The volatile oils adulterated with fatty oil leave a permanent fatty stain when evaporated on paper. With high boiling and difficultly volatile oils, however, similar residues are left, which may give rise to mistakes. Fatty oil is insoluble in 90 percent alcohol.2) For the separation of the fatty oil from the volatile oil, the latter is distilled off by steam distillation or removed by evaporation in an open dish on the water bath. However, it should be remembered that some volatile oils, such as bergamot, lemon, orange, anise and star-anise oil, leave a residue of several percent even when they are not adulterated. This is also true of resinified oils.
1) The specific gravities of alcohols of varying strengths are given on p. 568.
2) Castor oil only is soluble in 90 p. c. alcohol, but is insoluble in 70 p. c. alcohol and in petroleum ether. Mention should also be made of the slight optical activity of castor oil, aD about +5°.
The presence of the fat may be shown qualitatively in the residue by heating with potassium bisulphate in a test tube. Penetrating odors of acrolein show its presence. By igniting the residue on a platinum foil the characteristic odor of burning fat is noticed.
As the fatty oils give saponification numbers which lie between 180 and 200, the amount of the fat added can be determined quantitatively by saponification either directly in the volatile oil itself or in the distillation residue.
Oils that have been adulterated with cocoa nut oil solidify wholly or in part in a freezing mixture. Cocoa nut oil has been found in cananga oil, citronella oil and palmarosa oil and detected in this manner.