EDIBLE OILS AND FATS. 25 



(b) DETERMINATION. 



(1) Standardizing the sodium thiosulfate solution. 



Place 20 cc of the potassium bichromate solution, to which has been added 10 cc of 

 the solution of potassium iodid, in a glass-stoppered flask. Add to this 5 cc of strong 

 hydrochloric acid. Allow the solution of sodium thiosulfate to flow slowly into the 

 flask until the yellow color of the liquid has almost disappeared. Add a few drops 

 of the starch paste, and with constant shaking continue to add the sodium thiosulfate 

 solution until the blue color just disappears. The number of cubic centimeters of 

 thiosulfate solution used multiplied by 5 is equivalent to 1 gram of iodin. 



Example: Twenty cubic centimeters of bichromate solution required 16.2 cc sodium 

 thiosulfate; then 16. 2X5=81 = number cubic centimeters of thiosulfate solution 

 equivalent to 1 gram of iodin. Then 1 cc thiosulfate solution =0.0127 gram of iodin. 

 Theory for decinormal solution of sodium thiosulfate 1 cc =0.0127 gram of iodin. 



(2) Weighing the sample. a 



Weigh about 1 gram of fat or 0.500 gram of oil b on a small watch crystal or by 

 other suitable means. The fat is first melted, mixed thoroughly, poured onto the 

 crystal and allowed to cool. 



Introduce the watch crystal into a wide-mouth 16-ounce bottle with ground-glass 



stopper. 



( 3 ) A bsorption of iodin. 



The fat or oil in the bottle is dissolved in 10 cc of chloroform. After complete 

 solution has taken place, 30 cc of the iodin solution are added in the case of fats, or 

 from 40 to 50 cc d in the case of oils. Place the bottle in a dark place and allow to 

 stand, with occasional shaking, for three hours. e This time must be closely adhered 

 to in order to get good results. The excess of iodin should be at least as much as is 

 absorbed. 



(4) Titration of the unabsorbed iodin. 



Add 20 cc of the potassium iodid solution, and then 100 cc of distilled water to the 

 contents of the bottle. Wash any iodin which may be noticed upon the stopper 

 back into the bottle with the potassium iodid solution. Titrate the excess of iodin 

 with the sodium thiosulfate solution, which is added gradually, with constant shak- 

 ing, until the yellow color of the solution has almost disappeared. Add a few drops 

 of starch paste, and continue the titration until the blue color has entirely disap- 

 peared. Toward the end of the reaction stopper the bottle and shake violently, so 

 that any iodin remaining in solution in the chloroform may be taken up by the 

 potassium iodid solution. The excess of sodium thiosulfate solution should be 

 sufficient to prevent a reappearance of any blue color in the flask for five minutes. 



(5) Setting the value of iodin solution by thiosulfate solution. 



At the time of adding the iodin solution to the fat, two bottles of the same size as 

 those used for the determination should be employed for conducting the operation 

 described above, but without the presence of any fat. In every other respect the 



"The writer has found it unsatisfactory to weigh so small amounts of fat in flask as directed in the 

 A. O. A. C. methods. 



> With drying oils which have a very high absorbent power, 0.100 to 0.200 gram should be taken. 



See appendix, p. 150. 



'F. Ulzer, Jour. Soc. Chem. Ind., 1898, 17, 276, says iodiu should be in excess about twice the amount 

 that is absorbed. The solution loses strength with age, but can be used so long as 35 cc of decinor- 

 mal thiosulfate neutralize 25 cc iodin solution. 



The time allowed does not give the complete iodin absorption power of an oil or fat and can not 

 be compared with determinations where six to twelve hours have been used. It tfivt-s very satisfac- 

 tory comparative results, but the time factor nuM ). very closely adhered to. 



