154 Roberts — Action of Reducing Agents on Iodic Acid. 



undertaken with potassium iodide, iodic acid, and dilute hydro- 

 chloric acid. Potassium iodide was chosen as the reducing 

 agent on account of the simplicity of its oxidation products. 

 Approximately -^ solutions of iodic acid and potassium iodide 

 were used and dilute hydrochloric acid of specific gravity 1*05. 

 The latter acid was so dilute that it could be added in appar- 

 ently indefinite quantities to the iodic acid without producing 

 any perceptible odor or change of color. 



With 10 cc of iodic acid and half that volume of hydrochloric 

 acid, if the potassium iodide was added gradually from a 

 burette, the addition of 4 or 5 cubic centimeters gave a clear, 

 light yellow liquid in which no free iodine could be proved 

 either by starch or chloroform. As the amount of potassium 

 iodide increases, the liquid gradually becomes orange until 

 finally particles of iodine are precipitated out and accumulate 

 until they reach the quantitative yield represented by the com- 

 mon equation : 



5KI + HI0 3 + 5HC1 = 3H 2 + 5KC1 + 3l 2 



These phenomena seem to indicate that the reaction takes 

 place in two steps, giving first the iodine chloride, and then 

 the decomposition of this by means of more potassium iodide 

 setting free iodine. Even after solid particles of iodine appear 

 in the liquid, starch paste is not colored blue, showing the 

 well-known disturbing action of the chloride of iodine. That 

 the intermediate product is essentially the monochloride of 

 iodine instead of the trichloride may be inferred from the 

 superior stability of the former in dilute solutions containing 

 hydrochloric acid, since most authorities agree that the tri- 

 chloride is completely broken up in dilute solutions into the 

 monochloride, iodic acid and hydrochloric acid. If any tri- 

 chloride were first formed and underwent this decomposition, 

 the final form of the reaction would be the same as if the 

 monochloride were the sole product. This inference was 

 further confirmed by later results and by the following experi- 

 ment. 



The clear yellow liquid obtained by putting together 10 

 cubic centimeters of iodic acid, half that volume of hydrochloric 

 acid, and 4 cubic centimeters of potassium iodide was agitated 

 with benzene which is said to dissolve iodine trichloride with 

 a dark cherry-red color.* The result was a barely perceptible 

 pinkish tinge to the benzene, which left no appreciable residue 

 on evaporation. After separating the benzene and adding 

 ether, the color was quickly taken from the aqueous solution, 

 and the ether became yellow and, on evaporation, left a con- 

 siderable quantity of a volatile reddish-brown liquid. This 



* Ladenburg, Handworterbuch, v, 359. 



