A REVISION OF THE ATOMIC WEIGHT OF IODINE. 



109 



monium formate. Sample C was precipitated as silver chloride once, electro- 

 lyzed once, and finally precipitated with formic acid. All three samples were 

 fused in a current of hydrogen on a lime boat. 



In order to determine whether an even more gradual addition of silver nitrate 

 would influence the extent of the occlusion, in Analysis 42 the silver was intro- 

 duced by means of a large funnel provided with a fine spout. 



In one of the foregoing analyses the attempt was made to determine the ratio 

 between iodine and silver iodide by adding an excess of silver nitrate and weigh- 

 ing the precipitate. Although only 5 mg. excess of silver nitrate were added, 

 while the total volume of the liquid was over 2 liters, the precipitate was i mg. 

 heavier than it should have been as calculated from the result of the titration, 

 showing that the occlusion of silver nitrate by the silver iodide may take place 

 even after precipitation. In order to avoid this difficulty, in succeeding anal- 

 yses, the silver iodide was collected without the addition of silver nitrate, after 

 washing with pure water; then the dissolved silver iodide was determined in the 

 filtrate and wash-water, by comparing the precipitates produced by the addi- 

 tion of silver nitrate with those formed in standard solutions of hydriodic acid. 

 As a matter of fact, the filtrates were always found to be practically free from 

 silver iodide, while the wash- water contained from 0.2 to 0,4 of a mg. per liter. 

 This difference in solubility of the silver iodide in the filtrate and wash-waters 

 is probably due to colloidal solubility of this salt in pure water. The loss on 

 fusion of the silver iodide and the asbestos shreds were determined as usual. The 

 almost absolute lack of opalescence in the filtrate when treated with silver 

 nitrate or hydriodic acid is a good proof of the complete absence of chlorine 

 and bromine, since both silver bromide and silver chloride are considerably 

 more soluble than the iodide. 



The Atomic Weight OF Iodine. Series VIII. I:AgI. 



Finally, in order to determine the ratio of silver to silver iodide, the fil- 

 trate and wash- waters were evaporated to very small bulk, until the greater 

 part of the nitric acid had been expelled, and then the amount of silver in the 

 residue was determined nephelometrically, after dilution to 25 c.c, by adding 

 an excess of hydriodic acid and comparing with standard solutions of silver. 



