With certain oils, especially anise, star anise, fennel and rue oils, the congealing point affords a good means for judging the quality. With the first three oils a high congealing point shows a large content of anethol, with rue oil one of methyl nonyl ketone.
In isolated cases the melting point has been recommended for the determination of volatile oils. However, it is little suited to this end, since the volatile oils are not individual compounds but mixtures which reveal no sharp melting point throughout the entire mixture, but soften at first and then gradually change to a clear liquid. As a result no fixed melting point can be observed, but a temperature interval between which the oil melts. Contrasted with the melting point, the congealing point can be clearly and sharply observed.
The determination of the congealing point can be very well performed with Beckmann's well-known apparatus for the determination of the molecular weight by the lowering of the freezing point. A few slight changes make it especially suited for this purpose. They consist principally in doing away with the cork connections which hinder the free inspection of the mercury thread of the thermometer. The laboratory of Schimmel & Co.1) has the form shown in fig. 69. The battery jar A serves as the receptacle for the cooling liquid or freezing mixture. The glass tube B hanging in the metal cover serves as an air jacket for the freezing tube C and prevents the premature congealing of the oil to be tested. The freezing tube C is wider at the top and becomes narrower at the place where it rests on the edge of the tube B. In order to retain C in fixed position, three glass projections are fastened on the inside of the tube B, about 5 cm. below its upper edge. The thermometer, which is graduated into half degrees is held in position in a metal plate by three springs which allow of sliding the thermometer up or down.
1) Report of Schimmel & Co. October 1898, 43.
Fig. 69. 1/4 der wirklichen GroBc.
To carry out the test, the battery jar is filled with either water and pieces of ice or with chopped ice, according to the reduction of temperature desired. Only occasionally it is necessary to employ a freezing mixture of ice and salt. Then pour into the freezing tube so much of the oil to be tested that it stands at a height of about 5 ccm. in the tube and bring the thermometer, which must not touch the sides of the tube at any place, into the liquid. During the cooling the undercooled oil is to be protected from disturbances, which would produce a premature congealation.1) When the oil has been sufficiently undercooled, crystallization is to be induced by rubbing and scratching the thermometer against the sides of the tube. If this proceeding should not prove successful a small crystal of congealed oil or some solid anethol or methyl nonyl ketone is brought into the liquid, when congealation will take place with liberation of heat. The solidification is hastened by continued stirring with the thermometer, the mercury thread of which rises rapidly and finally reaches a maximum which is called the congealing point of the oil.
The oil should be sufficiently undercooled, otherwise the congelation will proceed too slowly thus rendering the observation more difficult. Again, the oils should not be undercooled too much, otherwise the results observed will be too low. Practically useful results are obtained when the oils are undercooled about 5°. It is desirable always to work under like conditions in order that the various qualities of the respective oils may be judged from a common view point. Schimmel & Co. subcool as follows:
Anethol to + 16°, Anise oil to +12°.
Staranise oil to -f10°, Fennel oil to + 3°.
1) A premature congealation often takes place when the oil has not been filtered clear, as suspended dust particles may give rise to congealation.
In certain cases it is desirable to record the temperature to which an oil was subcooled.
It should be mentioned that reference is had to the congealing temperature of oil of rose. This refers to the temperature at which the paraffin crystals are formed when the oil is slowly cooled.