362. Cottonseed Oil, Bechi’s Test.—Crude, fresh cottonseed oil, when not too highly colored, and generally the refined article, may be distinguished from other oils by the property of reducing silver salts in certain conditions. The reaction was first noticed by Bechi and has been the subject of extensive discussions.[328]
The process as proposed by Bechi has been modified in many ways but apparently without improving it. It is conducted as follows: One gram of silver nitrate is dissolved in 200 cubic centimeters of ninety-eight per cent alcohol and forty cubic centimeters of ether and one drop of nitric acid added to the mixture. Ten cubic centimeters of the oil are shaken in a test tube with one cubic centimeter of this reagent, and then with ten cubic centimeters of a mixture containing 100 cubic centimeters of amyl alcohol and ten of colza oil. The mixture is divided into two portions, one of which is put aside for future comparison and the other plunged into boiling water for fifteen minutes. A deep brown or black color, due to the reduction of silver, reveals the presence of cottonseed oil.
In this laboratory the heating is accomplished in a small porcelain dish on which is often deposited a brilliant mirror of metallic silver. The white color of the porcelain also serves as a background for the observation of the coloration produced. In most instances a green color has been noticed after the reduction of the silver is practically complete. Unless cottonseed oil has been boiled or refined in some unusual way, the test, as applied above, is rarely negative. The reduction of the silver is doubtless due to some aldehydic principle, present in extremely minute quantities, and which may be removed by some methods of technical treatment. The silver nitrate test therefore is reliable when the reduction takes place, but the absence of a distinct reaction may not in all cases prove the absence of cottonseed oil.
363. Milliau’s Process.—Milliau has proposed the application of the silver salt directly to the free fat acids of the oil instead of to the oil itself.[329] About fifteen cubic centimeters of the oil are saponified with alcoholic potash in the usual manner, 150 cubic centimeters of water added to the dish and the mixture boiled until the alcohol is evaporated. The fat acids are freed by the addition of decinormal sulfuric acid and as they rise to the surface in a pasty condition are removed with a spoon. The free acids are washed with distilled water. The water is drained off and the free acids dissolved in fifteen cubic centimeters of ninety-two per cent alcohol and two cubic centimeters of a three per cent solution of silver nitrate. The test tube containing the mixture is well shaken and placed in a water-bath, out of contact with light, and left until about one-third of the alcohol is evaporated. Ten cubic centimeters of water are added, the heating continued for a few minutes and the color of the supernatant fat acids observed. The presence of cottonseed oil is revealed by the production of a lustrous precipitate which colors the fat acids black. In some cases the process of Milliau gives better results than the original method of Bechi, but this is not always the case. It does away with the use of amyl alcohol and colza oil, but its manipulation is more difficult. In all doubtful cases the analyst should apply both methods.
364. Detection of Sesame Oil.—Milliau has pointed out a characteristic reaction of this oil which may be used with advantage in cases of doubtful identity.[330] The identification is based on the fact that the free acids of sesame oil, or some concomitant thereof, give a rose-red color when brought in contact with a solution of sugar in hydrochloric acid.
The analytical process is conducted as follows: About fifteen grams of the oil are saponified with alcoholic soda and when the reaction is complete treated with 200 cubic centimeters of hot water and boiled until the alcohol is removed. The fat acids are set free with decinormal sulfuric acid and removed with a spoon as they rise to the surface in a pasty state, in which condition they are washed by shaking with water in a large test tube. When washed, the acids are placed in an oven at 105° until the greater part of the water is evaporated and the acids begin to become fluid. At this point they are treated with half their volume of hydrochloric acid saturated with finely ground sugar. On shaking the mixture, a rose color is developed which is characteristic of the sesame oil. Other oils give either no coloration or at most a yellow tint.
365. The Sulfur Chlorid Reaction.—Some vegetable oils, when treated with sulfur chlorid, give a hard product similar to elaidin, while lard does not. This reaction is therefore helpful in discriminating between some vegetable and animal glycerids. The process which is described by Warren has been used with some satisfaction in this laboratory.[331]
Five grams of the oil or fat are placed in a tared porcelain dish and treated with two cubic centimeters of carbon bisulfid and the same quantity of sulfur chlorid. The dish is placed on a steam-bath and its contents stirred until the reaction is well under way. The heating is continued until all volatile products are evaporated, the hard mass being well rubbed up to facilitate the escape of imprisoned vapors. The powdered or pasty mass is transferred to a filter and washed with carbon bisulfid to remove all unaltered oil. The washing with carbon bisulfid is hastened by pressure and about 200 cubic centimeters of the solvent should be used. After drying, the weight of insoluble matter is obtained and deducted from the total weight of the sample used.
The color and tenacity of the hard, insoluble portion are characteristic. The quantitive part of the operation appears to have but little value, but applied qualitively in this laboratory it produces hard, leathery masses with cotton, olive and peanut oils, and but little change in lard and beef fats. Qualitively applied, the process is conducted as described above but without making the weighings. In this instance it is as easy of application as the process of Bechi and is deserving of greater attention than has been given it by analysts.
In the combination which takes place between the sulfur and the fat it is probable that only addition products are formed, since the quantity of alkali required for saponification is not diminished by previously treating the fat with sulfur chlorid.[332] The reactions which take place are probably well represented by the following equations, in which oleic acid is treated with sulfur chlorid: