28 FATS, OILS, AND WAXES 



being repeated three times. The oil is finally filtered and 

 washed on the filter paper with boiling water until the filtrate 

 is no longer acid, whereupon it is dried in an oven and weighed. 



About 5 grams of the acetylated product are next saponi- 

 fied by boiling with alcoholic potash * as described under the 

 determination of the saponification value. The alcohol is then 

 evaporated off, and the resulting soap is dissolved in water. 



Dilute sulphuric acid (i : lo) is then added in excess and 

 the solution is steam distilled until 600-700 c.c. of water have 

 passed over. The distillate is titrated with N/io caustic potash 

 using phenolphthalein as indicator ; the number of cubic centi- 

 metres required for neutralization multiplied by 5'6i and 

 divided by the weight of fat taken gives the acetyl value. 



Further information regarding the nature of a given fat 

 may be obtained by investigating the relative amounts of the 

 saturated and unsaturated acids. This may be effected by 

 saponification and conversion of the resulting soap into lead 

 soaps by means of lead acetate ; making use of the greater 

 solubility of the lead soaps of unsaturated acids in ether, 

 these may be separated from the lead soaps of the saturated 

 acids. The saturated and the unsaturated acids respectively 

 may then be set free from their lead soaps and examined. 



SPONTANEOUS CHANGES IN FATS. 

 Rancidity. — Most fats when exposed to air and light sooner 

 or later become rancid, acquiring an unpleasant taste and 

 smell. The actual cause of this change is as yet but little 

 understood, though it appears probable that it is the result of 

 the combined action of a number of different factors such as 

 oxygen, light, moisture, bacteria and enzymes ; the complex 

 fats, and possibly also the small quantities of proteins and 

 other impurities contained in them, are thereby broken down 

 into simpler bodies such as the lower volatile fatty acids and 

 aldehydes. Similarly, but little is known as to the chemical 

 changes involved in the process of becoming rancid ; it is 

 frequently true that a considerable quantity of free acid is 



* Prepared by dissolving about 32 grams of go per cent stick potash 

 in the least quantity of water and diluting to i litre with 96 per cent 

 alcohol ; the solution should be filtered after standing for twenty- four hours. 



