322 PRACTICAL PHYSIOLOGY 



EXPERIMENT. Dissolve 1 grm. of fat (butter) in as little alcohol as possible 

 (with the addition of ether, if necessary), and, after adding a few drops of 



phenolphthalein titrate with ^ KOH. The result is expressed as the number of 

 milligrams of KOH required to neutralise the fatty acid of 1 grm. of fat. In the 

 subjoined table the result is calculated as oleic acid on the basis that 1 c.c. y^ 



KOH equals 0'0282 grm. 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 polish 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 grm. of melted and filtered fat. By means of a pipette add exactly 25 c.c. 

 alcoholic potash, a sample of which has just previously been titrated against 



- HC1, using phenolphthalein as an 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 water bath for half an hour, and shake frequently. Then 



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



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 milligrams of KOH required to saponify 



1 grm. fat. 1 c.c. | KOH contains 0'028 gr. 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. 314). The iodine value is of great im- 

 portance in physiological investigations, since by it we can form an estimate of 

 the relative amount of unsaturated fatty acids in fats. Its determination involves 

 the use of carefully standardised solutions, and is too complicated for description 

 here. 



VII. The Reichert-Meissl value indicates the amount of volatile soluble 

 fatty acids present. It is of great value in testing the purity of butter, because 

 this contains a considerable proportion of such acids, whereas the cheaper fats, 

 which are sometimes used as substitutes for butter, do not contain much of them. 



EXPERIMENT. 5 grm. melted fat is saponified with alcoholic potash, the 

 alcohol evaporated, and the resulting soap dissolved in water acidified with 

 sulphuric acid, and distilled. The distillate, which contains the volatile acids, is 



collected in a flask and titrated with y^- NaOH, the result being expressed as the 



number of c.c. of decinormal acid contained in the distillate from five grammes of 

 fatty substance. 



