124 MASS. EXPERIMENT STATION BULLETIN 166. 



repeated thorough shaking until the blue color has entirel}^ disappeared, 

 leaving a bright green solution. As four times the titration is equivalent 

 to 1 gram of iodine, the iodine value of 1 cubic centimeter of thiosulfate 

 can be readily calculated. 



In theory, 1 cubic centuneter N/10 Na2S2035 H2O is equivalent to 

 .012692 gram of iodine. 



The following is the reaction : — 



K2Cr207 + 14 HCl+6 KI =2 CrCU+S KC1+6I+7 H2O 



61 : KiCrjOr :: 1 : x 



761.52 : 294.20 :: 1 : 0.38633 gram in 100 c.c. 



Wijs-Hubl Method.^ — The amount of material to be taken for this deter- 

 mination varies inversely with its iodine number (see table). From 0.30 to 

 0.45 gram of drying or fish oil, 0.45 to 0.70 gi-am of a semidrying oil, 0.55 to 

 0.90 gram of a nondrying oil, or 0.60 to 2 grams of fat are brought into a 

 300 cubic centimeter Erlenmeyer flask, together with 20 cubic centimeters 

 of carbon tetrachloride. After complete solution, 50 cubic centimeters of 

 iodine solution, accurately measured with a burette, are added and the 

 flask well stoppered and allowed to stand three to four hours, ^ with occa- 

 sional shaking, in a refrigerator at a temperature below 10° C. A rapid 

 bleaching of the solution indicates insufficient iodine. An excess equal to 

 the amount absorbed is deemed necessary for the attaimnent of constant 

 results. The cork stopper for the flask should be rolled until soft and 

 pliable, and moistened with potassium iodide to prevent loss of iodine by 

 volatilization. At the end of the absorption period, 50 cubic centimeters 

 of cold, recently boiled distilled water and 10 cubic centimeters of potas- 

 sium iodide are added to the contents of the flask, and the excess iodine 

 titrated with sodium thiosulfate. The thiosulfate is run in gradually, with 

 constant shaking, until the brownish yellow color of the solution has been 

 largely destroyed; then 2 cubic centimeters of starch paste are added and 

 the titration continued until the blue color has entirely disappeared. 

 Towards the end of the reaction the flask should be stoppered and shaken 

 vigorously, so that any iodine in the carbon tetrachloride mil be taken up 

 by the potassium iodide. The "bleached" condition should hold for a 

 considerable time with the flask stoppered, although the blue color will 

 develop again, due to the splitting off of iodine. Several blank determi- 

 nations should be run with every series of tests. The difference between 

 the titration of the blank and that of the excess iodine is the thiosulfate 

 equivalent of the fat, which multiphed by the factor (obtained as de- 

 scribed) and divided by the weight of fat taken gives the percentage of 

 iodine absorbed. 



Limit of error, 0.25 per cent. 



Synopsis of Reaction. — Solution with carbon tetrachloride. 



» Original Hubl method. Dingler's Polytech. Jour., 253, p. 281. 



» According to Lewkowitsch one-half hour is sufficient for all oils and fats having an iodine 

 number below 100, one hour for semidrying oils and two to six hours for drying oils. — Analysis 

 of Oils, Fats and Waxes, 1, p. 407, (1913). 



