416 JPeirce — Gravimetric Determination of Selenium. 



it appeared subsequently that the elements having colorless 

 ions had come together. And in Thomsen's system the same 

 was also true of the elements having colored ions. 



5. While there is good reason for believing that in solution 

 the ions are separated so as to no longer affect each others' 

 vibrations (see sec. 1, supra), it is also certain that they remain 

 within each others' range of influence, so that they cannot be 

 considered as free. Fitzgerald has shown that this conclusion 

 is in conformity with theory and experimental evidence has 

 been given above, proving that it is also in conformity with 

 fact. 



Aet. XLYIII. — The Gravimetric Determination of Sele- 

 nium ; by A. W. Peirce. 



[Contributions from the Kent Chemical Laboratory of Yale College. — L.] 



The method generally in use for the gravimetric determina- 

 tion of selenious acid is to precipitate the selenium with sul- 

 phurous acid in presence of hydrochloric acid and to weigh the 

 elementary selenium. Precipitation by this method, however, 

 is slow and incomplete in many cases, so that it is always neces- 

 sary to treat the filtrate a second time with sulphurous acid and 

 to digest for some time. To obviate the necessary delay in 

 this process, I Jhave tried the effect of substituting potassium 

 iodide as the reducer instead of the sulphurous acid, adopting 

 the idea from several recent volumetric methods for the deter- 

 mination of selenium* in which an iodide in acid solution is 

 used to reduce the selenious acid, and in which the liberated 

 iodine, caught in various ways, is titrated and taken as a 

 measure of the selenium. 



Varying amounts of selenium dioxide prepared according to 

 the method described in former articles, by dissolving pure 

 selenium in nitric acid, removing any selenic acid formed by 

 barium hydroxide, and subliming in a current of dry oxygen, 

 were dissolved in Erlenmeyer flasks, and the solution was 

 acidified with hydrochloric acid. Potassium iodide was added 

 and the selenium was precipitated in the form of a red powder. 

 Boiling for ten minutes served to remove most of the liberated 

 iodine and to change the selenium into the black modification, 

 which was collected upon an asbestos felt, washed, dried at 

 100° to a constant weight, and weighed. Early experiments 



* Muthman and Schaefer, Ber. d. d. chem. (resell., xxvi, 1008; Gooch and Rey- 

 nolds, this Jour., 1, 254; Gooch and Peirce, this Jour., fourth series, i, 31. 



