C. F. Roberts — Blue Iodide of Starch. 423 



is formed by the union of the colorless, or yellow, compound 

 C 6 H 10 O 6 I with hydriodic acid or one of its salts. He affirms 

 that a pure solution of iodine and starch gives no blue 

 color, but that the liquid must contain an iodide, a trace of 

 which is sufficient to bring about the desired result. 



My own study of the blue iodide of starch may be con- 

 sidered under two heads : 



1st. Its Decompositions. 



2d. The Conditions of its Formation. 



1. Decomposition of Starch Blue. 



(a) By heat. The effect of heat on the blue iodide of 

 starch is well known. If moderately heated, the blue color 

 returns on cooling, but if strongly heated in an open vessel, it 

 remains permanently colorless. The explanations of this 

 change, however, have been varied, some authorities stating 

 that all of the iodine is driven off by the stronger heat, others 

 that it is converted into iodic acid, and still others that the 

 product of the reaction is hydriodic acid. Stocks affirms the 

 last, and a very simple experiment seems to support this view. 

 If to the cooled, colorless liquid, a few drops of iodic acid be 

 added, the clear blue color is immediately restored. This ex- 

 periment was performed in a platinum vessel, and the same 

 results were obtained whether iodic acid were used alone or 

 mixed with dilute sulphuric acid, though the latter acid by 

 itself has no effect in restoring the color. That unaltered 

 starch remains in this solution after heating, is proved by the 

 fact that a drop of iodine colors the cold solution blue. 



While the fact that hydriodic acid is split off from the 

 iodide of starch under the influence of heat cannot be used as 

 an argument either for or against Mylius's views, these phe- 

 nomena are perhaps what might be expected if his views be 

 correct, The first effect of heat seems to be to produce a kind 

 of dissociation, the hydriodic acid splitting off from the rest 

 of the molecule and going back into it when cooled, but upon 

 longer heating the iodine which is directly united with the 

 starch molecule is either driven from the liquid or converted 

 into hydi'iodic acid. Upon the addition of iodic acid, some 

 iodine is set free which, along with the hydriodic acid, unites 

 with the starch to form the blue color. These phenomena are 

 thus shown to be in harmony with Mylius's theory, although 

 no proof of its correctness. 



(b) By iodic acid. If some blue iodide of starch be pre- 

 cipitated and well washed with dilute sulphuric acid, and then 

 iodic acid added, the solid dissolves to an apparently colorless 

 liquid. That this solution, however, contains free iodine, may 

 be proved by adding chloroform, which becomes tinged a de- 

 cided pink. This would indicate that the iodic acid acted by 



