30 FATS, OILS, AND WAXES 



their glycerol esters the fats. By elaborating the reaction, 

 Hlibl converted it into one of the most valuable criteria at 

 present known for the detection and estimation of unsaturated 

 acids in fats, and the so-called " iodine value " provides an 

 excellent 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 substances containing respectively 25 grams of 

 iodine in water 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. 



ip) A sodium thiosulphate solution containing roughly 24 



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



this solution is accurately determined as follows : Twenty 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 lO per cent solution 



of potassium iodide and 5 c.c. of concentrated hydrochloric 



acid. The resulting 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 i c.c. of thiosul- 



0*2 

 phate 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. 



{e) A starch solution freshly prepared by boiling up a 

 suspension of 0*5 gram of starch in 50 c.c. of water. 



The determination of the iodine value is carried out as 

 follows : — 



From 0-15 to o-i8 gram of a drying or marine animal oil, 



