C. F. Roberts — Blue Iodide of Starch. 427 



from hydriodic acid cannot produce that color, although in my 

 experiments I have not been able to make an aqueous solution 

 of iodine which did not turn starch blue, and which did not 

 contain hydriodic acid. 



We pass now to the third argument of Mylius quoted above, 

 viz: the deterrent action of chlorine and iodic acid on the 

 formation of starch blue. 



In regard to the action of chlorine, Stocks quotes the sug- 

 gestion of Miller that it may be due to the formation of iodine 

 chloride, which is well known to have a destructive effect on 

 the blue iodide of starch. But even granting this as an 

 explanation, two questions remain to be answered ; first, how 

 is the iodine chloride formed, and second, what is its action on 

 starch blue? It can hardly be supposed that the chlorine 

 withdraws the iodine bodily from the organic molecule. A 

 much more natural assumption would be that the chlorine de- 

 composes hydriodic acid, setting free iodine ; and here we 

 have sufficient cause for the destruction of the blue color. But 

 granting that the reaction may go farther, and the iodine thus 

 set free unite with an excess of chlorine to form iodine chlo- 

 ride, is not this also a body which might be expected to act 

 by destroying hydriodic acid? I have found, in fact, that 

 when iodine chloride is added to starch blue, iodine is set free, 

 the result which might be expected if the iodine chloride acts 

 upon the hydriodic acid in the compound according to the 

 reaction : 



HI + IC1 = HC1 + I 2 



This experiment was performed as follows : Iodine chloride 

 was prepared by putting together potassium iodide and hydro- 

 chloric acid in an excess of iodic acid. It was then separated 

 from the other bodies by extraction with ether, and to the 

 ethereal solution, starch blue was added. The color was taken 

 from the starch, and the ether became darker and brown. To 

 prove that this change in the ether was really due to free 

 iodine, it was left standing over pure water for some time, 

 with occasional shaking. Finally, some of this aqueous por- 

 tion was shaken with chloroform, to which it gave the charac- 

 teristic pinkish tinge. As iodine chloride gives no color to 

 chloroform, this shows conclusively that iodine had been set free. 



There remains to be considered only the action of iodic acid 

 on the iodide of starch. Its action on the precipitated blue 

 after formation has already been referred to, so that at this 

 point, we need only consider its action in preventing the 

 formation of starch blue. 



It is an undeniable fact that when dilute solutions of iodine 

 are used, iodic acid in sufficiently large quantities prevents the 

 formation of starch blue both when used alone, and when used 



Am. Joue. Sci.— Third Series, Vol. XLVII, No. 282.— June, 1894. 

 29 



