464 FOODS AND FOOD ADULTERANTS. 



of thiosulphate solution used multiplied by 5 is equivalent to 1 gram 

 of iodine. 



Iodide of potassium solution. One part of iodide of potassium in ten 

 parts of water. 



Starch paste. One gram of starch in fine powder suspended in 100 

 parts of water and heated to the boiling-point. The paste must be 

 cooled to the temperature of the room before using. 



Manipulation. The quantity of fat to be used is determined by its 

 nature. If it consist largely of cotton oil one-half gram is sufficient. 

 If it be mostly pure lard, one gram may be taken. A measured quan- 

 tity of the fat corresponding to the weight desired is run into a recently- 

 weighed glass-stoppered flask, and after a few minutes the weight of 

 the flask and oil taken with precautions already noted. The fat is now 

 dissolved in lOcc of chloroform ; from 20 to 30cc of the iodine solu- 

 tion are then added from a burette. If the solution is not perfectly clear, 

 more chloroform should be added. The amount of iodine employed 

 should be large enough to leave an excess of 8 or lOec uuabsorbed at 

 the end of the reaction. It is important, to secure comparative results, 

 to have the amount of iodine in excess in each case approximately the 

 same. This can be easily secured by a preliminary determination of the 

 approximate amount of iodine absorbed. At the same time two blank 

 determinations are made to determine the strength of the iodine solu 

 tion; the manipulation in all cases being the same as in those samples 

 containing the fat, save that the fat is omitted. After standing for two 

 hours from 10 to 20cc of the iodide potassium solution are added and 

 loOcc of distilled water, and the liquids thoroughly shaken together. 

 The decinormal solution of thiosulphate of soda is, added until the yel- 

 low color of the liquid has almost disappeared. The titration is con- 

 tinued after the addition of a few drops of starch paste until the blue 

 color has entirely disappeared. 



Example. Grams. 



Weight of flask 74.1288 



Weight of flask, phis fat 74.8168 



Weight of fat , 6880 



After the addition of lOcc of chloroform and 30cc iodine solution the 

 flask was allowed to stand two hours. Twenty cubic centimeters of the 

 iodide solution were then added and 150cc of water, and the titration 

 made with deci-normal thiosulphate solution, using starch paste as in- 

 dicator. The amount of thiosulphate used was 21.2cc. The strength 

 of the iodine solution determined by blank experiment was lOcc of the 

 iodine solution=19.4cc of thiosulphate solution. Since 30cc of the 

 iodine solution were used the amount of dcci- normal thiosulphate solu- 

 tion necessary to combine with the whole of the iodine would be 58.2cc ; 

 then 58.2cc 21.2cc = 37cc, the number of cubic centimeters equiva- 

 lent to the iodine absorbed by the fat. In a deci-normal thiosulphate 

 solution each cubic centimeter equals .0127 grams of iodine; the total 

 amount of iodine absorbed therefore was 37 x .0127 = .4G90g. Then the 

 percentage of iodine absorbed = .4690 x 100 H- .688 = C8.L'<>. 



