54: Gooch and Blumenthal — Use of Selenic Acid. 



Art. VII. — The Use of Selenic Acid in the Determination 

 of Bromine Associated with Chlorine in Haloid Salts / by 

 F. A. Gooch and P. L. Blumenthal. 



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



When an aqueous solution of bromine is boiled, reaction 

 takes place in slight degree between that element and the 

 water, according to the equation 



3Br 2 + 3H Q O^Z±5HBr + HBr0 3 . 



After the greater part of the bromine has been removed 

 from the system by boiling, a small portion may still remain, 

 in stable equilibrium for a given concentration, as shown in 

 the equation. In further concentration by boiling, more water , 

 and free bromine are removed, bromic acid reacts with hydro- 

 bromic acid to form bromine, according to the reverse action 

 above, and a new equilibrium is established for each new con- 

 centration. Eventually, all the bromic acid might be reduced, 

 and all the bromine be liberated as such 7 but for the fact that 

 hydrobromic acid also tends to volatilize from the solution. 



The satisfactory separation of bromine from chlorine by the 

 action of differential oxidizers upon the haloid salts depends 

 upon the realization of several conditions ; first, the presence 

 of a suitable reducing agent in the solution, so that if a bromate 

 is formed, it may be instantaneously decomposed ; second, the 

 prevention of the volatilization of hydrobromic acid, by work- 

 ing in sufficiently dilute solution, by condensing the steam with 

 a reflux, or by passing the vapors given off through a fresh 

 solution of the oxidizer ; third, the choice of an oxidizer which 

 liberates bromine from bromides with the formation of a 

 reduction product not appreciably active toward bromine under 

 the final conditions. No substance is known which accom- 

 plishes the separation at all concentrations, and it is, therefore, 

 necessary to select a suitable oxidizer which will decompose 

 bromides without attacking chlorides in a solution of regulated 

 concentrations. In such action the differential margin must 

 be sufficiently large to admit of the practical discontinuance 

 of the oxidation after the bromine is completely removed, and 

 before the hydrochloric acid is attacked. The concentration 

 of free acid in the solution is of especial importance, since it 

 controls the rate of formation and volatilization of the halogen 

 acids, and the rate of liberation of bromine. 



Selenic acid (H 2 Se0 4 ) may react with chlorides* and with 

 bromides according to the reversible reactions 



2HC1 + H 2 Se0 4 ^^:H 2 Se0 3 + H 2 + CI, 

 2HBr + H 2 Se0 4 ^Z>H 2 Se0 3 + H 2 + Br 2 

 * Petterson, Zeitschr. anal. Chem., xii, 287. 



