A REVISION OF THE ATOMIC WEIGHTS OF IODINE AND SILVER. 1 23 



every trace of water could be expelled from the pentoxide at this temperature, 

 and since it seemed probable that the amount of water retained would vary 

 with the temperature and time of treatment, the conditions in the diflFerent 

 experiments were made as nearly as possible identical. 



The details of manipulation were as follows: The crystals of iodic acid were 

 powdered in a new smooth agate mortar together with a small quantity, about 

 2 per cent of the weight of the acid, of the phase produced in the first stage in the 

 dehydration. This step was of advantage in catalyzing the first stage in the 

 dehydration, for frequently, when the precaution of thus inoculating the iodic 

 acid was omitted, the temperature reached 130° before dehydration began, 

 whereas when this precaution is taken the water first appears as low as 85°, 

 Since th^ iodic acid itself melts at 110° when in contact with the phase pro- 

 duced at this temperature, it seemed better to prevent the acid from reaching 

 this temperature until the first stage in the dehydration was past. As a matter 

 of fact, preliminary water determinations in the dried material indicated a 

 slightly larger proportion of water in acid which had not been inoculated with 

 the second phase. 



The weighed boat, containing from 6 to 10 gm. of the iodic acid, was next 

 heated in a slow current of air at 90° to 1 10° until the first two thirds of the water 

 of composition had been given off by the iodic acid and had been expelled from 

 the tube by the current of dry air. The temperature was then raised until the 

 second portion of water was given off. Usually the pentoxide began to form 

 at about 220°. There is no evidence of fusion at this point, even if the tempera- 

 ture rises much above 220° before all the remaining water is expelled. After 

 all the water had disappeared the temperature was raised to 240° and main- 

 tained at this point for 4 hours. Then the boat was transferred to the weigh- 

 ing-bottle, allowed to stand in the desiccator with the counterpoise for some 

 time, and was weighed. 



DETERMINATION OF IODINE IN IODINE PENTOXIDE. 



The analysis of the iodine pentoxide for iodine was effected by dissolving the 

 substance in water, reducing the iodic acid solution to hydriodic acid with a 

 suitable reducing agent, and titrating the hydriodic acid against a weighed 

 amount of silver. 



The operation in detail was as follows: After being weighed, the boat with 

 its contents was placed in a large thick-walled flask and immediately covered 

 with about 500 c.c. of the purest water, in which it slowly dissolved. Even when 

 the pentoxide was slightly colored the solution was colorless to the eye and abso- 

 lutely clear. The weighing-bottle was rinsed with water and the rinsings were 

 added to the main solution in the flask. In a number of early experiments the 

 reduction was accomplished by slowly pouring the dilute solution of iodic acid 

 into a solution of a slight excess of sulphurous acid. Very nearly the theoretical 

 amount of the purest silver was weighed out and dissolved in redistilled 



