imni.] PUBLIC DOCUMENT — No. 31. 135 



color (iodiuc) luis been largely destroyed, then 2 cubic ceuti- 

 nieters of stareh paste are added and the titration continued 

 niitil llic 1)1 ue ]);irtic'les have entirely disappeared from the 

 resulting brii;lit green solution. As five times the titration is 

 etpiivalent to 1 gram of iodine, the iodine value of 1 cubic centi- 

 meter of thiosulfate is easily calculated. 



In theory, 1 cubic centimeter N^/10 Na2S203 5 aq. is equiva- 

 lent to .012()07 of a gram of iodine. 



The following is the reaeti(m: — 



K.CroO; + 14 UC\ + G KI = 2 CrCla + 8 KCl +61 + 7 H,0 



6l:K,Cr,07 : : l:x. 



761.82: 294.50 : : 1: .38657 g. in a 100 c.c. 



Method. — One gram of fat (.2 of a gram of a drying or fish 

 oil, .3 of a gram of a semidrying oil, or .4 of a gram of nondry- 

 ing oil ) is brought into a 300 cubic centimeter Erlenmeyer flask 

 and 10 cubic centimeters of carbon tetrachloride added. After 

 complete solution, 30 cubic centimeters of iodine solution, accu- 

 rately measured with a burette, are added and the flask well 

 stoppered and allowed to stand two hours ^ in a cool, dark place, 

 with occasional shaking. A rapid bleaching of the solution indi- 

 cates insufficient iodine. A considerable excess is said to be 

 necessary for the attainment of constant results. The tempera- 

 ture should not exceed 20° C, as heat seems to cause a secondary 

 reaction and certainly destroys the accuracy of the determina- 

 tion. ]\Ioistening of the stopper with potassium iodide solution 

 will prevent loss of iodine by volatilization. At the end of the 

 absorption period 100 cubic centimeters of cold, recently boiled 

 distilled water and 10 cubic centimeters of potassium iodide 

 solution are added to the contents of the flask, and the excess 

 of iodine titrated with sodium thiosulfate solution. The thio- 

 sulfate 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 titra- 

 tion continued until the blue particles have entirely disappeared. 

 Towards the end of the reaction the fla.sk should be stoppered 

 and shaken violently, so that any iodine in the carbon tetra- 



1 According to Wijs, one hour is sulliclent for any oil or fat and fifteen to thirty minutes 

 for nondrying and semidrying oils. 



