| and G = | 92 | (7). |
| 3M + 38 |
N per cent of neutral fat yields, therefore, on saponification, the following theoretical quantities of fat acids F, and glycerol G expressed as parts per hundred.
| F = N × | 3M | (8); |
| 3M + 38 |
| and G = N × | 92 | (9). |
| 3M + 38 |
Formula (9) expresses also the total yield of glycerol from any given sample. For a further discussion of this part of the subject a work of a more technical character may be consulted.[323]
353. Determination of a Free Fat Acid in a Fat.—The principle of the method rests upon the comparative accuracy with which a free fat acid can be titrated with a set alkali solution when phenolphthalien is used as an indicator. Among the many methods of manipulation which the analyst has at his command there is probably none more simple and accurate than that depending on the solution of the sample in alcohol, ether, chloroform, or carbon tetrachlorid. Any acidity of the solvent is determined by separate titration and the proper correction made. Either an aqueous or alcoholic solution of the alkali may be used, preferably the latter. The alkaline solution may be approximately or exactly decinormal, but it is easier to make it approximately so and to determine its real value before each operation by titration against a standard decinormal solution of acid. About ten grams of the sample and fifty cubic centimeters of the solvent will be found convenient quantities.
Example.—Ten grams of rancid olive oil dissolved in alcohol ether require three and eight-tenths cubic centimeters of a solution of alcoholic potash to saturate the free acid present. When titrated with decinormal acid the potash solution is found to contain 25.7 milligrams of potassium hydroxid in each cubic centimeter. The specific gravity of the oil is 0.917 and the weight used therefore 9.17 grams. Then the total quantity of potassium hydroxid required for the neutralization of the acid is 25.7 × 3.8 = 97.7 milligrams.
The acid value A is therefore:
| A = | 3.8 × 25.7 | = 10.6 |
| 9.17 |
It is customary to regard free acid as oleic, molecular weight 282. On this assumption the percentage of free acids in the above case is found by the formula