1 86 Biological Chemistry. 



hydrolysis from various fats differ greatly in the degree of 

 saturation that is, in the number of double bonds. They 

 will differ greatly, therefore, in their capacities to form 

 addition products with halogens. The iodine value is 

 amount of iodine expressed as a percentage of the weight 

 of the fat taken, which will combine with the fat under 

 certain standard conditions of experiment. Even satu- 

 rated acids will decolorize iodine solutions (with formation 

 of substitution products), but they will not do so as rapidly 

 as acids which are unsaturated. It is therefore necessary 

 to choose certain standard conditions for determination of 

 the iodine value, which, valuable as it is for characterizing 

 fats, must be regarded as a somewhat empirical number. 

 The method of determining this factor was worked out by 

 Von Hiibl, but the modified and short method of Wijss 

 is now generally employed. The solution of Wijss is 

 made by dissolving iodine trichloride (9 '4 grammes) and 

 iodine (7*2 grammes) in glacial acetic acid (1 litre). The 

 amount of iodine in this solution is ascertained by titration 

 with sodium thiosulphate.* To determine the iodine 

 value of a fat, a weighed amount of the latter is dis- 

 solved in carbon tetrachloride and allowed to stand in a 

 dark place for one hour with excess of the Wijss reagent. 

 After this interval, the amount of free iodine remaining is 

 estimated by titration with thiosulphate solution. From 

 the difference between the titration number thus obtained, 

 and that of the same amount of the reagent before keeping 

 with the fat, the amount of iodine which has combined 

 under the conditions of experiment can be ascertained. 



The " iodine numbers " have been largely used in 

 biological research. Leathes, for example, estimated the 

 iodine numbers of the liver fat after animals had ingested 

 large quantities of various other fats, and found that this 



* For methods refer to a text-book on quantitative analysis. 



