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



with other acids, but a much smaller amount of iodic acid is 

 necessary if mixed with another acid, as for example, sulphuric 

 acid. This may be explained either on the assumption of a 

 more rapid action when sulphuric acid is used, as Landolt* has 

 found to be the case in the reaction between iodic acid and 

 sulphurous oxide, or in the case of ordinary iodine solutions 

 prepared and kept in glass vessels, it may be supposed due to 

 the fact that the liquid contains iodides and iodates derived 

 from the glass. I have tried bringing together extremely 

 dilute solutions of iodic acid and of potassium iodide, and 

 found that no perceptible amount of iodine is set free, though 

 in more concentrated solutions, the iodine is set free in nearly 

 the quantitative yield represented by the equation : 

 5KI + 6HI0 3 = 5KI0 3 + 3H 2 + 3I 2 



In this reaction, however, the amount of iodine found is always 

 a trifle too low, probably due to the fact above shown, viz: 

 that in very dilute solutions, the iodide and iodic acid do not 

 react on each other, except in presence of another acid. The 

 dilute sulphuric acid alone when added to starch blue does not 

 bleach the color, but diminishes the intensity to an appreciable 

 degree. 



The amount of iodic acid necessary to prevent the forma- 

 tion of the blue color depends not only on the amount of other 

 acid present but also on the order in which the reagents are 

 put together. Thus I have found in several cases that, with a 

 definite amount of iodic acid and iodine, there was no color 

 produced if the iodine, iodic acid, and sulphuric acid were 

 mixed first, and left standing for some minutes or half an hour 

 before adding the starch ; whereas, if the starch were put in 

 first and the other three constituents added directly, a blue 

 color would result which could only be destroyed on long- 

 standing and shaking, if at all. 



In general, the amount of iodic acid necessary to prevent 

 the formation of the starch blue seems in very great excess of 

 that which would be necessary to destroy all of the hydriodic 

 acid which can possibly be present. It is only by following 

 the precautions suggested above that the iodic acid needed can 

 be made to bear any reasonable relation to this hydriodic acid. 

 I have dissolved iodine in a mixture of iodic and sulphuric 

 acids, and added starch and obtained immediately a blue color, 

 though naturally not as deep and intense as would be obtained 

 without the iodic acid. This solution was as strong as possible 

 of iodine, being made up with powdered iodine in a platinum 

 dish, and containing always some of the undissolved solid. 

 The liquid poured off from this, whether filtered or not, and 



* Ber. d. d. cliem. Gesell., xx, 745. 



