C. F. Roberts — Blue Iodide of Starch. 429 



even when it had considerable iodic acid added to it, refused 

 under any circumstances to give a colorless liquid with starch ; 

 and the same was found true even when it was left standing 

 with additional iodic acid for half an hour before the starch 

 was added. 



These facts and the fact that the purest possible, freshly 

 prepared aqueous solution of iodine may be shown to contain 

 hydriodic acid, have suggested to me the possibility that hydri- 

 odic acid may be constantly forming in traces when iodine dis- 

 solves in water, and that, therefore, no aqueous solution can be 

 free from it whether kept in glass or not, though there is no 

 doubt that the glass may greatly increase the amount. So 

 many facts seem to point to the truth that hydriodic acid or 

 one of its salts is taken up as such into the molecule of the 

 blue iodide of starch that the fact of the large amount of 

 iodic acid necessary to prevent its formation can hardly be 

 considered a sufficient argument against that belief, though we 

 naturally question what may be the reason for this necessary 

 excess. There seem to me to be two possible explanations, 

 either the iodic acid destroys hydriodic acid much less readily 

 in dilute solutions than has been generally supposed, or some 

 hydriodic acid is constantly being formed in an iodine solution, 

 and perhaps the correct explanation lies in the union of these 

 two. 



Landolt* has made some investigation of the reaction be- 

 tween iodic acid and hydriodic acid in dilute solutions, and has 

 found that in an extremely dilute solution, nineteen seconds 

 may elapse before the formation of iodine makes itself mani- 

 fest by the blue color given to starch. I have used even 

 diluter solutions than he, and found that with '000009 grams 

 iodic acid and '000033 grams potassium iodide in 20 "cubic 

 centimeters of water, no color appeared with starch until seve- 

 ral minutes had elapsed if only a small quantity of sulphuric 

 acid was present, whereas if one-fourth of the liquid consisted 

 of dilute sulphuric acid, a blue tinge appeared immediately. 

 The completeness of the reaction in a given time, then, is 

 seen to depend on the amount of sulphuric acid present, and 

 there is nothing to show whether it is finally complete or whether 

 there may be left traces of undecomposed hydriodic acid. 



If we assume that hydriodic acid is constantly being formed 

 in traces in an iodine solution, and remember that the reaction 

 between hydriodic acid and iodic acid is not an immediate one 

 in the presence of a large quantity of water, we can under- 

 stand why a considerable excess of iodic acid must be used in 

 order to destroy completely the last traces of hydriodic acid 

 and thus entirely prevent the formation of starch blue. 



*Ber. d. d. chem. Gesell., xix, 1317. 



