oil in a dry test tube of 18 to 20 nun. diameter. Cool in water or in a suitable freezing mixture, the temperature of which should be about 5 lower than the supposed congealing point of the liquid. To initiate congelation, rub the inner walls of the tube with a thermometer, or add a small amount of the substance previously solidified by excessive freezing. The thermometer should be rubbed quickly up and down in the mixture in order to cause a rapid congelation throughout, witli its subsequent liberation of heat. The temperature should be read frequently; at first the rise of temperature is rapid, but soon approaches a constant value for a brief interval of time. This value is taken as the congealing point of the oil.

The process described above should be repeated several times to assure obtaining the true congealing point.

The thermometer used should be calibrated in 0.1^o units and should be accurately standardized. A thermometer covering the range of -5^o to +50^o is satisfactory for most determinations.

Before the oil is tested, it should be thoroughly dried with sodium sulfate, since the presence of small amounts of water will often materially lower the congealing point.

In the case of sassafras oils, it is well to initiate the congelation by the addition of a small piece of solid safrole since sassafras oil can be congealed only with great difficulty if no "seed" is used.

For a more exact determination of the congealing point, the test tube containing the supercooled oil may be insulated by means of an air jacket. This is frequently of particular importance when determining congealing points which are much below room temperature, as, for example, the congealing point of euclayptus oils. Gildemeister and Hoffmann recommend the use of the Beckman apparatus,⁴¹ frequently used for the determination of molecular weights by the lowering of the freezing point. The use of a larger sample (up to 100 cc.) may make the congealing point sharper.

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38 The so-called "congealing point" of rose oil is not a true congealing point, but is determined by the same method as that used for titer determinations in fixed oils. (See

"Special Tests and Procedures” p. 329.)

39 The melting point is usually used for crystalline solids.

40 This maximum will be the "congealing point" of the pure crystalline compound.

41 "Die atherischen Ole," 3d Ed. Vol. I, 707.