28 FATS, OILS, AND WAXES 



the liquid is acidified with 6o c.c. of sulphuric acid (sp. gr. 1-3) 

 as described above. After the shaking with light petroleum 

 is completed, 50 c.c. of 94 per cent alcohol are added instead 

 of the salt solution ; this has the effect of accelerating the 

 separation of the petroleum layer which otherwise might take 

 a long time. 



Owing to the relatively small solubility of stearic acid in 

 light petroleum the method may give too low a result in the 

 case of substances very rich in stearin ; the result should, there- 

 fore, be checked by a second estimation in which the number 

 of shakings with petroleum are increased two or three fold. 

 Leathes * has modified and considerably improved this method. 



Kumagawa and Sutof have found that the following 

 method gives good results : Two to five grams of the dry sub- 

 stance J are heated on a water bath for two hours with 25 c.c. 

 of 5 N sodium hydroxide (20 grams in 100 c.c.) in a covered 

 beaker. The mixture is then transferred to a separating funnel 

 and acidified with 30 c.c. of 20 per cent hydrochloric acid. 

 The fatty acids set free are taken up with ether, and the 

 ethereal solution is filtered through asbestos and evaporated. 

 The residue, which contains colouring matter, lactic acid and 

 other substances as well as fatty acids, is dried for some 

 hours at 50°, and then taken up with light petroleum, where- 

 upon the impurities separate out in resinous form. After 

 filtering through asbestos the petroleum is distilled off, and 

 the residue, consisting of almost pure fatty acids, is dried at 

 50° to constant weight. 



QUANTITATIVE METHODS EMPLOYED FOR THE 

 CHARACTERIZATION OF FATS. 



The following estimations are in common use for the 

 commercial valuation of fats : — 



(i) The Acid Number. 



This is the number of milligrams of potassium hydroxide 

 required for the neutralization of the free acids in a sample 

 of fat. 



* Leathes : " The Fats," London, 1910. 

 + Kumagawa and Suto : " Biochem. Zeit.," 1908, 8, 212. 

 + Yoshitaka Schimidzu (" Bioch. Zeit.," 1910,28, 237) recommends using 

 undried material since drying leads to a loss of fat, probably from oxidation. 



