CONSTITUENTS OF PLANTS 353 



and oils. Specific tests may be made to detect certain oils, such 

 as cottonseed oil. 



The specific gravity is of importance. 



The index of refraction is the measure of the extent to which 

 the fat bends a ray of light passing through it. It affords a 

 rapid method for testing the purity of some oils. 



The saponification value is estimated by saponifying a weighed 

 quantity of fat with a solution containing a known amount of 

 alkali, and estimating the unused alkali by titration with an in- 

 dicator and an acid of known strength. It is usually expressed 

 in terms of milligrams of alkali neutralized by one gram fat. 



The volatile acids are estimated by saponifying a weighed quan- 

 tity of fat, liberating the fatty acids with a non-volatile acid, and 

 distilling off the volatile acids with water. The distillate is 

 titrated with alkali of known strength. This method is especially 

 valuable for butter, since it is the only ordinary fat which contains 

 glycerides of volatile fatty acids. 



The iodine value is estimated by treating a weighed amount of 

 fat with a solution containing a known amount of iodine. After 

 sufficient time, the uncombined iodine is titrated and so estimated. 

 The iodine combines with the unsaturated fats, and not with the 

 saturated, so that the quantity of iodine absorbed depends on the 

 quantity, and the condition of unsaturation of the fats present. 

 The iodine number is the milligrams of iodine which combine with 

 one gram of oil. The iodine number is a valuable index to the 

 nature and purity of many oils. 



Free fatty acids are often present in the ether extract. If the 

 substance is old, and the fat in it has become rancid, a large part 

 of the ether extract may consist of free fatty acids. They come 

 from the hydrolysis or decomposition of the fats into fatty acids 

 and glycerol. If acids of low boiling point are present, the free 

 acids are partly volatilized when the substance is dried before 

 being extracted with ether, or when the ether extract is dried. 

 They are best determined by extracting the substance without 

 drying, and titrating the etheral solution with a standard solution 

 of caustic potash, with the addition of alcohol. 



