IODINE ABSORPTION. 333 



They give figsires as follows : — 



E. \V. T. .Jone.s prefers, instead of using an acid of estimated 

 strength, to te.st 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 

 almost completely deposited on allowing' the acetic acid to cool. 



The Iodine and Bromine Absorption. 



Von Hubl'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 eondition.s, 

 combine with iodine and bromine. 



For the iodine absorption, it lia.s been shown that the presence 

 of iodine chloride is necessary. 



The process is worked as follows : — 



Rearjents. — 13 grammes of iodine arc dissolved in pure '.)'.) per 

 (.cut. acetic acid, and chlorine passed in till the stren^'th of tlii> 

 solution is doubled; this point is siuuply shown In- a change 

 of colour. 



Deci-normal sodium thiosulphate solution. Dissolve 'io grauunes 

 of pure sodium thiosulphate solution and 1 gramme of salicylic 

 acid in 1 litre of water. .VUow this to stand a few davs and 

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

 To standardise the solution, about O'J:") irrauime of resublimed 

 iodine is accurately weighed in a small stoppered flask, about 

 2 grammes of potassium iodide and "2 c.c. of water are added, 

 aud the flask uently 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 iodine 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 1 if tie 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 

 at least 100 c.c. ; 10 c.c. of carbon tetrachloride are added, and 



