Dissolving the Soap.—After the removal of the alcohol the soap should be dissolved by adding 100 cubic centimeters of recently boiled distilled water, or eighty cubic centimeters when aqueous potassium hydroxid has been used for saponification, and warming on the steam-bath, with occasional shaking, until the solution of the soap is complete.
Setting free the Fat Acids.—When the soap solution has cooled to about 60° or 70°, the fat acids are separated by adding forty cubic centimeters of dilute sulfuric acid solution containing twenty-five grams of acid in one liter, or sixty cubic centimeters when aqueous potassium hydroxid has been used for saponification.
Melting the Fat Acid Emulsion.—The flask is restoppered as in the first instance and the fat acid emulsion melted by replacing the flask on the steam-bath. According to the nature of the fat examined, the time required for the fusion of the fatty acid emulsions may vary from a few minutes to several hours.
The Distillation.—After the fat acids are completely melted, which can be determined by their forming a transparent, oily layer on the surface of the water, the flask is cooled to room temperature, and a few pieces of pumice stone added. The pumice stone is prepared by throwing it, at a white heat, into distilled water, and keeping it under water until used. The flask is connected with a glass condenser, [Fig. 102], slowly heated with a naked flame until ebullition begins, and then the distillation continued by regulating the flame in such a way as to collect 110 cubic centimeters of the distillate in, as nearly as possible, thirty minutes. The distillate should be received in a flask accurately marked at 110 cubic centimeters.
Fig. 102.—Apparatus for the Distillation of Volatile Acids.
Titration of the Volatile Acids.—The 110 cubic centimeters of distillate, after thorough mixing, are filtered through perfectly dry filter paper, 100 cubic centimeters of the filtered distillate poured into a beaker holding about a quarter of a liter, half a cubic centimeter of phenolphthalien solution added and decinormal barium hydroxid solution run in until a red color is produced. The contents of the beaker are then returned to the measuring flask to remove any acid remaining therein, poured again into the beaker, and the titration continued until the red color produced remains apparently unchanged for two or three minutes, The number of cubic centimeters of decinormal barium hydroxid solution required should be increased by one-tenth to represent the entire distillate.
The number thus obtained expresses, in cubic centimeters of decinormal alkali solution, the volatile acidity of the sample. In each case blank distillations of the reagents used should be conducted under identical conditions, especially when alcoholic alkali is used for saponification. It is difficult to secure alcohol which will not yield a trace of volatile acid in the conditions named. The quantity of decinormal alkali required to neutralize the blank distillate is to be deducted from that obtained with the sample of fat.
351. Determination of Soluble and Insoluble Fat Acids.—The volatile fat acids are more or less soluble in water, while those which are not distillable in a current of steam are quite insoluble. It is advisable, therefore, to separate these two classes of fat acids, and the results thus obtained are perhaps more decidedly quantitive than are given by the distillation process just described. The methods used for determining the percentage of insoluble acids are essentially those of Hehner.[320] Many variations of the process have been proposed, especially in respect of the soluble acids.[321]
The process, as conducted in this laboratory and approved by the Association of Official Agricultural Chemists, is as follows: