This reaction shows itself with nearly all vegetable oils but those which have been chemically treated either for the purpose of bleaching, or for the removal of the acidity, do not respond to the test at all, or else in a feeble manner, and that only after standing some time. Lard, goose fat, tallow, deer fat, butter fat, etc., show no change in color on being treated with this reagent, either with or without the addition of alkali. The presence of a small quantity of vegetable oil betrays itself by the appearance of the above mentioned coloration, the intensity of which forms an approximate measure of the amount of vegetable oil present in the sample. In experiments with suspected lards, which deviated in their iodin absorption numbers from those of genuine lard, the results were concordant, the color deepening as the iodin figure rose. The mineral fats (paraffin, vaselin) are without action on this reagent, and the only animal fat which reduces it is codliver oil.
In like manner some samples of lard may be found which exhibit a deportment with this reagent similar to that shown with vegetable oils, and tallow and lard oil have been shown to give more distinct reactions than some of the vegetable oils.[272]
The phosphomolybdic acid may be prepared by precipitating a solution of ammonium molybdate with sodium phosphate and dissolving the washed precipitate in a warm solution of sodium carbonate. The solution is evaporated to dryness and the dry residue subjected to heat. If a blue coloration be produced it may be discharged by adding a little nitric acid and reheating. The residue is dissolved in water, acidified with nitric and made of such a strength as to contain about ten per cent of the substance.
319. Coloration with Picric Acid.—If to ten cubic centimeters of oil a cold saturated solution of picric acid in ether be added and the latter be allowed to evaporate slowly, the acid remains dissolved in the oil, to which it communicates a brown color.
Pure lard, after the evaporation of the ether, appears of a citron-yellow color; if cottonseed oil be present, however, the mixture assumes a brown-red color.[273]
320. Coloration with Silver Nitrate.—A modification of Bechi’s method of reducing silver nitrate, given further on, has been proposed by Brullé.[274] The reagent employed consists of twenty-five parts of silver nitrate in 1,000 parts of alcohol of ninety-five per cent strength. Twelve cubic centimeters of the oil to be examined and five of the reagent are placed in a test tube, held in a vessel containing boiling water, and the ebullition continued for about twenty minutes. At the end of this time an olive oil, even if it be an impure one, will show a beautiful green tint. With seed oils the results are quite different. Cotton oil submitted to this treatment becomes completely black. Peanut oil shows at first a brown-red coloration and finally a somewhat green tint, losing its transparency. Sesamé oil is distinguished by a red-brown tint very pronounced and remaining red. Colza oil takes on a yellowish green coloration, becomes turbid and is easily distinguished in its reaction from olive oil. In mixtures of olive oil with the other oils, any notable proportion of the seed oils can be easily determined by the above reactions. Natural butter treated with this reagent retains its primitive color. That containing margarin becomes a brick-red and as little as five per cent of margarin in butter can be detected by this test. With ten per cent the tint is very pronounced.
321. Coloration with Stannic Bromid.—This reagent is prepared by adding dry bromin, drop by drop, to powdered or granulated tin held in a flask immersed in ice water, until a persistent red color indicates that the bromin is in excess. In the application of this reagent three or four drops of it are added successively to a little less than that quantity of the oil, the mixture well stirred and set aside for a few minutes. The unsaponifiable matters of castor oil give a green color when thus treated, sandal wood oil a blood-red color and cedar oil a purplish color.[275]
322. Coloration with Auric Chlorid.—The use of auric chlorid for producing colorations in oils and fats was first proposed by Hirschsohn.[276] One gram of auric chlorid is dissolved in 200 cubic centimeters of chloroform and about six drops of this reagent added to five cubic centimeters of the oil to be tested. In the case of cottonseed oil a beautiful red color is produced.
I have found that even pure lards give a trace of color sometimes with this reagent, and therefore the production of a slight red tint cannot in all cases be regarded as conclusive of the presence of cottonseed oil.[277]
In general, it may be said that the color reactions with fats and oils have a certain qualitive and sorting value, and in any doubtful case they should not be omitted. Their value can only be established by comparison under identical conditions with a large number of fats and oils of known purity. The analyst must not depend too confidingly on the data found in books, but must patiently work out these reactions for himself.