QUANTITATIVE METHODS 27 



(6) The Acetyl Value. 



This is a measure of the amount of hydroxyl groups which 

 a fat contains ; its value depends upon the fact that compounds 

 containing an alcoholic hydroxyl group react with acetyl 

 chloride or acetic anhydride so as to replace the hydrogen of 

 the hydroxyl by the acetyl group (CH3CO — ) as shown by 

 the equation — 



ROH + ch'co>^ ^ ROCOCH3 + CH3COOH 



If the resulting acetyl derivative is saponified by means of 

 caustic potash it breaks up as follows :■ — 



ROCOCH3 + KOH = ROH + CH3COOK, 



and it is possible to determine the number of milligrams of 

 caustic potash which are thus utilized in combining with the 

 acetyl groups to form potassium acetate. 



The number of milligrams of potash required for the 

 saponification of the acetyl derivative obtained from i gram 

 of the fat is termed the acetyl value of that fat. 



Castor oil and grape-stone oil have particularly high acetyl 

 values which in the castor oil is due to the presence of the 

 hydroxy acid known as ricinoleic acid. 



The following are the acetyl values of some of the more 

 important oils, fats, and waxes : — 



The following method, due to Lewkowitsch, has been 

 adopted as the standard process. 



About 10 grams of the fat are boiled in a round-bottomed 

 flask under a reflux condenser for two hours with twice their 

 weight of acetic anhydride. The mixture is then poured into 

 a litre flask and boiled for half an hour with 500-600 c.c. of 

 water, a slow stream of carbon dioxide being conducted into 

 the liquid all the while to prevent bumping. After cooling, 

 the upper layer of water is siphoned off and the lower oily 

 layer is again boiled with water as above, the whole process 



