i 5 4 MILK AND DAIRY PRODUCTS PART 



pure fat will absorb. Thus a measure of the amount of 

 oleic acid (in the case of butter) present in the fat can be 

 obtained. 



(a) Hull's 1 Method for Determining the Iodine Number. 

 The required solutions are : 



(!) Iodine- Mercuric Chloride. Twenty-five g. of iodine in 500 c.c. of 

 95 per cent, alcohol (free from fusel oil) ; also 30 g. of mercuric chloride 

 in 500 c.c. of the same alcohol. The two solutions are kept separate, 

 and at least forty-eight hours before being used equal quantities are 

 mixed together. The mixture may be kept for some time, however, if 

 5 per cent, of concentrated hydrochloric acid (sp. gr. 1'19) is added 

 to it. 2 



(2) Sodium Thiosulphate. This solution contains about 25 g. per litre ; 

 the exact strength can be determined by Volhard's method. 3 "874 g. of 

 pure potassium bichromate, which "has been recrystallised several times, 

 is dissolved in one litre of distilled water. 15 c.c. of a lOper cent, potassium 

 iodide solution are then brought into a bottle with ground-glass stopper 

 along with 5 c.c. of concentrated hydrochloric acid and 100 c.c. of water. 

 Then 20 c.c. of the bichromate solution are added under vigorous 

 shaking. Each cubic centimetre of bichromate frees O'Ol g. of iodine. 

 The contents of the flask are then titrated with thiosulphate from a 

 burette until the dark brown colour becomes faint yellow, at which 

 point a few drops of starch paste are added, and the titration continued 

 until the blue colour disappears. 



As the thiosulphate rapidly changes, it must be standardised against 

 the above-mentioned bichromate solution, which is very durable. 



The actual determination of the iodine number is carried out 

 as follows : 



O'8-l g. of clear filtered butter fat is weighed out into a 

 specially designed flask with ground-glass stopper. 3 This 

 flask, although it holds 270 c.c., only weighs from 40-50 g. 

 The fat is dissolved in 15 c.c. of chloroform, and 50 c.c. of the 

 mixed iodine-mercury solution added from a burette. If the 

 liquid, after stirring, is not quite clear, a little more chloroform 

 must be added ; and if, on the other hand, the colour becomes 

 fainter after a short time, more iodine -mercury solution must 

 be added. Enough iodine must be added to impart a dark 

 brown colour to the liquid even after one and a half to two 

 hours. Generally the reaction is complete at the end of 

 several hours, but it is best to allow twenty-four hours to 

 elapse, during which time the liquid is kept in a dark place at 

 a temperature of 15-18 C. At the end of this time 30 c.c. of 

 10 per cent, potassium iodide solution are added, the mixture 



1 Dingier' s Polyt. Journal, Vol. CCLIII, p. 281. 



2 Waller, Chem. Zeitung., 1895, p. 1,831. > 



3 To be obtained from Joh. Greiner, Munich. 



