Part 67

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. 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:

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.

=346. Saponification Equivalent.=—Allen defines the saponification equivalent as the number of grams of fat saponified by one equivalent, viz., 56.1 grams of potassium hydroxid. The saponification equivalent is readily calculated from the saponification value using it as a divisor and 56100 as a dividend. Conversely the saponification value may be obtained by dividing 56100 by the saponification equivalent. No advantage is gained by the introduction of a new term so nearly related to saponification value.

=347. Saponification Value of Pure Glycerids.=—The theoretical saponification values of pure glycerids are given in the following table.

Molecular Saponification Name. Symbol. weight. value. Butyrin C₃H₅(O.C₄H₇O)₃ 302 557.3 Valerin C₃H₅(O.C₅H₉O)₃ 344 489.2 Caproin C₃H₅(O.C₆H₁₁O)₃ 386 438.3 Caprin C₃H₅(O.C₁₀H₁₉O)₃ 554 305.0 Laurin C₃H₅(O.C₁₂H₂₃O)₃ 638 263.8 Myristin C₃H₅(O.C₁₄H₂₇O)₃ 722 233.1 Palmitin C₃H₃(O.C₁₆H₃₁O)₃ 806 208.8 Stearin C₃H₅(O.C₁₈H₃₅O)₃ 890 189.1 Olein C₃H₅(O.C₁₈H₃₃O)₃ 884 190.4 Linolein C₃H₅(O.C₁₈H₃₁O)₃ 878 191.7 Ricinolein C₃H₅(O.C₁₈H₃₃O₂)₃ 932 180.6 Euricin C₃H₅(O.C₂₂H₁₄O)₃ 1052 160.0

From the above table it is seen that in each series of glycerids the saponification equivalent falls as the molecular weight rises.

=348. Acetyl Value.=—Hydroxy acids and alcohols, when heated with glacial acetic acid, undergo a change which consists in substituting the radicle of acetic acid for the hydrogen atom of the alcoholic hydroxyl group. This change is illustrated by the equations below:

For a Fat Acid:

Ricinoleic acid. Acetic anhydrid. C₁₇H₃₂(OH).COOH + (C₂H₃O)₂O =

Acetyl-ricinoleic acid. Acetic acid. C₁₇H₃₂(O.C₂H₃O)COOH + HC₂H₃O₂.

For an Alcohol:

Cetyl alcohol. Acetic anhydrid. Cetyl acetate. Acetic acid. C₁₆H₃₃.OH + (C₂H₃O)₂O = C₁₆H₃₃.C₂H₃O + HC₂H₃O₂.

Determination.—The method of determining the acetyl value of a fat or alcohol has been described by Benedikt and Ulzer. The operation is conducted on the fat acids and not on the glycerids containing them.

The insoluble fat acids are prepared as directed in paragraph =340=.

From twenty to fifty grams of the fat acids are boiled with an equal volume of acetic anhydrid, in a flask with a reflux condenser, for two hours. The contents of the flask are transferred to a larger vessel of about one liter capacity, mixed with half a liter of water and boiled for half an hour. To prevent bumping, some bubbles of carbon dioxid are drawn through the liquid by means of a tube drawn out to a fine point and extending nearly to the bottom of the flask. The liquids are allowed to separate into two layers and the water is removed with a syphon. The oily matters are treated several times with boiling water until the acetic acid is all washed out. The acetylated fat acids are filtered through a dry hot jacket filter and an aliquot part, from three to five grams, is dissolved in absolute alcohol. After the addition of phenolphthalien the mixture is titrated as in the determination of the saponification value. The acid value thus obtained is designated as the acetyl acid value. A measured quantity of alcoholic potash, standardized by seminormal hydrochloric acid, is added, the mixture boiled and the excess of alkali determined by titration. The quantity of alkali consumed in this process measures the acetyl value. The sum of the acetyl acid and the acetyl values is the acetyl saponification value. The acetyl value is therefore equal to the difference of the saponification and acid values of the acetylated fat acids. In other words, the acetyl value indicates the number of milligrams of potassium hydroxid required to neutralize the acetic acid obtained by the saponification of one gram of the acetylated fat acids.

Example.—A portion of the fat acids acetylated as described, weighing 3.379 grams, is exactly neutralized by 17.2 cubic centimeters of seminormal potassium hydroxid solution, corresponding to 17.2 × 0.02805 = 0.4825 gram of the hydroxid, hence 0.4825 × 1000 ÷ 3.379 = 142.8, the acetyl acid value of the sample.

After the addition of 32.8 cubic centimeters more of the seminormal potash solution, the mixture is boiled to saponify the acetylated fat acids. The residual potash requires 14.2 cubic centimeters of seminormal hydrochloric acid. The quantity of potash required for the acetic acid is therefore 32.8 - 14.3 = 18.5 cubic centimeters or 18.5 × 0.02805 = 0.5189 gram of potassium hydroxid. Then 0.5189 × 1000 ÷ 3.379 = 153.6 = acetyl value of sample. The sum of these two values, viz., 142.8 and 153.6 is 296.4, which is the acetyl saponification value of the sample. As with the iodin numbers, however, it is also found that acids of the oleic series give an acetyl value when treated as above, and it has been proposed by Lewkowitsch to determine, in lieu of the data obtained, the actual quantity of acetic acid absorbed by fats. This is accomplished by saponifying the acetylated product with alcoholic potash and determining the free acetic acid by distillation, in a manner entirely analogous to that used for estimating volatile fat acids described further on.

The rôle which the acetyl value plays in analytical determinations is interesting, but the data it gives are not to be valued too highly.

=349. Determination of Volatile Fat Acids.=—The fat acids which are volatile at the temperature of boiling water, consist chiefly of butyric and its associated acids occurring in the secretions of the mammary glands. Among vegetable glycerids cocoanut oil is the only common one which has any notable content of volatile acids. The boiling points of the above acids, in a pure state, are much higher than the temperature of boiling water; for instance, butyric acid boils at about 162°. By the expression volatile acids, in analytical practice, is meant those which are carried over at 100°, or a little above, with the water vapor, whatever be their boiling point. The great difficulty of removing the volatile from the non-volatile fat acids has prevented the formulation of any method whereby a sharp and complete separation can be accomplished. The analyst, at the present time, must be content with some approximate process which, under like conditions, will give comparable results. Instead, therefore, of attempting a definite determination, he confines his work to securing a partial separation and in expressing the degree of volatile acidity in terms of a standard alkali. To this end, a definite weight of the fat is saponified, the resulting soap decomposed with an excess of fixed acid, and a definite volume of distillate collected and its acidity determined by titration with decinormal alkali. The weight of fat operated on is either two and a half or five grams.

Numerous minor variations have been proposed in the process, the most important of which is in the use of phosphoric instead of sulfuric acid in the distillation. An extended experience with both acids has shown that no danger is to be apprehended in the use of sulfuric acid and that on the whole it is to be preferred to phosphoric.

The process as used in this laboratory and as adopted by the official agricultural chemists is conducted as follows:

=350. Removal of the Alcohol.=—The saponification is accomplished in the manner already described, (=341-344=) and when alcoholic potash is used proceed as follows:

The stopper having been laid loosely in the mouth of the flask, the alcohol is removed by dipping the flask into a steam-bath. The steam should cover the whole of the flask except the neck. After the alcohol is nearly removed, frothing may be noticed in the soap, and to avoid any loss from this cause or any creeping of the soap up the sides of the flask, it should be removed from the bath and shaken to and fro until the frothing disappears. The last traces of alcohol vapor may be removed from the flask by waving it briskly, mouth down, to and fro.