QUANTITATIVE METHODS 25 



of iodine thus absorbed by lOO 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 0/ iodine chloride and 7-2 

 grams of finely powdered iodine in separate flasks in about 200 

 c.c. of gently warmed glacial acetic acid. The two solutions 

 are then united in a I litre graduated flask and made up to 

 the mark with more glacial acetic acid. 



This solution should be standardized on the following day 

 by mixing 20 c.c. of it with 10 c.c. of lO per cent potassium 

 iodide solution and titrating the free iodine by means of the 

 standard thiosulphate. 



The actual determination of the iodine value is carried out 

 as follows : — 



From 0-2-0-4 gram of fat should be carefully weighed and 

 dissolved in 10 c.c. of pure carbon tetrachloride (which has 

 been shown by a blank test not to absorb iodine) ; 25 c.c. of 

 the iodine solution are then added, and the flask is stoppered 

 and set aside in the dark for one or two hours. The liquid is 

 then transferred to a larger flask, the smaller flask being 

 washed out thoroughly by means of 10 c.c. of potassium 

 iodide solution and water until the total volume is about 

 300 c.c. The solution is then titrated with the thiosulphate. 

 The difference between this reading and the amount required 

 by 25 c.c. of the iodine solution is a measure of the iodine 

 absorbed by the amount of fat. 



The values obtained by Wijs's method are, as a rule, rather 

 higher than those obtained by the Hubl method. 



Appended is a list of iodine values of some important fats : — 



