24 FATS, OILS, AND WAXES 



[e] A starch solution freshly prepared by boiling up a 

 suspension of 0-5 gram of starch in 50 c.c. of water. 



The determination of the iodine value is carried out as 

 follows : — 



From 0-15 to o-i8 gram of a drying or marine animal oil, 

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

 drying oil or 0-8 to l-o 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 {c) ; 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 mix- 

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

 mercuric iodide is produced, more potassium iodide solution 

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

 dissolved in the chloroform and part in the potassium iodide 

 solution, 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 continued until the dark blue colour is destroyed. 

 Twenty-five c.c. of the original Hiibl iodine solution, which 

 had been left in a stoppered bottle with 10 c.c. chloroform and 

 kept in the dark for the same length of time as the bottle 

 containing the sample of the fat, 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 



