C. F. Roberts — Blue Iodide of Starch. 425 



(1) An aqueous solution of iodine cannot color starch blue, 

 but the color appears immediately if a trace of hydriodic acid 

 or potassium iodide be added. 



(2) Iodine solutions which color starch blue contain hydri- 

 odic acid or one of its salts. 



(3) The presence of substances which destroy hydriodic acid, 

 as, for example, chlorine and iodic acid, prevents the formation 

 of starch blue. 



These different statements will be considered in the order in 

 which they are here quoted, but (1) and (2) are so nearly iden- 

 tical that they cannot be kept separate. 



I have never succeeded in obtaining aqueous solutions of 

 iodine and starch which, upon being mixed, did not produce a 

 blue color, although great care was taken to avoid the possi- 

 bility of any alkali from glass getting into the solutions. 

 According to Mylius, this is the great cause of the presence of 

 iodide in an iodine solution. In my experiments, the iodine 

 was powdered, left standing for some time in a mixture of 

 iodic and sulphuric acids, then washed thoroughly with water, 

 and finally dissolved in water, all of these processes being car- 

 ried on in a platinum dish. An emulsion of arrow-root, also 

 well washed, was made in a platinum dish, and the two solu- 

 tions were mixed in a platinum crucible. Notwithstanding all 

 of these precautions, the blue color appeared immediately. 

 This would seem to indicate that Mylius's views were errone- 

 ous, but on the other hand, I was able to prove the presence 

 of hydriodic acid in my aqueous solution, even thus carefully 

 prepared. I did this by shaking the solution with chloroform 

 and separating the two liquids with a separating funnel, 

 and repeated this until the chloroform added remained per- 

 fectly colorless. Then, upon the addition of a few drops of 

 iodic acid, and shaking, the chloroform became faintly but 

 undoubtedly tinged with pink. It is proved, then, that this 

 liquid which colored starch contained hydriodic acid, but it 

 still remains an open question whether this iodide was an im- 

 purity in the iodine, not removed by its repeated washings, 

 which seems hardly probable, or whether it is formed when- 

 ever iodine dissolves in water, and is therefore, in traces, a 

 necessary accompaniment of every iodine solution. 



That glass has a more powerful action on an iodine solution 

 than has generally been supposed cannot however be doubted. 

 My own attention has been drawn to this fact by the difference 

 in behavior of the liquid which results when starch blue is 

 heated in a glass tube and that which is formed when the same 

 compound is heated in a platinum vessel. In the first case the 

 cold, colorless solution is turned blue by the addition of any 

 acid, whereas in the latter case, iodic acid is the only one 



