184 Gooch and Peirce — Method for the 



receiver in the first stage of the process, together with the 

 small amount remaining in the flask, was titrated with sodium 

 thiosulphate and taken as the measure of the selenium dioxide 

 acted upon originally. In this way we obtained the following 

 results : 





Se0 2 corresponding 





Se0 2 



to iodine found in 





taken. 



the distillate. 



Error. 



grm. 



grm. 



grm. 



0-0366 



0372 



0-0006 + 



0366 



0-0377 



0-0011 + 



0-1098 



0-1090 



0-0008 — 



0-1098 



0-1101 



0*0003 + 



It is obvious from these results that the iodine fiually found in 

 the receiver is actually an accurate measure within reasonable 

 limits of the selenium dioxide originally put into V 1 and 

 volatilized therefrom by the action of phosphoric acid and 

 potassium bromide. 



When tellurium dioxide is subjected to similar treatment 

 the phenomena are different. The solution containing the 

 tellurous acid, potassium bromide, and phosphoric acid, in the 

 proportions used in the experiments with selenious acid, colors 

 at about the same degree of concentration at which the solu- 

 tion containing the selenious acid began to darken. As the 

 concentration progresses the color deepens, ruby red crystals 

 (probably hydrated tellurium tetrabromide) form, which accu- 

 mulate upon the walls of the flask and turn yellow, and when 

 the volume of the solution is diminished to 15 cm 3 a green 

 vapor begins to distil. During the process no iodine is set free 

 in the Drexel bottle, and upon stopping the boiling we found 

 that the addition of potassium iodide to Y 2 liberated no iodine, 

 even when the boiling had gone so far that a trace of the green 

 vapor had condensed and run into the water in the flask. In 

 view of these facts it seemed probable that the process of 

 treatment which we have described might be applied to the 

 determination of selenium associated with tellurium. In the 

 following experiments, made to test the point, tellurium dioxide 

 was weighed out and dissolved in strong potassium hydroxide, 

 the alkali was neutralized and the precipitate thus formed was 

 redissolved by phosphoric acid, and 20 cm 3 of the acid of 

 sp. gr. 1*70 were added in excess. To the solution were added 

 definite amounts of selenium dioxide taken in a standardized 

 solution and 1 grm. of potassium bromide, and the whole was 

 introduced into the first flask, Y 1 , with enough water to make 

 the entire volume of the solution 50 cm 3 . The second flask, 

 Y 2 , contained 10 cm 3 of water, and the Drexel bottle and trap 

 were charged with a solution of potassium iodide. Carbon 



