32 FATS, OILS, AND WAXES 



O'2 to 0*3 gram of a semi-drying oil, 0-3 to 0-4 gram of a non- 

 drying oil or O'8 to 1*0 gram of a solid fat are accurately 

 weighed from a weighing bottle by difference into a 500-800 

 c.c. bottle, provided with a well-ground stopper, and dissolved 

 in 10 c.c. of the chloroform (V) ; 25 c.c. of the iodine solution 

 (a) are then run in, and the stopper, which is moistened with 

 potassium iodide solution (d) to prevent loss of iodine by 

 volatilization, is inserted. If a clear solution is not obtained 

 more chloroform must be added. The bottle is then left to 

 stand in the dark and if the dark brown colour should disappear 

 after two hours or less, another 25 c.c. of the iodine solution 

 must be added, as it is essential that there should be a con- 

 siderable excess of iodine. In the case of solid fats and non- 

 drying oils the reaction can be considered as being complete 

 after six to eight hours, but in the case of drying oils or fish 

 oils twelve to eighteen hours are necessary. After the com- 

 pletion of this time from 15 to 20 c.c. of the potassium iodide 

 solution (d) are added, and, after thorough shaking, the mixture 

 is diluted with 400 c.c. of water. If a red precipitate of mer- 

 curic iodide is produced, more potassium iodide solution should 

 be added. The excess of free iodine, part of which is dis- 

 solved in the chloroform and part in the potassium iodide solu- 

 tion, is then titrated by shaking with the standardized sodium 

 thiosulphate solution until only a faint yellow colour remains. A 

 little of the starch solution is now added and the titration is con- 

 tinued until the dark blue colour is destroyed. Twenty -five c.c. 

 of the original iodine solution are then titrated in a similar 

 way with the sodium thiosulphate, and the difference in the 

 two results gives the amount of iodine absorbed. The amount 

 of iodine thus absorbed by 100 grams of the fat gives the 

 iodine value. 



The values obtained by the Hiibl method are generally 

 considered to be very reliable and concordant, but the method 

 is somewhat tedious, and for this reason the more rapid 

 method of Wijs * is preferable. 



The iodine solution required for this method is obtained 

 by separately dissolving 9*4 grams of iodine chloride and 7*2 

 grams of finely powdered iodine in separate flasks in about 200 



*Wijs: "Zeit. anal. Chem.," 1898, 277; "Zeit. Unters. Nahr. Genussm.," 

 1902, 497. 



