ii BUTTER 155 



shaken, and 100 c.c. water added. If a red precipitate of 

 mercuric iodide should separate, then the amount of potassium 

 iodide is insufficient and must be increased. Sodium thio- 

 sulphate is then run in from a burette until the colour of the 

 solution, and also of the layer of chloroform, is only faintly 

 yellow, at which point the starch paste is added. The iodine 

 solution must be standardised afresh each time by titration 

 against sodium thiosulphate, and the latter against potassium 

 bichromate. 



With each determination a blank must be made, the reagents 

 being used exactly in the manner described above, but without 

 the fat, of course. The result of the blank must be deducted 

 when calculating the iodine number. 



Most careful and uniform manipulation is required in 

 carrying out the method described above, for the smallest 

 error in the reading means a big difference in the final result. 

 For this reason the same burettes, pipettes, &c., must always 

 be used. 



Generally the iodine number for pure butter fat falls between 

 25'7 and 49 ; in the case of individual cows, in some special 

 instances it may vary between 25 and 53. 



(b) Wijs l Method for Determining the Iodine Number. 



Wijs found that in Htibl's solution it was the iodic acid 

 which was the active component, and he sought to use this 

 acid as the source of iodine. When, however, the solution 

 came to be kept, and also during the reaction, difficulties arose 

 owing to the decomposition of the acid. Wijs therefore chose 

 a solution of iodine chloride in 99 per cent, acetic acid, and this 

 can satisfactorily replace Hubl's solution. It is prepared by 

 dissolving 13 g. of iodine in 1 litre of acetic acid (Acidum aceticum 

 glaciale 99 per cent. Merck) and the strength of the solution 

 determined. Thereupon chlorine gas, which has been freed 

 from hydrochloric acid by washing, is led slowly into the 

 solution until the contents have doubled. This solution is a 

 very constant one, and when it is used the time required for 

 the experiment is only short, three to four or at most ten 

 minutes. The acetic acid used in preparing the solution must 

 be absolutely pure. , 



1 Ber. der deutsch. chem. Gesellschaft, 1898, p. 750. 



