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



which has this power. In case sulphuric acid is added to the 

 liquid before heating in the test-tube, the color is not restored 

 by any acid, even iodic. 



Although I have found it impossible to mix carefully pre- 

 pared solutions of iodine and starch without getting a blue 

 color, if only a drop or two of iodine be added to an excess of 

 starch, the blue formed where the liquids first touch, disap- 

 pears in the mass of the liquid, and then the addition of hydri- 

 odic acid brings out the blue color distinctly. This is in 

 harmony with Mylius's statements, and also with the fact that 

 in working with dilute solutions in a large bulk of alkaline 

 liquid, the delicacy of the starch test for iodine is increased by 

 the addition of two or three grams of potassium iodide. 



In a further effort to prepare iodine, free from iodides, I 

 have taken an iodine solution, shaken it with chloroform, 

 rinsed with water, and finally drawn off the chloroform solu- 

 tion. Then, upon adding pure water and shaking some of the 

 mixture with starch, the starch remains uncolored. If, how- 

 ever, this mixture of iodine in chloroform and pure water be 

 heated and then cooled, or if it be exposed to the sunlight in a 

 platinum dish for about an hour, the mixture acquires the 

 power of coloring starch immediately, and the colorless aqueous 

 portion of yielding iodine to chloroform when treated with 

 iodic acid. The appearance of free iodine, indicated by the 

 chloroform, showed that hydriodic acid had been formed by 

 the iodine and water on heating or in sunlight. 



It has just been shown that if a solution of iodine in chloro- 

 form be washed with water, separated from the aqueous por- 

 tion, and starch added, there is no immediate change of color ; 

 but the blue color may be brought out in three different ways : 



(1) By addition of a drop of dilute hydriodic acid. 



(2) By heating and then cooling the liquid. 



(3) By long standing. 



The evidence of (1) is a direct support of Mylius's views, and 

 (2) and (3) may be considered as indirectly indicative of the 

 same, since they give conditions under which hydriodic acid 

 might presumably be developed. Further evidence in the same 

 direction may be found in the following experiment. An 

 emulsion of starch in chloroform was made, and to this a solu- 

 tion of iodine in chloroform added. The color remained un- 

 altered, but the addition of a little potassium iodide, either in 

 the solid form or in aqueous solution, immediately produced 

 the blue color. The addition of pure water failed to give the 

 blue color until after standing for several hours. 



There seems, then, to be sufficient evidence that all solu- 

 tions which form starch blue contain an iodide, and also, con- 

 sidering the experiments with chloroform, that solutions freed 



