BAXTER. — A REVISION OF THE ATOMIC WEIGHT OF IODINE. 421 



ture iu the air has been shown by Stas to absorb considerable oxygen.* 

 Silver prepared iu a similar fashion by investigators in this laboratory 

 has always been found to be pure,t and since two different samples 

 were used in the following work with identical results, there can be no 

 doubt of the purity of that employed here. Furthermore, one syn- 

 thesis of silver chloride, carried out with a portion of this silver by 

 Wells in the investigation of Richards and Wells upon the atomic 

 weight of chlorine, yielded the value 35.467, which is identical with the 

 average of other similar determinations made with his own silver. 



The chief impurities in commercial iodine are halogens of lower 

 atomic weight and iodide of cyanogen. These were removed by dis- 

 solving iodine in a strong solution of half its weight of potassic iodide, 

 and distilling the greater portion of the iodine from a retort into a flask 

 cooled with cold water. The iodine thus obtained was next converted 

 into hydriodic acid by covering it with considerable water and passing 

 through the solution a stream of sulphuretted hydrogen. This gas was 

 generated by the action of dilute sulphuric acid upon ferrous sulphide, 

 and was purified by bubbling through three gas washing bottles contain- 

 in<r water and through two towers filled with beads moistened with 

 water. The reaction between the sulphuretted hydrogen and the iodine 

 results chiefly in the formation of hydriodic acid and the precipitation of 

 sulphur, although a small quantity of sulphuric acid is always produced. 

 Iodide of cyanogen Avith sulphuretted hydrogen yields hydrocyanic acid, 

 hydriodic acid, and sulphur.J The solution was first boiled for a short 

 time until the sulphur had clotted together, and the sulphur was removed 

 by filtration. The clear solution was then boiled for several hours in 

 order to eliminate the hydrocyanic acid. § Finally the hydriodic acid 

 was partially converted into iodine by distilling the solution with potassic 

 permanganate. This permanganate must also have had the effect of 

 oxidizing any organic impurities. Since in the reaction three eighths of 

 the iodine remain in the form of iodides, the resulting iodine was thus 

 subjected to a second distillation from an iodide. The product was 

 again converted into hydriodic acid by means of hydrogen sulphide, and 

 this again into iodine by recrystallized potassic permanganate which was 

 free from even a trace of chlorine. Since in the case of bromine it has 

 already been shown that two distillations from a pure bromide are 



* CEuvres Completes, 3, 125. 



t Proc. Amer. Acad., 28, 22 ; 29, 64 ; 31, 173; 39, 249. 



t Dainnier, Ilandb. d. anorg. Cliein., II, 1, 432. 



§. Richards and Singer, Amer. Chem. Jour., 27, 205. 



