Part 68

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.

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. Many variations of the process have been proposed, especially in respect of the soluble acids.

The process, as conducted in this laboratory and approved by the Association of Official Agricultural Chemists, is as follows:

Preparation of Reagents.—Sodium Hydroxid Solution.—A decinormal solution of sodium hydroxid is used. Each cubic centimeter contains 0.0040 gram of sodium hydroxid and neutralizes 0.0088 gram of butyric acid (C₄H₈O₂).

Alcoholic Potash Solution.—Dissolve forty grams of good caustic potash in one liter of ninety-five per cent alcohol redistilled over caustic potash or soda. The solution must be clear and the potassium hydroxid free from carbonates.

Standard Acid Solution.—Prepare accurately a half normal solution of hydrochloric acid.

Indicator.—Dissolve one gram of phenolphthalien in 100 cubic centimeters of ninety-five per cent alcohol.

Determination.—Soluble Acids.—About five grams of the sample are placed in the saponification flask already described, fifty cubic centimeters of the alcoholic potash solution added, the flask stoppered and placed in the steam-bath until the fat is entirely saponified. The operation may be facilitated by occasional agitation. The alcoholic potash is always measured with the same pipette and uniformity further secured by allowing it to drain the same length of time (thirty seconds). Two or three blank experiments are conducted at the same time.

In from five to thirty minutes, according to the nature of the fat, the liquid will appear perfectly homogeneous and, when this is the case, the saponification is complete and the flask is removed and cooled. When sufficiently cool, the stopper is removed and the contents of the flask rinsed with a little ninety-five per cent alcohol into an erlenmeyer, of about 200 cubic centimeters capacity, which is placed on the steam-bath together with the blanks until the alcohol is evaporated.

The blanks are titrated with half normal hydrochloric acid, using phenolphthalien as indicator, and one cubic centimeter more of the half normal hydrochloric acid than is required to neutralize the potash in the blanks is run into each of the flasks containing the fat acids. The flask is connected with a reflux condenser and placed on the steam-bath until the separated fat acids form a clear stratum on the upper surface of the liquid. The flask and contents are cooled in ice-water.

The fat acids having quite solidified, the liquid contents of the flask are poured through a dry filter into a liter flask, taking care not to break the cake. Between 200 and 300 cubic centimeters of water are brought into the flask, the cork with the condenser reinserted and the flask placed on the steam-bath until the cake of acid is thoroughly melted. During the melting of the cake of fat acids, the flask should occasionally be agitated with a rotary motion in such a way that its contents are not made to touch the cork. When the fat acids have again separated into an oily layer, the flask and its contents are cooled in ice-water and the liquid filtered through the same filter into the same liter flask as before. This treatment with hot water, followed by cooling and filtration of the wash water, is repeated three times, the washings being added to the first filtrate. The mixed washings and filtrate are made up to one liter, and 100 cubic centimeters thereof in duplicate are titrated with decinormal sodium hydroxid. The number of cubic centimeters of sodium hydroxid required for each 100 cubic centimeters of the filtrate is multiplied by ten. The number so obtained represents the volume of decinormal sodium hydroxid neutralized by the soluble fat acids of the fat, plus that corresponding to the excess of the standard acid used, viz., one cubic centimeter. The number is therefore to be diminished by five, corresponding to the excess of one cubic centimeter of half normal acid. This corrected volume multiplied by 0.0088 gives the weight of soluble acids as butyric acid in the amount of fat saponified.

Insoluble Acids.—The flask containing the cake of insoluble fat acids from the above determination and the paper through which the soluble fat acids have been filtered are allowed to drain and dry for twelve hours, when the cake, together with as much of the fat acids as can be removed from the filter paper, is transferred to a weighed evaporating dish. The funnel, with the filter, is then placed in an erlenmeyer and the paper thoroughly washed with absolute alcohol. The flask is rinsed with the washings from the filter paper, then with pure alcohol, and the rinsings transferred to the evaporating dish. The dish is placed on the steam-bath until the alcohol is evaporated, dried for two hours at 100°, cooled in a desiccator and weighed. It is again placed in the air-bath for two hours, cooled as before and weighed. If there be any considerable decrease in weight, reheat two hours and weigh again. The final weighing gives the weight of insoluble fat acids in the sample, from which the percentage is easily calculated.

The quantity of non-volatile and insoluble acids in common glycerids is from ninety-five to ninety-seven parts in 100. The glycerids yield almost the same proportion of fat acids and glycerol when the acids are insoluble and have high molecular weights. When the acids are soluble and the molecular weight low the proportion of acids decreases and that of glycerol increases.

In the following table will be found the data secured by quantitive saponification and separation of soluble and insoluble acids found in the more common glycerids:

Yield per 100 parts Molecular weight of of glycerid. Glycerid. Fat acid. Glycerid. Fat acid. Fat acid. Glycerol. Stearin Stearic 890 284 95.73 10.34 Olein Oleic 884 282 95.70 10.41 Palmitin Palmitic 806 256 95.28 11.42 Myristin Myristic 722 228 94.47 12.74 Laurin Lauric 638 200 94.95 14.42 Caprin Capric 594 172 93.14 15.48 Caproin Caproic 386 116 90.16 23.83 Butyrin Butyric 302 88 87.41 30.46

The general expression for the saponification of a neutral fat is C₃H₅O₃.R₃ + 3H₂O = 3R.OH + C₃H₈O₃, in which R represents the acid radicle. It is evident from this that the yield of more than 100 parts of fat acids and glycerol given by glycerids is due to the absorption of water during the reaction.

=352. Formulas for General Calculations.=—For calculating the theoretical yields of fat acids and glycerol, the following general formulas may be used:

Let M = the molecular weight of the fat acid: K = saponification value: F = the quantity of free fat acids in the glycerid: N = the quantity of neutral fat in the glycerid: A = the number of milligrams of potassium hydroxid required to saturate the free acid in one gram of the sample.

The free acid is determined by the method given below.

M grams of a fat acid require 56100 milligrams of potassium hydroxid for complete neutralization while F grams corresponding to 100 grams of fat are saturated by 100 × A milligrams of the alkali.

Then M : 56100 = F : 100A.

AM Whence F = ------ (1). 561

Likewise since M grams of fat acid require the quantity of potassium hydroxid mentioned above we have: