The details of saponification with sulfuric acid are of no interest from an analytical point of view.[307]
341. Saponification in an Open Dish.—The simplest method of saponifying fats is to treat them with the alkaline reagent in an open dish. In all cases the process is accelerated by the application of heat. Vigorous stirring also aids the process by securing a more intimate mixture of the ingredients. This method of decomposing glycerids, however, is not applicable in cases where volatile ethers may be developed. These ethers may escape saponification and thus prevent the formation of the maximum quantity of soap. While not suited to exact quantitive work, the method is convenient in the preparation of fat acids which are to be the basis of subsequent analytical operations, as, for instance, in the preparation of fat acids for testing with silver nitrate. Large porcelain dishes are conveniently used and the heat is applied in any usual way, with care to avoid scorching the fat.
342. Saponification under Pressure.—The method of saponification which has given the best satisfaction in my work and which has been adopted by the Association of Official Agricultural Chemists is described below.[308]
Reagents.—The reagents employed are a solution of pure potash containing 100 grams of the hydroxid dissolved in fifty-eight grams of recently boiled distilled water, alcohol of approximately ninety-five per cent strength redistilled over caustic soda, and sodium hydroxid solution prepared as follows:
One hundred grams of sodium hydroxid are dissolved in 100 cubic centimeters of distilled water. The caustic soda should be as free as possible from carbonates, and be preserved from contact with the air.
Apparatus.—A saponification flask; it has a round bottom and a ring near the top, by means of which the stopper can be tied down. The flask is arranged for heating as shown in [Fig. 101]. A pipette graduated to deliver forty cubic centimeters is recommended as being more convenient than a burette for measuring the solutions: A pipette with a long stem graduated to deliver 5.75 cubic centimeters at 50°.
Manipulation.—The fat to be examined should be melted and kept in a dry warm place at about 60° for two or three hours, until the water has entirely separated. The clear supernatant fat is poured off and filtered through a dry filter paper in a jacket funnel containing boiling water. Should the filtered fat, in a fused state, not be perfectly clear, it must be filtered a second time. The final drying is accomplished at 100° in a thin layer in a flat bottom dish, in partial vacuum or an atmosphere of inert gas.
The saponification flasks are prepared by thoroughly washing with water, alcohol, and ether, wiping perfectly dry on the outside, and heating for one hour at the temperature of boiling water. The hard flasks used in moist combustions with sulfuric acid for the determination of nitrogen are well suited for this work. The flasks should be placed in a tray by the side of the balance and covered with a silk handkerchief until they are perfectly cool. They must not be wiped with a silk handkerchief within fifteen or twenty minutes of the time they are weighed or else the electricity developed will interfere with weighing. The weight of the flasks having been accurately determined, they are charged with the melted fat in the following way:
Fig. 101.—Apparatus for Saponifying under Pressure.