in Titrations by Sodium Thiosulphate. 



293 



acid, and the iodine set free is titrated with sodium thiosul- 

 phate. In this procedure the thiosulphate is taken up by the 

 iodine as it is added to the solution, so that the danger of any 

 action between the thiosulphate and the acid is out of the 

 question. It was shown in a former paper from this labora- 

 tory * that this simple method is inaccurate on account of the 

 incompleteness of reduction in the cold and in presence of the 

 iodine evolved. In a later article also from this laboratory f 

 it was shown that selenium may be completely precipitated 

 and determined with accuracy gravimetrically provided the 

 amount of potassium iodide' employed is enormously in excess 

 of that theoretically required. This suggests naturally the trial 

 of very large excesses of potassium iodide in the process of 

 Muthmann and Shafer. The details of experiments made in 

 this manner are given in the following table. 









Table VII. 















HC1 









Se0 2 





Vol. of 



sp. gr. 



Se0 2 







used. 



KL 



solution. 



(112). 



found. 



Error. 





grm. 



grm. 



cm 3 



cm 3 



grm. 



grm. 



(1) 



•0553 



10 



150 



10 



•0558 



•0005 + 



(2) 



•0574 



5 



u 



a 



•0567 



•0007 — 



(3) 



•0683 



5 



« 



a 



•0683 



•0000 ± 



(*) 



•0487 



5 



a 



a 



•0484 



•0003 — 



(5) 



•2617 



10 



a 



u 



•2589 



•0028 — 



It is obvious that for small quantities of selenium dioxide the 

 accuracy of the process is very much increased by the use of 

 large amounts of iodide, though, of course, the difficulty in 

 reading the end reaction due to the presence of precipitated 

 red selenium still remains ; but the process is still inaccurate 

 when large amounts of selenium dioxide are employed. 



In conclusion I wish to thank Prof. F. A. Gooch for his 

 kind advice and assistance. 



*Tbis Journal, Gooch. and Reynolds, vol. 1, 254. 

 f Peirce, this Journal, vol. i, 1896, p. 416. 



