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.[315] 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[316] or five grams.[317]
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.[318]
The process as used in this laboratory and as adopted by the official agricultural chemists is conducted as follows:[319]
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.