The addition of alcohol to a volatile oil always results in a lowering of the specific gravity. When an oil containing alcohol is dropped into water, the drops do not remain clear and transparent, as is the case with pure oils, but become opaque and milky.
For the definite identification of alcohol the suspected oil is heated until it just begins to boil,3) and the first few drops that come over are collected in a test tube and filtered, to remove any oil globules which may also have come over, through a filter moistened with water. The filtrate is made strongly alkaline with dilute potassa solution, and treated, after heating to 50 to 60°, with a solution of iodine in potassium iodide, until the solution remains slightly yellow. If alcohol is present small crystals of iodoform will separate in a short time at the bottom of the liquid. It must here be remembered that other bodies, such as aldehydes, acetone and acetic ether also yield iodoform under the conditions given.
1) African copaiba balsam oils, the presence of which can be ascertained by the identification of cadinene (see p. 329), are dextrogyrate. According to the observations thus far recorded, the angle varies between +16°51' and +22°26'. Dextrogyrate gurjunbalsam oils are also said to occur.
2) Liebig's Annalen 369 (1909), 56.
3) Mot all of the alcohol is driven over by heating on the water bath.
Larger amounts of alcohol may be removed from volatile oils by shaking with water, from which the alcohol may be again removed by distillation and identified by the iodoform reaction. If the shaking out is done in a graduated cylinder, the increase of the aqueous layer corresponds approximately to the amount of the alcohol.
It is better, however, to use sodium chloride solution or glycerin, because with these the two layers separate better, and a more accurate reading is possible.
The alcohol content may also be approximately calculated if the specific gravity is determined before and after shaking with water.
If d is the specific gravity of the oil (at 15°), D the specific gravity of the oil shaken with water, and s the specific gravity of the alcohol,1) the alcohol content of the oil in percent follows from the formula
(D-d).100 / D - s
As already mentioned on page 599 the amount of the alcohol in an oil which in itself does not give a methyl number, may be determined quantitatively by Zeisel's methoxyl method.