242 III. CHEMISTRY OF NEUTRAL FATS 



hydroxide required, to neutralize the free fatty acids in one gram of oil or fat. 

 This value may also be expressed as the percentage of oleic acid. In the 

 latter case, 1 ml. of 0.1 N alkali is considered equivalent to 0.0282 g. of 

 oleic acid. 



When the acid number is subtracted from the saponification number, 

 the difference will eciual the potassium h3'^droxide ac^tually required for the 

 saponification of the fat. This is sometimes referred to as the ester number. 

 In the case of most refined oils, as well as of crude oils of high quality, the 

 saponification and ester numbers are practically identical. 



(6) Iodine Number 



The iodine number is defined as the number of grams of iodine absorbed 

 by one hundred grams of fat or oil. Under certain conditions, iodine is ab- 

 sorbed quantitatively by unsaturated acids or triglycerides at the point of 

 unsaturation. Thus, each molecule of oleic acid will combine with 2 atoms 

 of iodine, while one molecule of triolein will absorb 6 atoms of this halogen. 

 Trilinolein reacts with 12 atoms and trilinolenin with 18 atoms of iodine. 

 Whereas the reaction gives theoretical results with oleic or linoleic acids, 

 this is not the case with linolenic acid, in which a value of only 223 has been 

 found, in contrast to a theoretical result^^^ of 274. Much lower than theo- 

 retical values are obtained where a proximity exists between the double 

 bond and the carboxyl group, as well as where the double bonds are con- 

 jugated.'^* 



The iodine number is especially helpful in determining the ability of a fat 

 to be hardened by hydrogenation. Coupled with the thiocyanogen 

 number, it is an invaluable aid in ascertaining the usefulness of a fat to 

 serve as a drying oil. Irrespective of these uses, it is also important in the 

 identification of fats, as it is one of the constants less readily affected by the 

 treatment of the oil than are some other constants. 



Since free iodine reacts with fat very slowly, it is usually combined with 

 other substances to facilitate the reaction. One of the earliest reagents for 

 this use was the so-called Hiibl reagent, which contained an alcoholic solution 

 of iodine in the presence of mercuric chloride. '^^ Later modifications were 

 the Wijs reagent^^"'^'^ in which iodine monochloride was employed in 

 glacial acetic acid, and the Hanus method, ^^- which involved the use of 

 iodine monobromide in place of the iodine monochloride. The latter com- 

 pound has the advantage of being more easily prepared than the chloride. 



'" W. R. Bloor, Biochemistry of the Fatty Acids, Reinhold, New York, 1943. 



'28 B. P. Caldwell and F. A. Piontkowski, /. Am. Chem. Soc, 56, 2086-2089 (1934). 



329 Baron Hubl, Dinglers polytech. J., 253, 281-295 (1884). 



"0 J. J. A. Wijs, Ber., 31, 750-752 (1898). 



3" J. J. A. Wijs, Z. Untersuch. Nahr. Genussm., 1, 561 (1898). 



332 J. Hanus, Z. Untersuch. Nahr. Genussm., 4, 913-920 (1901). 



