QUANTITATIVE METHODS 23 



(4) Iodine Value. 



It was first observed by Hiibl that an alcoholic solution 

 of iodine containing mercuric chloride reacted at ordinary 

 temperatures both with the free unsaturated acids and with 

 their glycerol esters the fats. By elaborating this reaction, 

 Hubl formulated the so-called " iodine value " which provides 

 a method of characterizing a fat. 



For the determination of the iodine value of a fat the 

 following solutions are required : — - 



[a) An iodine solution made by mixing together equal 

 volumes of two solutions containing respectively 25 grams 

 of iodine and 30 grams of mercuric chloride in 500 c.c. of 

 96 per cent alcohol. The two solutions should be mixed 

 together about twenty-four hours before use, as the resulting 

 mixture alters its strength considerably during the first few 

 hours after it has been made. 



{h) A sodium thiosulphate solution containing roughly 48 

 grams of crystallized salt in i litre of water ; the strength of 

 this solution is accurately determined as follows : 20 c.c. of a 

 potassium bichromate solution containing 3"8657 grams of the 

 pure salt dissolved in i litre of water are run into a stoppered 

 bottle containing 10 c.c. of a 10 per cent solution of potassium 

 iodide and 5 c.c. of concentrated hydrochloric acid. The re- 

 sulting brown solution, if carefully made, should contain 

 exactly 0-2 gram of iodine ; it is at once titrated by means of 

 the thiosulphate solution, and, supposing x c.c. were required 

 to decolorize it then it follows that l c.c. of thiosulphate is 



equivalent to — gram of iodine. 



[c) Chloroform or carbon tetrachloride, the purity of which 

 should be tested by mixing 20 c.c. of it with 20 c.c. of the 

 iodine solution and titrating the free iodine two or three hours 

 after ; the amount found should be exactly the same as that 

 contained in 20 c.c. of the iodine solution to which no chloro- 

 form or carbon tetrachloride has been added. 



[d) A 10 per cent solution of potassium iodide made by 

 dissolving i part of the iodide in 10 parts of water. 



