340. Saponification.—In many of the analytical operations which are conducted on the glycerids it is necessary to decompose them. When this is accomplished by the action of a base which displaces the glycerol from its combination with the fat acids, the resulting salts are known as soaps and the process is named saponification. In general use the term saponification is applied, not only strictly, as above defined, but also broadly, including the setting free of the glycerol either by the action of strong acids or by the application of superheated steam. In chemical processes the saponification of a glycerid is almost always accomplished by means of soda or potash lye. This may be in aqueous or alcoholic solution and the process is accomplished either hot or cold, in open vessels or under pressure. It is only important that the alkali and glycerid be brought into intimate contact. The rate of saponification is a function of the intimacy of contact, the nature of the solvent and the temperature. For chemical purposes, it is best that the decomposition of the glycerid be accomplished at a low temperature and for most samples this is secured by dissolving the alkali in alcohol.
In respect of solvents, that one would be most desirable, from theoretical considerations, which acts on both the glycerids and alkalies. In the next rank would be those which dissolve one or the other of the materials and are easily miscible, as, for instance, carbon tetrachlorid for the glycerid and alcohol for the alkali. As a rule, the glycerid is not brought into solution before the saponification process is commenced. Instead of using an alcoholic solution of sodium or potassium hydroxid the sodium or potassium alcoholate may be employed, made by dissolving metallic sodium or potassium in alcohol. It is probable, however, that a little water is always necessary to complete the process.
If a fat be dissolved in ether and treated with sodium alcoholate, a granular deposit of soap is soon formed and the saponification is completed in twenty-four hours. As much as 150 grams of fat can be saponified with ten grams of metallic sodium dissolved in 250 cubic centimeters of absolute alcohol.[305] For practical purposes the alcoholic solution of the hydroxid is sufficient.
The chemical changes which fats undergo on saponification are of a simple kind. When the process is accomplished by means of alkalies, the alkaline base takes the place of the glycerol as indicated in the following equation:
| Triolein 884. | Potassium hydroxid 168. | ||
| C₃H₅(O.C₁₈H₃₃O)₃ | + | 3KOH | = |
| Potassium oleate 960. | Glycerol 92. | ||
| (KO.C₁₈H₃₃O)₃ | + | C₃H₅(OH)₃. | |
The actual changes which take place in ordinary saponification are not so simple, however, since natural glycerids are mixtures of several widely differing fats, each of which has its own rate of decomposition. Palmitin and stearin, for instance, are saponified more readily than olein and some of the saponifiable constituents of resins and waxes are extremely resistant to the action of alkalies. The above equation may be regarded as typical for saponification in aqueous or alcoholic solutions in open dishes or under pressure. If the alkali used be prepared by dissolving metallic sodium or potassium in absolute alcohol (sodium alcoholate or ethoxid) the reaction which takes place is probably represented by the equation given below:
C₃H₅(O.C₁₈H₃₃O)₃ + 3C₂H₅.ONa = C₃H₅(ONa)₃ + 3C₁₈H₃₃O.O.C₂H₅,
in which it is seen that complete saponification cannot occur without the absorption of some water, by which the sodium glyceroxid is converted into glycerol and sodium hydroxid, the latter compound eventually uniting with the ethyl ether of the fat acid.[306]
Glycerids are decomposed when heated with water under a pressure of about sixteen atmospheres or when subjected to a current of superheated steam at 200°. The reaction consists in the addition of the elements of water, whereby the glyceryl radicle is converted into free glycerol and the fat acid is set free. The chemical change which ensues is shown below:
C₃H₅(O.C₁₈H₃₃O)₃ + 3H₂O = 3C₁₈H₃₄O₂ + C₃H₅(OH)₃.