4:18 Peirce — Gravimetric Determination of Selenium. 



Later experiments under similar conditions, excepting that 

 the volume of the liquid in which the precipitation took place 

 was very much increased, so that the tendency on the part of 

 the selenium to include the iodide might be diminished, gave 

 errors entirely within satisfactory limits though always posi- 

 tive. 





Se, taken 













as Se0 2 



Se found 



KI 



Volume 



Error 





grm. 



grm. 



grm. 



cm 3 . 



grm. 



(10) 



0-2853 



0-2861 



7 



900 



0-0008 + 



(11) 



0-3189 



0-3192 



8 



400 



0-0003 + 



(12) 



0-3318 



0-3324 



7 



500 



0-0006 + 



(13) 



0-3798 



0-3805 



7 



500 



0-0007 + 



(14) 



0-4252 



0-4259 



7 



350 



0-0007 + 



(15) 



0-4430 



0-4434 



10 



450 



0-0004 + 



It is sufficient to dilute to 400 cm 3 before acidifying with 

 hydrochloric acid and adding potassium iodide to an amount 

 about three grams in excess of that actually required. Boiling 

 for 10 to 20 minutes will change the selenium to the black 

 modification and remove most of the iodine. The process of 

 precipitation and filtering can be completed in half an hour. 

 The selenium is dried at 100° to a constant weight. 



When the selenium occurs in the higher form of oxidation 

 the reduction follows the same course, though iodine is not lib- 

 erated until the solution is quite warm ; but at the end of the 

 usual time of boiling the action is complete. The following 

 shows the results obtained by acting in the manner described 

 on selenic acid. 



Se0 3 taken 



Se taken 



Se found 



KI 



Volume 



Error. 



grm. 



grm. 



grm. 



grm. 



cm 3 . 



grm. 



0-1709 



0-1063 



0-1065 



5 



500 



0*0002 + 



0-1709 



0-1063 



0-1062 



5 



375 



o-oooi — 



0*3231 



0-2010 



0-2017 



5 



350 



0-0007 + 



0*5005 



0-3115 



0-3126 



6 



500 



0-0011 + 



Evidently this method will not distinguish between selenious 

 and selenic acids, but it will be found of much value in point 

 of time saved in the determination of either separately, or of 

 the total selenium in case both occur together. 



The kindly advice and suggestions of Prof. F. A. Gooch are 

 gratefully acknowledged. 



