300 PRACTICAL PHYSIOLOGY 



the .unsaturated fatty acids, as these are soluble in ether, whereas the 

 precipitate consists of the soaps of the solid fatty acids. 



Fat Values 



I. Melting Point. Use ordinary method, see p. 209. 



II. Specific Gravity. 



EXPERIMENT. Melt small amounts of butter and of oleomargarine 

 and drop the melted fats into alcohol at room temperature (15C.). 

 The butter will sink, but oleomargarine will float since it is composed of 

 fats of lower specific gravity than those of butter. 



III. Acid value indicates the amount of free fatty acid which is 

 present in the fat. The value rises when fats become rancid. 



EXPERIMENT. Dissolve 1 gm. of fat (butter) in as little neutral 

 alcohol as possible (with the addition of ether, if necessary), and, after 



N 



adding a few drops of phenolphthalein titrate with -y^ KOH. The 



result is expressed as the number of milligrams of KOH required to 

 neutralise the fatty acid of 1 gm. of fat. In the subjoined table the 



N 

 result is calculated as oleic acid on the basis that 1 c.c. r^ KOH = 



0-0282 gm. oleic acid. 



IV. The Saponification Value. This is a measure of the total amount 

 of fatty acid (both free and combined) contained in the fat. The fat is 

 saponified with a known amount of caustic potash which is in excess of 

 that required to produce complete saponification, and the caustic potash 

 which is not neutralised in the process is ascertained by titration against 

 standard acid. 



EXPERIMENT. Weigh a dry, clean, wide-mouthed Erlenmeyer flask, 

 and add to it 2 gms. of melted and filtered fat. By means of a pipette 

 add exactly 25 c.c. alcoholic potash, a sample of which has just 



N 

 previously been titrated against -~- HC1, using phenolphthalein as 



indicator. Close the flask with a cork having a wide glass tube passing 

 through it. This serves as a reflux condenser. Place the flask on a 

 boiling waterbath for half an hour, and shake frequently. Then 

 remove the flask, add 1 c.c. phenolphthalein solution and titrate against 



N 



~n HC1. The difference between the amount of acid now required 



and the amount of acid corresponding to 25 c.c. of the alcoholic potash, as 

 determined by the previous titration, corresponds to the amount of 

 fatty acids. The result is usually calculated in terms of the number of 



N 

 milligrams of KOH required to saponify 1 gm. fat. 1 c.c. -~ KOH 



contains 0-028 gm. KOH. 



V. The Ester (ether) value represents the amount of fatty acid which 

 is combined with glycerine. It is obtained by deducting the acid 

 value (III) from the saponification value (IV). 



VI. The Iodine value is the percentage amount of iodine which a 

 weighed quantity of fat can absorb. This is proportional to the amount 

 of unsaturated fatty acid (oleic, etc.) in the fat (see p. 222). The iodine 

 value is of great importance in physiological investigations, since by it 

 we can form an estimate of the relative amount of unsaturated fatty 



