Theoretical Analysis of Changes in the System’s Composition in the Course of Oxidative Electrolysis of Bromide Solution: pH Dependence

Abstract

Changes in the indicator electrode potential (at zero current) E and (quasi) equilibrium composition of aqueous solution in the anode chamber of the model electrolyzer, which initially contained 0.5 M concentration of bromide anions, provided that the solution was kept at a constant pH and constant (together with the gas phase above it) total number of Br atoms in all its compounds, are calculated. Theoretical analysis was carried out for four different hypotheses regarding the possible extent of electrolysis and the nature of the processes are theoretically analyzed. They are: (1) no formation of bromine compounds with positive oxidation states occurs, i.e., electrolysis only leads to the formation of molecular bromine in its various forms (the dissolved state of Br2, as well as phases of liquid bromine $${\text{Br}}_{{\text{2}}}^{{{\text{liq}}}}$$ , and bromine vapor in the gas space above the $${\text{Br}}_{{\text{2}}}^{{{\text{vap}}}}$$ solution); (2) oxidation of bromide ions leads to the formation of bromine compounds in its oxidation state up to +1 inclusive; (3) the process proceeds with the formation of both bromate ion (BrO3-) and compounds of bromine with lower oxidation states in solution ( $${\text{Br}}_{{\text{3}}}^{ - },$$ $${\text{Br}}_{{\text{5}}}^{ - },$$ Br2, $${\text{Br}}_{{\text{2}}}^{{{\text{liq}}}},$$ $${\text{Br}}_{{\text{2}}}^{{{\text{vap}}}},$$ BrO–, HBrO); (4) in addition to the components specified in clause (3), the formation of the perbromate anion $$\left( {{\text{BrO}}_{{\text{4}}}^{ - }} \right)$$ is also taken into consideration. All electrochemical and chemical reactions involving bromine-containing species have been taken into consideration in the hypothesis framework of the system’s evolution (1), (2), (3), or (4), are assumed to be in a (quasi)equilibrium state. Predictions for all hypotheses (1), (2), (3), or (4) have been compared at three different pH values of the solution (2, 6 and 10 of Br-containing anolyte composition’s evolution in the course of electrolysis.