174: G. Edgar — lodometric Estimation of Vanadic Acid. 



Art. X. — The lodometric Estimation of Vanadic Acid, 

 Chromic Acid and Iron in the Presence of One Another / 

 by Graham Edgar. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cxciv. ] 



In a previous paper from this laboratory* it has been shown 

 that vanadic and chromic acids may be estimated in the presence 

 of one another by a process based upon the differential reducing 

 action of hydrobromic and hydriodic acids. The present paper 

 will show that the use of processes of differential reduction 

 may be extended to the estimation of three constituents ; in the 

 present case, vanadic acid, chromic acid and iron. 



A preliminary investigation was made in order to determine 

 whether vanadic acid and iron might be estimated iodometrically 

 by distillation, first with hydrobromic acid and last with hydri- 

 odic acid, the liberated halogen being absorbed in potassium 

 iodide and determined after each distillation. In the first dis- 

 tillation the vanadic acid should be reduced to the tetroxide, 

 according to the equation, 



V 2 5 + 2HBr = Vfi 4 + H 2 + Br 2 . 

 In the second distillation, the vanadium tetroxide should be 

 reduced to trioxide and the ferric salt to ferrous salt, according 

 to the equation, 



V 2 4 + Fe 2 3 + 4HI = V 2 3 + 2FeO + 2H 2 + 2l 2 . 

 It is evident that both constituents may be calculated from 

 the amount of halogen liberated in the two reductions. 

 Experiments upon solutions of sodium vanadate and ferric 

 chloride, carried out in the manner to be described later, gave 

 the results shown in Table (I). 



If this process be carried out in the presence of chromic 

 acid the reduction by hydrobromic acid should proceed accord- 

 ing to the equation, 



V 2 5 + 2Cr0 3 + 8HBr = V 2 4 + O 2 3 -I- 4H 2 + 4Br 2 , 

 while the reduction by hydriodic acid should proceed as before, 

 according to the equation, 



V 2 4 + Fe 2 3 + 4HI = V 2 3 + 2FeO + 2H 2 +2l 2 . 



If the halogen liberated in the two reductions be separately 

 determined, we have two equations, the first giving the sum of 

 the vanadic and chromic acids, and the second giving the sum 

 of the vanadic acid and the iron ; the halogen liberated by the 

 vanadium being the same in each case. If then either the 

 vanadium, iron or chromium be estimated separately, we obtain 

 a third equation, from which, with the aid of the first two, all 

 three constituents may be calculated. 



In this investigation an attempt was made to estimate the 

 vanadium in a separate portion of the solution by reduction 



* Edgar, this Journal, xxvi, p. 333, 1908. 



