p. E. Browning— Reduction of Vanadic Acid. 187 



at the end of about fifteen minutes the iodine has ceased to 

 bleach, showing the oxidation to be complete. I have gener- 

 ally allowed the flasks to stand a few minutes longer to be sure 

 of a complete oxidation. The excess of iodine is then destroyed 

 by standard arsenious acid solution, against which the iodine 

 has been previously standardized, starch added and the color 

 brought back with a few drops of iodine. The amount of 

 arsenious acid solution used for the bleaching of the iodine, in 

 terms of iodine subtracted from the whole amount of iodine used, 

 gives the amount of iodine necessary to oxidize the vanadium 

 from the condition of tetroxide to that of pentoxide. The 

 results follow in the table. 





72O5 taken. 



V2O5 found. 



Error. i 



\mount KI. 



H2so4e-o 





grm. 



grm. 



grm. 



grm. 



cml 



(1) 



0-1699 



0-1690 



0-0009 — 



1 



10 



(2) 



0-1704 



0-1699 



0-0005 — 



1 



10 



(3) 



0-1706 



0-1700 



0-0006 — 



1 



10 



(4) 



0-1702 



0-1692 



0-0010- 



1 



10 



(5) 



0-3613 



0-3620 



0-0007 + 



1-5 



10 



(6) 



0-1805 



0-1803 



0-0002- 



1 



10 



m 



0-3614 



0-3620 



0-0006 + 



1-5 



10 



(8) 



0-1811 



0-1814 



0-0003 + 



1 



10 



(9) 



0-1807 



0-1815 



0-0008 + 



1 



10 



(10) 



0-3613 



0-3620 



0-0007 + 



1-5 



10 



(11) 



0-3679 



0-3674 



0-0005- 



1-5 



10 



(12) 



0-3612 



0-3608 



0-0004- 



1-5 



10 



(13) 



0-2893 



0-2907 



0-0014— 



1-5 



10 



(14) 



0-3456 



0-3448 



0-0008 — 



1-5 



10 



(15) 



0-3453 



0-3448 



0-0005- 



1-5 



10 



(16) 



0-3907 



0-3912 



0-0005 + 



2 



10 



(17) 



0-3908 



0-3898 



0-0010- 



1 



10 



(18) 



0-3906 



0-3921 



0-0015 + 



2 



10 



(19) 



0-3909 



0-3912 



0-0003 + 



1-5 



10 



The results are on the whole satisfactory, the average error 

 of all the determinations being less than 0*0002—. It will be 

 noticed that large amounts of potassium iodide tend to give 

 plus errors, probably due to the tendency of the hydriodic acid 

 to hold iodine. 



The action of the hydrobromic acid was tried in exactly the 

 same manner. It was found that when the residual volum© of 

 the liquid in the flask reached 25^°^' the blue color appeared, 

 and the absence of free bromine was proved by holding a paper 

 moistened with potassium iodide in the steam. If the boiling 

 is not carried to the point indicated, where the blue color 

 appears, the results come low, showing incomplete reduction. 

 The results follow in the table. 



