Rape and flaxseed oils absorb a part of the spectrum but do not affect the rest of it. The spectroscope is of little practical utility in oil analysis.[263]
311. Critical Temperature of Solution.—The study of the critical temperature of solution of oils has been made by Crismer, who finds it of value in analytical work.[264] If a fatty substance be heated under pressure, with a solvent, e. g., alcohol, it will be noticed that as the temperature rises the meniscus of separation of the two liquids tends to become a horizontal plane. If at this point the contents of the tube be thoroughly mixed by shaking and then be left at rest, a point will soon be reached at which the two liquids again separate, and this point is distinctly a function of temperature. Following is a description of a convenient method of determining the critical temperature of the solution of fats and oils for experimental purposes. Tubes are prepared for holding the reagents in such a way that, after the introduction of the fat and alcohol, they can be easily sealed. The capacity of these tubes should be about five cubic centimeters. They should be charged with about one cubic centimeter of the dry filtered fat and about twice that quantity of ninety-five per cent alcohol. Care should be exercised to avoid touching the upper sides of the tube with the reagents. When charged the tubes are sealed in the flame of a lamp and attached to the bulb of a delicate thermometer in such a manner as to have the surface of its liquid contents even with the top of the bulb. The tube is conveniently fastened to the thermometer by a platinum wire. For duplicate determinations two tubes may be fastened to the same thermometric bulb. The apparatus thus prepared is placed in a large vessel filled with strong sulfuric acid. The operator should be careful to protect himself from the danger which might arise from an explosion of the sealed tubes during heating. It is advisable in all cases to observe the reaction through a large pane of clear glass. The bath of sulfuric acid is heated by any convenient means and an even temperature throughout the mass is secured by stirring with the thermometer and its attachments. When the meniscus which separates the two liquids becomes a horizontal plane the thermometer is removed and the liquid in the tubes well mixed until it appear homogeneous. The thermometer is replaced in the bath, which is allowed to cool slowly, and the phenomena which take place in the sealed tubes are carefully noted. The critical temperature of solution is that at which the two liquids begin to separate. This moment is easily noted. It is, moreover, preceded by a similar phenomenon taking place in the capillary part of the tube which retains a drop of the mixture on shaking. In this droplet an opalescence is first noted. In the mass of the liquid this opalescence, a few seconds afterwards, is observed to permeate the whole, followed by the formation of zones and finally of the reappearance of the meniscus of separation between the two liquids. The temperature at this moment of opalescence preceding the separation of the liquid is the critical temperature of solution. With alcohol of 0.8195 specific gravity, at 15°.5 (ninety-five per cent), the observed critical temperatures for some of the more common fats and oils are as given below:
| Butter fat | 100°.0 | |
| Oleomargarin | 125°.0 | |
| Peanut | oil | 123°.0 |
| Cotton | ” | 116°.0 |
| Olive | ” | 123°.0 |
| Sesamé | ” | 121°.0 |
| Colza | ” | 132°.5 |
| Mutton tallow | 116°.0 | |
| Beef marrow | 125°.0 | |
| Nut oil | 100°.5 | |
When the alcohol is not pure or if it be of a different density from that named, the numbers expressing the critical temperature of solution will vary from those given above.
312. Polarization.—The pure glycerids are generally neutral to polarized light. In oils the degree of polarization obtained is often variable, sometimes to the right and sometimes to the left. Olive oil, as a rule, shows a slight right hand polarization. Peanut, sesamé, and cottonseed oils vary in polarizing power, but in no case is it very marked. Castor oil polarizes slightly to the right.
In determining the polarizing power of an oil it should be obtained in a perfectly limpid state by filtration and observed through a tube of convenient length, as a rule, 200 millimeters. The deviation obtained may be expressed in divisions of the sugar scale of the instrument or in degrees of angular rotation.
313. Turbidity Temperature.—The turbidity temperature of a fat, when dissolved in glacial acetic acid, as suggested by Valenta, may prove of some diagnostic value.[265] The fats are dissolved, with the aid of heat, in glacial acetic acid and, on slowly cooling, the temperature at which they become turbid is observed. The following data observed by Jones are given for comparison.[266]
The numbers represent the turbidity temperature of the fat when treated with the glacial acetic acid, and allowed to cool slowly. Butter fat, from 40° to 70°, mostly from 52° to 65°; oleomargarin, 95° to 106°; rape oil, 101°; sesamé oil, 77°; linseed oil, 53° to 57°; lard oil, 96°; olive oil, 89°; peanut oil, 61° to 88°.
It is important in this test that the acetic acid be absolutely glacial. About three cubic centimeters of the glacial acetic acid, and three of the fat, should be used.