Chemical Constituents of the Animal Body. 185 



weight of these acids. If a fat is boiled with excess of an 

 alcoholic solution of potassium hydroxide, the mixture at 

 the completion of the saponification will consist of glycerol, 

 potassium soap, and excess of potassium hydroxide. Now, 

 if a fat yields on hydrolysis relatively large amounts of 

 an acid of low molecular weight, such as butyric acid, 

 there will be less free potassium hydroxide after the com- 

 pletion of the hydrolysis of one gramme of fat than there 

 would be if the fat yielded only acids of higher molecular 

 weight, such as stearic acid. The " saponification value " 

 denotes the number of milligrams of potassium hydroxide 

 which is required for the saponification of one gramme of a 

 fat. This number is determined by heating a weighed 

 amount of a fat with excess of semi-normal alcoholic 

 solution of alkali, and determining the excess of alkali 

 remaining in the liquid after heating for about half an 

 hour, by titration with standard acid in the presence of 

 phenol-phthalein. 



The following are some typical saponification numbers 



Beef suet 193-2-200 



Lard 195-4-200 



Mutton fat 195 



Palm oil 242-250 



Linseed oil 191-195 



Olive oil 185-196 



Butter .;,.,. fc, f;: ;^. . . 227 



Certain fats and oils contain a small amount of free 

 acid. 



This should be determined before estimation is made 

 of the saponification number. The " acid number " is the 

 number of milligrams of potassium hydroxide necessary 

 to neutralize to phenol-phthalein the free acid in 1 gramme 

 of fat. 



(b) The Iodine Number. The acids obtained by 



