CHEMISTRY — BAXTER. 189 



Baxter, Gregory P., Harvard University, Cambridge, Massachusetts. Grant 

 No. 451. Researches upon atomic weights. (For previous reports see 

 Year Books Nos. 3, 4, 5, and 6.) $1,000. 



The analysis of iodine pentoxide, commenced by Mr. G. S. Tilley, has 

 been continued by Professor Baxter. Iodic acid, made by the action of pure 

 fuming nitric acid upon the purest iodine in vessels of fused quartz, was 

 purified by crystallization in quartz vessels. Portions of about 10 grams of 

 the acid were carefully dehydrated in a platinum boat in a current of dry air, 

 the temperature finally reaching 240° C., and the resulting iodine pentoxide 

 was weighed. Then it was dissolved in water, reduced in dilute nitric acid 

 solution by means of hydrazine, and finally it was precipitated in about 

 thirtieth normal solution with a nitric-acid solution of a weighed amount of 

 silver, a very few tenths of a milligram in excess of the necessary quantity 

 being used. This excess of silver was determined gravimetrically after 

 evaporation of the entire filtrate to small volume. It was found impossible 

 to use sulphurous acid as the reducing agent, owing to occlusion of silver 

 sulphate. The ratio of silver to iodine pentoxide was found in seven experi- 

 ments to be 0.646212. Assuming the iodine pentoxide to have been pure and 

 using the ratio of silver to iodine already determined by Baxter, 0.848843, the 

 atomic weight of silver referred to oxygen 16.000 is calculated to be 107.837, 

 which is undoubtedly a minimum value. 



However, the iodine pentoxide was found to contain traces of moisture, 

 and during the present year the water content of the pentoxide after drying 

 at 240° was first thoroughly investigated. Weighed quantities of the pen- 

 toxide were decomposed by heating in a current of dry air, and, after removal 

 of iodine by condensation and then by a layer of hot silver, the water was 

 collected in a phosphorus pentoxide tube and weighed. The water content of 

 the iodine pentoxide was found to vary only slightly when the conditions of 

 heating the substance were somewhat varied, and was constant at 0.023 per 

 cent, when the pentoxide was heated as in the analyses for iodine. If a 

 correction for this moisture is applied, the atomic weight of silver becomes 

 107.847. 



The possibility of the adsorption of air upon the surface of so porous a 

 material as iodine pentoxide which has been made by efflorescence of iodic 

 acid led to experiments to test this point. From the gain in weight when 

 air was admitted to an exhausted tube containing iodine pentoxide, the 

 density of the pentoxide was found to be almost exactly that found by dis- 

 placement of toluol. If the pentoxide adsorbs air, a lower density is to be 

 expected when determined by displacement of toluol. 



Finally, a second series of analyses of iodine pentoxide by titration against 

 silver were carried out, with new samples both of iodic acid and of silver. 



13— YB 



