If these oxygenated oils are mixed with oils free from oxygen, for instance, oil of turpentine, they show exactly the same behavior as oils free from oxygen; the nitroprusside of copper is not decomposed and retains its gray-green color. If, for instance, oil of cloves is mixed with oil of turpentine, the red coloration by nitroprusside of copper does not appear.
III. Hager's alcohol and sulphuric acid test.—Bring into a test-tube of about 0.5 inch diameter, five to six drops of the oil to be tested and twenty-five to thirty drops of pure concentrated sulphuric acid, and mix the two fluids by shaking, whereby either no heating takes place or a scarcely perceptible one, or the heating is strong or very vigorous and in some cases increased to the evolution of vapors. The mixture is either clear or turbid. After complete cooling, add to the mixture eight to ten cubic centimeters of 90 per cent. alcohol, and after closing the tube with the finger, shake vigorously. The mixture now shows a different color, is clear or turbid, and the deposit formed after standing for one day is also differently colored and either soluble or insoluble in boiling alcohol.
The mixture of oil, sulphuric acid and alcohol is perfectly clear and transparent with oils of bitter almonds, fennel, clove and rose; with anise-seed oil and star anise-seed oil only the alcoholic layer over the mixture of sulphuric acid and oil is clear. The mixture of oil, acid and alcohol is slightly turbid or nearly clear with oils of valerian, peppermint and field thyme. With most of the other volatile oils occurring in commerce, the mixture is more or less milky turbid. Heating of the oil and acid mixtures does not take place with pyrogenous oils (petroleum, benzine) or only to a very slight degree, as with oils of peppermint and mustard.
IV. Hager's guaiacum reaction[3] serves for the detection of oil of turpentine in a volatile oil. By pouring upon as much guaiacum, freshly powdered, as will lie upon the point of a small knife, in a test-tube 1 cubic centimeter (25 drops) of spike oil, and heating nearly to boiling over a petroleum lamp, the oil after being removed from the flame and allowing the undissolved resin to settle, shows a yellow color. By now pouring upon an equal quantity of guaiacum in another test-tube 25 drops of spike oil and 5 drops of rectified oil of t from the flame shows a dark violet color. Various other oils behave in the same manner as spike oil, and hence a content of oil of turpentine can be readily detected in them. Other oils do not exhibit this behavior; but this can be remedied by adding, in testing for oil of turpentine, a few drops of an oil of the first class.
The guaiacum reaction is an ozone reaction and with reference to this, the volatile oils may be divided into three classes:—
a. Oils inclining to the formation of ozone.—Foremost of these is oil of turpentine, especially when rectified. Oils of tansy, rue, mint, juniper, zedoary, etc., show considerably less inclination.
b. Oils which, especially when heated, directly incite the oil of turpentine to form ozone, and to color guaiacum violet or blue.—Such oils are many kinds of oil of citronella, oils of spike, calamus, cedar, etc.
c. Oils with a content of oil of turpentine, which remain indifferent towards guaiacum.—To such oils, if to be tested for oil of turpentine, with the assistance of the guaiacum reaction, a few drops of an oil of the second class have to be added.
V. Hübl's iodine method.—Mr. C. Barenthin has applied Hübl's iodine method for fixed oils to the examination of volatile oils. He uses the following solutions:—
1. Fifty grammes iodine and 60 grammes of mercuric chloride in a liter of alcohol freed from fusel oil, and let stand for 12 hours.