I. Test with iodine.—This test is based upon the fact that some oils violently detonate with iodine, while others develop heat and vapors, and others again remain indifferent. For this test pour upon about 0.19 gramme of dry iodine in a watch-crystal 4 to 6 drops of the oil to be examined.
1. A vigorous reaction (detonation) with considerable increase in the temperature and emission of vapors takes place with the following oils: oils of bergamot, lemon, lavender, nutmeg, orange peel, spike, turpentine, wormwood.
2. Such a reaction as mentioned under 1, does not take place with oils of bitter almonds, copaiba, calamus, clove, peppermint, rose.
3. Moderate heating and slight vapors are developed with oils of anise-seed, fennel, camomile, curly mint, marjoram, rosemary, sassafras, thyme.
When an oil of the second series becomes heated with iodine and evolves vapors, it may first of all be adulterated with cheaper oils. This may also be the case when an oil of the third series reacts violently with iodine and evolves vapors with strong heating. Formerly the iodine test was highly valued; it has, however, been shown to be unreliable since it is frequently dependent on the age of the oil.
In place of iodine, Rudolph Eck recommends a very dilute alcoholic iodine solution, which is not discolored by oils of turpentine, while other oils discolor it. Dissolve a drop of the oil to be examined in 3 cubic centimeters of 90 to 100 per cent. alcohol, and add a drop of the iodine solution. The latter is not discolored in the presence of an oil of turpentine. There are also, however, several volatile oils, which do not discolor the iodine solution. Mierzinski mentions the following: All cold-expressed oils from the Aurantiaceæ, further oils of coriander, caraway, galanga, rue, sassafras, rose, rosemary, anise-seed, fennel, calamus, neroli, angelica, wormwood. Hence, this reaction cannot be relied upon.
II. Hoppe's nitroprusside of copper test.—This test sometimes gives good results, but only with hydrocarbons absolutely free from oxygen and oxygenated oils. It is, therefore, not suitable for oils derived from the Aurantiaceæ. The process is as follows: Add to a small quantity of the oil to be examined in a perfectly dry test-tube, 2 to 5 milligrammes of pure nitroprusside of copper previously thoroughly dried and finely pulverized, shake vigorously and gradually heat to boiling. After boiling for a few seconds allow to cool. If the oil is free from oil of turpentine, or another oil containing no oxygen, the precipitate formed is brown, black, or gray, and according to the quantity of the reagent added and the original color of the oil, the supernatant oil will be differently colored and appear more or less dark. If, however, the oil is adulterated with oil of turpentine, the precipitate formed shows a handsome green or blue-green color, while the supernatant oil retains its original color or at the utmost acquires a very slightly darker one. The longer the oil is allowed to stand after settling, the more distinct and beautiful the color of the oil and of the precipitate appears. For the establishment and certain recognition of very small quantities of oil of turpentine in oxygenated oils, it is best to first add very little of the nitroprusside of copper to the oil to be tested, and a larger quantity only after being convinced either of the purity or adulteration of the oil. This is done to be able, on the one hand, better to judge the reaction, if the oil is pure, and, on the other, if it is adulterated, to establish such adulteration with certainty and to approximately estimate the quantity of oil of turpentine present. The less nitroprusside of copper is used, the better small quantities of oil of turpentine can be detected.
Nearly all volatile oils free from oxygen show the same behavior towards nitroprusside of copper; they decompose it, which is not the case with oxygenated oils. The behavior of the latter is shown in the following table:—
| Name of the oil | Color of the oil | Proportion of nitroprusside of copper to oil | Color of the oil after the experiment | Color of the Precipitate |
|---|---|---|---|---|
| Caraway | clear as water and colorless | 1:1000 parts | slightly yellowish | dirty gray. |
| Fennel | pale yellowish | 1:1000 " | brownish-yellow | black. |
| Dill | pale reddish-yellow | 1:1000 " | becomes first colorless, then yellowish | " |
| Anise-seed | pale yellow | 1:1000 " | yellow | " |
| Camomile (green) | yellowish | 1:1000 " | brownish-yellow | ash-gray. |
| Lavender | pale yellow | 1:1000 " | wine-yellow | slate-gray. |
| " | " | 1: 100 " | brown-yellow | " |
| Mint(curly) | colorless | 1:1000 " | wine-yellow | first gray, then black. |
| Peppermint | " | 1:1000 " | yellowish | black. |
| " | " | 1: 100 " | brownish-yellow | " |
| Balm | yellow | 1:1000 " | dark wine-yellow | " |
| Marjoram | colorless | 1:1000 " | yellowish | " |
| " | " | 1: 100 " | brown-yellow | " |
| Sage | slightly yellowish | 1:1000 " | wine-yellow | dark green. |
| " | " | 1: 100 " | brown-yellow | dark green, then nearly black. |
| Thyme(field) | " | 1:1000 " | brownish-yellow | slate-gray. |
| " | " | 1: 100 " | darkbrown-yellow | nearly black. |
| Wormwood | yellow-brown | 1:1000 " | dark brown | black. |
| Tansy | pale yellow | 1:1000 " | red-brown | dirty brown. |
| Milfoil | dark azure-blue | 1:1000 " | first pale blue, then dark green | gray-brown. |
| Cajeput | colorless | 1:1000 " | brownish-yellow | black. |
| Clove | slightly yellowish | 1:2000 " | rose-red and clear | slate-gray. |
| " | " | 1:1000 " | violet-red and clear | " |
| " | " | 1: 500 " | cherry-red and opaque | " |
| " | " | 1: 100 " | dark cherry-red and opaque | " |
| Cassia | brownish-yellow | 1:1000 " | brownish-red to hyacinth-red | black. |
| " | " | 1: 100 " | dark brown-red | " |
| Sassafras | yellowish | 1:1000 " | yellowish-brown | " |
| Star anise | pale yellow | 1:1000 " | dark wine-yellow | " |
| Valerian | pale greenish | 1: 100 " | brownish-yellow | " |
| Rue | slightly yellowish | 1: 100 " | brown-yellow | ash-gray. |
| Bergamotte | yellowish | 1:1000 " | dark yellow | " |
| " | " | 1: 100 " | brownish-red | " |