IODINE VALUE 2$ 



actually obtained in different cases agrees fairly closely with the 

 theoretical value calculated from the number of double and triple 

 bonds in the molecule. 



For example, in the case of oleic acid, CH3[CH2]7. CH : CH 

 [CHgJr.COOH, 2 gram atoms of iodine (2x127 grams) should 

 be theoretically required to saturate i gram molecule (282 grams) 

 of oleic acid. This gives 90-07 for the number of grams of iodine 

 per 100 grams oleic acid, while the iodine value actually found is 

 89-8 to 90-5. 



Organic acid. 



Oleic and Ricinoleic 

 Series 

 Linoleic 

 Linolenic 



Double or triple bonds. 

 One double bond 



One triple bond 

 Three double bonds 



Atoms of iodine^ per 

 molecule acid. 



Two 



Four 

 Six 



Hiibl's method for the determination of the iodine value of 

 unsaturated acids or their glycerides consists in adding to a known 

 weight of the fat dissolved in chloroform an excess of an alcoholic 

 solution of mercuric chloride containing a known amount of free 

 iodine per cubic centimetre. 



The excess of iodine . is then determined by titration with 

 standard thiosulphate solution. It is found that the presence of 

 the mercuric salt is essential for the reaction to proceed in a 

 regular, well-defined manner, although the actual role played by 

 this compound is not known. 



The Solution of Iodine and Mercuric Chloride is prepared as 

 follows : — Twenty-five grams iodine are dissolved in 500 c.c. 95 per 

 cent, alcohol, and then added to a solution of 30 grams mercuric 

 chloride, HgClg, in an equal quantity of the same spirit. The 

 amount of free iodine in the solution is found to vary constantly, 

 and should therefore be standardised by titration with thio- 

 sulphate immediately before use. 



The chloroform employed for the solution of the fat should be 

 previously tested by treating 10 c.c. with an equal volume of the 

 iodine solution, and titrating the free iodine after two hours 

 standing. The amount of free iodine should be unchanged. 



From -15 to -2 gram of linseed oil is accurately 

 weighed out, dissolved in 12 c.c. of pure chloroform, and 

 placed in a well-stoppered flask of 500 c.c. capacity. 

 From 25 to 30 c.c. of the iodine solution are then 



