Il8 RESEARCHES UPON ATOMIC WEIGHTS. 



The method actually employed was to reduce to hydriodic acid weighed 



amounts of iodine pentoxide, and to determine the weight of silver necessary 



sAg 

 to combine with the iodine. If the ratio of silver to iodine pentoxide, — ^ , is 



multipUed by that of iodine to silver, — , the per cent of iodine in the pent- 



Ag 



2I 

 oxide is obtained, and thence the ratio — • 



5O 



PURIFICATION OF MATERIALS FOR THE FIRST SERIES OF 



ANALYSES. 



IODIC ACID. 



Iodine pentoxide may be readily prepared by dehydration of iodic acid at a 

 comparatively low temperature. Of the various methods for preparing the 

 latter substance, by far the most satisfactory for the present purpose is the 

 oxidation of iodine with fuming nitric acid. The volatile by-products of the 

 reaction, oxides of nitrogen and water, as well as the excess of nitric acid, may 

 be removed by heating, and the acid may be crystallized from aqueous solution. 



Iodine was purified for most of the preparations by the methods already 

 described by one of us.^ These consisted usually in a preliminary distillation of 

 commercial iodine from solution in commercial aqueous potassium iodide. 

 The iodine was then twice reduced to hydriodic acid by hydrogen sulphide 

 and water, and the hydriodic acid was subsequently oxidized by very nearly 

 the theoretical quantity of recrystaUized permanganate, with intermediate 

 boiling of the hydriodic acid solution to eliminate cyanogen as hydrocyanic 

 acid.^ In this way, since only five-eighths of the iodine is set free from hydriodic 

 acid by permanganate, the iodine was three times distilled from a solution of an 

 iodide, the iodide being of greater purity in each succeeding distillation. It has 

 already been shown that three such distillations are sufficient to remove com- 

 pletely chlorine and bromine,^ so that there can be no question of the freedom of 

 the resulting iodine from those closely related elements. The final product 

 was once distilled with water into a Jena glass flask cooled with running water, 

 and the resulting solid was washed with the purest water upon a porcelain 

 Gooch crucible containing no mat. 



Pure fuming nitric acid was prepared by distilling the best commercial 

 product several times, with rejection of the first and last thirds, until the distil- 

 late, after dilution, gave no test for chloride with silver nitrate in a nephelo- 



1 Baxter: Froc. Amer. Acad., 40, 421 (1904); Jour. Amer. Chcm. Soc, 26, 1579; Zdt. anorg. 

 Chem., 43, 16. (See page 92.) 



2 Richards and Singer: Ajncr. Chem. Jour., 27, 205 (1902). 



' Baxter: Loc. cit.; also Proc. Amer. Acad., 41, 73 (1905); Jour. Amer. Chevi. Soc, 27, 876; 

 Zeit. anorg. Chem., 46, 36. 



