(d) A portion of the sample is well mixed with one-fourth of its weight of chromic acid; if pure, the oil will be converted into an opaque mass.
(e) Introduce 2 c.c. of the sample into a narrow graduated glass cylinder, add 0·1 gramme potassium dichromate, next 2 c.c. of a mixture of sulphuric and nitric acids, shake well, and then add 1 c.c. of ether; shake again and allow the mixture to stand at rest. Lively effervescence and evolution of nitrous fumes soon take place, and the oil rises to the surface, showing a characteristic coloration. Olive oil exhibits a green colour, whereas in presence of 5 per cent. of sesamé, arachis, cotton-seed, or poppy oil, the colours will vary from greenish-yellow to yellow or yellowish red. The coloration is more readily observed upon agitating the mixture with water and, setting it aside for a short time.
(f) Several portions of the oil are placed upon a porcelain slab and separately treated with a few drops of concentrated sulphuric acid, nitric acid, and a solution of potassium dichromate in sulphuric acid, and notice taken of the colours produced, comparative tests being simultaneously made with olive oil of undoubted purity.
(g) The presence of sesamé oil is readily detected by the formation of a deep green colour when the oil is agitated with a mixture of equal parts of nitric and sulphuric acid.
(h) Upon mixing samples containing cotton-seed oil with an equal volume of nitric acid (40° B.) a coffee-like colour is produced. Olive oil gives a pale green, rape and nut, a pale rose, and sesamé oil a white-coloured mixture.
The presence of rape- and cotton-seed oils may also be detected as follows:—Dissolve 0·1 gramme silver nitrate in a very little water, and add about 4 c.c. of absolute alcohol. This solution is added to the sample of olive oil to be tested, the mixture well shaken and put aside for one or two hours; it is then to be heated for a few minutes. If cotton-seed or rape-seed oil is present, the oily stratum which separates on standing will exhibit a brownish-red or blackish colour, due to the reduction of silver. Olive oil fails to cause an appreciable coloration. Experiments made by the author with samples of olive oil containing 10 per cent. of cotton-seed and rape-seed oils furnished the following results:—On standing one hour, without heating, the mixture containing cotton-seed oil showed a slightly dark colour, that adulterated with rape-seed oil a decidedly dark colour; upon the application of heat, the former exhibited a dark-red colour, while the latter turned quite black.
Maumené’s test.—This test is founded upon the fact that the elevation of temperature caused by mixing olive oil with strong sulphuric acid is considerably less than that produced with the oils commonly employed as its adulterants. With these latter an evolution of sulphurous acid generally takes place, which is not the case with pure olive oil. The best method of procedure is as follows:—10 c.c. of sulphuric acid (sp. gr. 1·844) are gradually added to 50 grammes of the sample, the mixture being constantly stirred with a small thermometer, and observations made of the maximum increase of temperature produced, as well as of the evolution of gas. When treated in this manner, genuine olive oil causes an elevation of about 42°; that given by various other oils, often added to it, ranges from 52° to 103°, and it is frequently possible to recognise their presence in admixtures by the high temperature produced. The following are the increases of temperature observed by L. Archbutt:—olive, 41-45; rape, 55-64; arachis, 47-60; sesamé, 65; cotton-seed (crude) 70; (refined), 75-76; poppy-seed, 86-88; menhaden, 123-128. In the Paris Municipal Laboratory an acid of 1·834 sp. gr. is used, and the following heating powers are regarded as standards:—For olive oil, 55·5°; for cotton-seed, 69·5° for nut, 62°; for sesamé, 66°; for poppy oil, 73°.
The application of Hubl’s test for butter (see p. [75]) is one of the most useful means for the detection of foreign oils in olive oil. The iodine absorption number of the pure oil is considerably below that of its most common adulterants.
The prevalence of the adulteration of olive oil has been abundantly demonstrated. Of 232 samples examined by the New York and Massachusetts State Boards of Health, 165 (71 per cent.) were spurious. It is a notorious fact that large quantities of cotton-seed oil are exported from the United States to France and Italy, much of which returns home under the guise of the genuine product of the olive.