IODINE ABSORPTION. 263 



They give figures as follows : 



Maximum. Minimum. Average. 



Butter fat, . . 39-0 C. 29-0C. 36-0 C. 



Margarine, . . 97-0C. 94-0 C. 95-0 C. 



E. W. T. Jones prefers, instead of using an acid of estimated 

 strength, to test it against a standard sample of butter, and to 

 dilute the acid so that it gives a temperature of turbidity of 

 60. Margarine then gives about 100. 



Hehner has found that this test depends almost entirely on 

 the glycerides of the saturated fatty acids present, as these are 

 deposited almost completely on allowing the acetic acid to cool. 



The Iodine and Bromine Absorption. 



Von Hiibl's Method: Wijs' Modification. This method 

 depends on the fact that acids of the oleic, linolic, and linolenic 

 series contain unsaturated bonds, and, under suitable conditions, 

 combine with iodine and bromine. 



For the iodine absorption, it has been shown that the presence 

 of iodine chloride is necessary. 



The process is worked as follows : 



Reagents. 13 grammes of iodine are dissolved in 1 litre of 

 pure 99 per cent, acetic acid, and chlorine passed in till the 

 strength of the solution is doubled ; this point is sharply shown 

 by a change of colour. It is advisable to add a little more iodine 

 till the colour is slightly brown to eliminate the presence of I C1 3 . 



Deci-normal sodium thiosulphate solution. Dissolve 25 

 grammes of pure sodium thiosulphate solution and 1 gramme of 

 salicylic acid in 1 litre of water. Allow this to stand a few days 

 and filter. This solution is permanent and does not alter in 

 strength. To standardise the solution, about 0-25 gramme of 

 resublimed iodine is weighed accurately in a small stoppered flask, 

 about 2 grammes of potassium iodide and 2 c.c. of water are 

 added, and the flask gently shaken till the iodine is dissolved. 

 The iodine solution is diluted with water, transferred to a larger 

 flask, and titrated with the sodium thiosulphate solution till the 

 yellow colour just disappears. This operation is repeated two 

 or three times. The mean strength of the solution deduced from 

 these experiments is noted on the label of the bottle. 



A 10 per cent, (approximate) solution of potassium iodide and 

 a starch paste solution, made by pouring an emulsion of 1 gramme 

 of starch in a little cold water into 200 c.c. of boiling water, and 

 boiling for ten minutes ; if a little mercuric iodide be added, 

 this solution is permanent. 



The process is performed as follows : About 0-5 gramme of 

 the fat is accurately weighed in a glass-stoppered flask holding 



