Selenious and Selenic Acids. 



33 



Se = 79-1, 0= 16 







H 2 S0 4 



Di-hydrogen- 











Initial 



Final 



half- 



potassium 



KI 



Se0 2 



SeO a 





volume. 



volume. 



strength. 



arseniate. 



taken. 



taken. 



found. 



Error. 



cm 3 . 



cm 3 . 



cm 3 . 



grm. 



grm. 



grm. 



grm 



grm. 



100 



35 



20 



2 



1-3277 



0-1280 



01275 



0-0005- 



it 



" 



u 



" 



1-0429 



0-0998 



0994 



0-0004- 



u 



u 



l( 



u 



1-0887 



0-1024 



0-1028 



0-0004 + 



'' 



u 



" 



11 



1-0405 



0-1036 



01028 



0-0008 — 



(( 



" 



u 



f< 



1-0721 



0-1030 



01029 



o-oooi — 



u 



II 



ti 



" 



0-9958 



0-1273 



01272 



0001 — 



125 



It 



" 



" 



2-0828 



0-1997 



0-2000 



0-0003 + 



" 



it 



" 



It 



2-2272 



0-2110 



02113 



00003 + 



II 



u 



u 



" 



2-1535 



0-2067 



0-2069 



0-0002 + 



150 



40 



" 



II 



2-6554 



0-2560 



02549 



o-ooii— 



175 



35 



K 



II 



3-2428 



0-3110 



0-3118 



0-0008 + 



it 



35 



" 



11 



3-2428 



0-3085 



0-3083 



0-0002- 



Obviously the reduction of selenious acid by this method is 

 regular and accurate. 



When similar treatment was applied to selenic acid it became 

 apparent that the arsenic acid attacked and destroyed the 

 iodide before the selenic acid had been completely reduced. It 

 is plain, therefore, that the selenic acid must be reduced to the 

 condition of selenious acid before its estimation by the iodide 

 method can be attempted. Ordinarily the simplest mode of 

 reducing selenic acid is by boiling it in solution with hydro- 

 chloric acid of definite strength,* but in this case the presence 

 of hydrochloric acid is precluded on account of the consequent 

 volatilization of arsenious chloride during the process of con- 

 centration in the subsequent treatment with the. iodide. It 

 has been shown, however, in a recent paper from this labora- 

 toryf that selenic acid is easily and completely reduced to 

 selenious acid by potassium bromide and sulphuric acid under 

 defined conditions. Moreover, arsenious bromide is not volati- 

 lized appreciably under the conditions. We made the attempt, 

 therefore, to effect the iodometric determination of selenic 

 acid by first reducing it to selenious acid by the bromide 

 process and then treating the residue with arsenic acid and 

 potassium iodide in the manner described. 



Selenic acid was prepared in standard solution by treating a 

 known weight of pure resublimed selenium dioxide by a strong 

 solution of potassium permanganate, in presence of a moderate 

 amount of sulphuric acid, until the purple color was distinctly 

 visible, dissolving the precipitated oxide of manganese by oxalic 

 acid, again adding permanganate until the final color of faintly 

 visible pink was permanent for a half-hour or more, and dilut- 

 ing to a fixed volume. Portions of the solution of selenic acid 

 were measured into counterpoised Erlenmeyer flasks of 300 cm 3 



* Gooch and Evans, this Journal, 1. 400. 

 f Gooch and Scoville, this Journal, 1. 402. 



Am. Jour. Sci. — Fourth Series, Yol. I, No. 1. — January, 1896. 

 3 



