IN LIGHT IN THE PRESENCE OF OXYGEN. CREIGHTON, 57 



to show that the process of the change is one of oxidation, 

 through the different oxygen compounds of iodine. The amount 

 of iodine as iodate in the solution into which oxygen was passed, 

 just after the last trace of iodine had disappeared, was found 

 to be 0.0000372 gram per cc. or the 500 cc. started with would 

 contain 0.0186 gram; that is, 4.87 per cent, of the original 

 amount of iodine. 



It is evident then that the amount of iodate present in the 

 solution will be greatest just after the solution becomes colour- 

 less; that is, never very much greater than 4.87 per cent. This 

 will readily be seen from the consideration that no appreciable 

 amount of iodate could exist, while there was any potassium 

 iodide or hydriodic acid present in a solution containing sul- 

 phuric acid. Also it has been show'n that the amount of iodate 

 decreases with time, after the solution loses its colour. 



It seemed a not unlikely explanation that the iodine might 

 be changed into periodates. It would appear a perfectly 

 natural process for the hydriodic acid to be oxidised to iodic 

 acid,, possibly through the intermediate formation of hypoiodous 

 acid, and this quickly transformed to some of the periodic 

 acids. Of course, there could not, and need not, be a'ny appre- 

 ciable amount of these intermediate substances present at any 

 time. In order to test this explanation the solution was exam- 

 ined for periodates. 



On adding a solution of silver nitrate to the acidified solu- 

 tio'n a slight milkiness appeared. 0.0062 gram of this precip- 

 itate yielded on heating 0.0031 gram of metallic silver, which 

 amount corresponds to the quantity of silver contained in silver 

 dimesoper iodate, Ag 4 I 2 O g + SH^O. However, it could not be 

 this substance, as the solution failed to yield iodine on reduction. 



Since from the manner in which the iodine disappeared, it 

 was believed that the iodine must have changed to some oxygen 

 compound, the effect of strong reducing agents were tried on 

 the solution. Zinc dust was added to the acid solution and 



