The pipette with a long stem, marked to deliver 5.75 cubic centimeters, is warmed to a temperature of about 50°. The fat, having been poured back and forth once or twice into a dry beaker in order to thoroughly mix it, is taken up in the pipette, the nozzle of the pipette having been previously wiped to remove any externally adhering fat, is carried to near the bottom of the flask and 5.75 cubic centimeters of fat allowed to flow into the flask. After the flasks have been charged in this way they should be re-covered with the silk handkerchief and allowed to stand for fifteen or twenty minutes, when they are again weighed.
343. Methods of Saponification.—In the Presence of Alcohol.—Ten cubic centimeters of ninety-five per cent alcohol are added to the fat in the flask, and then two cubic centimeters of the sodium hydroxid solution. A soft cork stopper is inserted and tied down with a piece of twine. The saponification is completed by placing the flask upon the water or steam-bath. The flask during the saponification, which should last one hour, should be gently rotated from time to time, being careful not to project the soap for any distance up its sides. At the end of an hour the flask, after having been cooled to near the room temperature, is opened.
Without the Use of Alcohol.—To avoid the danger of loss from the formation of ethers, and the trouble of removing the alcohol after saponification, the fat may be saponified with a solution of caustic potash in a closed flask without using alcohol. The operation is carried on exactly as indicated above for saponification in the presence of alcohol, using potassium instead of sodium hydroxid solution. For the saponification, use two cubic centimeters of the potassium hydroxid solution which are poured on the fat after it has solidified in the flask. Great care must be taken that none of the fat be allowed to rise on the sides of the saponifying flask to a point where it cannot be reached by the alkali. During the process of saponification the flask can only be very gently rotated in order to avoid the difficulty mentioned. This process is not recommended with any apparatus except a closed flask with round bottom. Potash is used instead of soda so as to form a softer soap and thus allow a more perfect saponification.
The saponification may also be conducted as follows: The alkali and fat in the melted state are shaken vigorously in the saponification flask until a complete emulsion is secured. The rest of the operation is then conducted as above.
344. Saponification in the Cold.—By reason of the danger of loss from volatile ethers in the hot alcoholic saponification, a method for successfully conducting the operation in the cold is desirable. Such a process has been worked out by Henriques.[309] It is based upon the previous solution of the fat in petroleum ether, in which condition it is so easily attacked by the alcoholic alkali as to make the use of heat during the saponification unnecessary. The process is conveniently conducted in a porcelain dish covered with a watch glass. Five grams of the fat are dissolved in twenty-five cubic centimeters of petroleum ether and treated with an equal quantity of four per cent alcoholic soda lye. The process of saponification begins at once and is often indicated by the separation of sodium salts. It is best to allow the action to continue over night and, with certain difficultly saponifiable bodies, such as wool fat and waxes, for twenty-four hours. In the case of butter fat an odor of butyric ether may be perceived at first but it soon disappears. After the saponification is complete, the excess of alkali is determined by titration in the usual way with set hydrochloric acid, using phenolphthalien as indicator. For the determination of volatile acids, the mixture, after saponification is complete, is evaporated rapidly to dryness, the solid matter being reduced to powder with a glass rod, after which it is transferred to a distilling flask and the volatile acids secured by the usual processes. In comparison with the saponification and reichert-meissl numbers obtained with hot alcoholic potash, the numbers given by the cold process are found to be slightly higher with those fats which give easily volatile ethers. On account of the simplicity of the process and the absence of danger of loss from ethers, it is to be recommended instead of the older methods in case a more extended trial of it should establish the points of excellence claimed above.
345. Saponification Value.—The number of milligrams of potassium hydroxid required to completely saturate one gram of a fat is known as the saponification value of the glycerid. The process of determining this value, as worked out by Koettstorfer and modified in the laboratory of the Department of Agriculture, is as follows:[310]
The saponification is accomplished with the aid of potassium hydroxid and in the flask and manner described in the preceding paragraph. About two grams of the fat will be found a convenient quantity. Great care must be exercised in measuring the alkaline solution, the same pipette being used in each case and the same time for draining being allowed in every instance. Blanks are always to be conducted with each series of examinations. As soon as the saponification is complete, the flask is removed from the bath, allowed to cool and its contents are titrated with seminormal hydrochloric acid and phenolphthalien as indicator. The number expressing the saponification value is obtained by subtracting the number of cubic centimeters of seminormal hydrochloric acid required to neutralize the alkali after saponification from that required to neutralize the alkali of the blank determinations, multiplying the result by 28.06 and dividing the product by the number of grams of fat employed.
Example.—Weight of sample of fat used 1.532 grams: Number of cubic centimeters half normal hydrochloric acid required to saturate blank, 22.5: Number of cubic centimeters of half normal hydrochloric acid required to saturate the alkali after saponification 12.0: Difference, 10.5 cubic centimeters:
Then 10.50 × 28.06 ÷ 1.532 = 192.3.
This latter number represents the saponification value of the sample.