360. Separation of Arachidic Acid.—Peanut oil is easily distinguished from other vegetable glycerids by the presence of arachidic acid.
The method used in this laboratory for separating arachidic acid is a modification of the usual methods based on the process as carried out by Milliau.[326] About twenty grams of the oil are saponified with alcoholic soda, using twenty cubic centimeters of 36° baumé soda solution diluted with 100 cubic centimeters of ninety per cent alcohol. When the saponification is complete, the soda is converted into the lead soap by treatment with a slight excess of a saturated alcoholic solution of lead acetate. Good results are also obtained by using dilute alcohol, viz., fifty per cent, instead of ninety per cent, in preparing the lead acetate solution.
While still warm the supernatant liquid is decanted, the precipitate washed by decantation with warm ninety per cent alcohol and triturated with ether in a mortar four times, decanting the ethereal solution in each instance. By this treatment all of the lead oleate and hypogaeate are removed and are found in the ethereal solution, from which they can be recovered and the acids set free by hydrochloric acid and determined in the usual way.
The residue is transferred to a large dish containing two or three liters of pure water and decomposed by the addition of about fifty cubic centimeters of strong hydrochloric acid. The lead chlorid formed is soluble in the large quantity of water present, which should be warm enough to keep the free acids in a liquid state in which form they float as a clear oily liquid on the surface. The free acids are decanted and washed with warm water to remove the last traces of lead chlorid and hydrochloric acid. The last traces of water are removed by drying in a thin layer in vacuo. Practically all of the acids, originally present in the sample except oleic and hypogaeic, are thus obtained in a free state and their weight is determined.
The arachidic acid may be separated almost quantitively by dissolving the mixed acids in forty cubic centimeters of ninety per cent alcohol, adding a drop of hydrochloric acid, cooling to 16° and allowing to stand until the arachidic acid has crystallized. The crystals are purified by washing twice with twenty cubic centimeters of ninety per cent and three times with the same quantity of seventy per cent alcohol. The residual impure arachidic acid is dissolved in boiling absolute alcohol, poured through a filter and washed with pure hot alcohol. The filtrate is evaporated to dryness and heated to 100° until a constant weight is obtained. From the above data, the percentages of oleic, hypogaeic, arachidic and other acids in the sample examined are calculated.
In the above process, owing to the pasty state of the lead soaps, the trituration in a mortar with ether is found troublesome. The extraction of the lead oleate and hypogaeate is facilitated by throwing the pasty ethereal mass on a filter and washing it thoroughly with successive portions of about fifty cubic centimeters of ether. By this variation, it was found by Krug in this laboratory, that less ether was required and a more complete removal of the lead oleate effected. The solution of the lead oleate is completed by about half a dozen washings with ether as above described. The extraction may also be secured by placing the lead soaps in a large extracting apparatus and proceeding as directed in paragraph [40]. The residue is washed from the filter paper into a large porcelain dish and decomposed as already described with hydrochloric acid. After the separation is complete, the mixture is cooled until the acids are solid. The solid acids are then transferred to a smaller dish, freed of water and dissolved in ether. The ethereal solution is washed with water to remove any traces of lead salt or of hydrochloric acid. After the removal of the ether, the arachidic acid is separated as has already been described.
The melting point of pure arachidic acid varies from 73° to 75°.
361. Detection of Arachis (Peanut) Oil.—Kreis has modified the usual process of Renard for the detection of arachis oil, by precipitating the solution of the fat acid with an alcoholic instead of an aqueous solution of lead acetate, in a manner quite similar to that described above.[327] The fat acids are obtained in the usual manner, washed with hot water and the acids from twenty grams of the oil dissolved in 100 cubic centimeters of ninety per cent alcohol. The solution is cooled in ice-water and the fat acids precipitated by the addition of fifteen grams of lead acetate dissolved in 150 cubic centimeters of ninety per cent alcohol. The precipitate, after standing for two hours, is separated by filtration through cotton wool and is extracted for six hours with ether. The residue is boiled with 250 cubic centimeters of five per cent hydrochloric acid until the fat acids appear as a clear oily layer upon the surface. The acids thus obtained are washed with hot water to remove lead chlorid, dried by pressing between blotting paper, dissolved in 100 cubic centimeters of ninety per cent alcohol, cooled to 15° and allowed to stand for several hours, after which time any arachidic acid present is separated by crystallization and identified in the usual manner.
When it is not important to obtain all of the acid present, the process may be simplified in the following manner:
The fat acids obtained from twenty grams of oil are dissolved in 300 cubic centimeters of ether and treated at the temperature of ice-water with a quantity of the alcoholic lead acetate solution mentioned above. Lead oleate remains in solution and the precipitate which forms after a few hours consists almost wholly of the lead salts of the solid fat acids. The precipitate is collected, washed with ether and identified in the usual manner.