34 



Gooch and Peirce — IodometriG Determination, etc. 



capacity and weighed, 1 grm. of potassium bromide was added, 

 and sulphuric acid in such quantity that the total amount of 

 the free acid should correspond to 20 cm 3 of the acid of half- 

 strength. The solution possessing a volume of 60 cm 3 to 

 100 cm 3 was boiled until the clear, colorless solution left when 

 the bromine vanished began to color again. Experience 

 showed that the reappearance of the brownish color is very 

 easily seen and that it is not safe to conclude that all free 

 bromine has been eliminated, under the conditions of dilution 

 and proportion, until this stage of concentration — which corre- 

 sponds to a volume of about 35 cm 3 — has been reached ; but 

 the distillation should not be poshed beyond the point at which 

 the returning color is noted. When this condition was reached 

 the solution was cooled, and treated exactly in the manner 

 described for the reduction of selenious acid. The neutraliza- 

 tion by acid potassium carbonate, after the final boiling, gener- 

 ally occasioned the precipitation of manganous carbonate, but 

 the precipitate did not interfere in the slightest with the 

 titration which followed. 



The following table comprises the determinations which 

 were made to test the accuracy of the iodometric determina- 

 tion of selenic acid by the combined processes of reduction. 



KI used 

 Se0 2 taken in second SeO a 



as HoSe0 4 . reduction. found. Error 



grm. grm. grm. grm. 



0*0378 0-6306 0*0380 0*0002 + 



0-0378 0-5643 0'0374 0*0004- 



0-0516 0-7136 0-0517 0001 + 



0-0503 0-7302 0*050S 0*0005 + 



0-0541 0*6671 0-0544 0*0003 + 



0-1007 1-3277 0-1011 0-0004 + 



0*1008 1*3277 0*1011 0*0003 + 



0-1007 1*2082 0*1005 0*0002 — 



0-1007 1*1684 0*1016 0*0009 + 



0*1007 1*0522 0-0999 0*0008 — 



0*1009 1*2679 0*1005 0*0004 — 



0*1031 1*1119 0*1032 0*0001 + 



0*1870 1*8720 0-1879 0*0009 + 



0-2014 1*9915 02020 0'0006 + 



0*2016 2*0745 0*2025 0*0009 + 



0*2059 1*8687 0*2064 0*0005+' 



It is plain, therefore, that selenic acid may be determined 

 iodometrically with accuracy by first reducing it to the condi- 

 tion of selenious acid by treatment with potassium bromide in 

 presence of sulphuric acid, in the manner described, and then 

 completing the reduction to the elementary condition by the 

 treatment with potassium iodide and potassium arseniate. 



