Electrochemically driven evolution of Br-containing aqueous solution composition

Abstract

There exist numerous Br-containing species in various oxidation states between −1 and +5 (Br−, Br3−, Br5−, Br2 in the solute and liquid states, BrO−, HBrO, BrO3−, etc.), which are mutually transformed via numerous protonation/deprotonation, chemical and redox/electrochemical processes. Composition of their aqueous solutions has been analyzed under conditions of thermodynamic equilibrium for various total concentrations of Br atoms inside the system, ctot (0.2–2.0Br-atommoldm−3) as a function of pH (from 2 to 8) and electrode potential. Br− anions are the dominant species at sufficiently negative electrode potentials while bromate anions are mostly present at high positive ones. Concentrations of the species in intermediate oxidation states existing within the medium potential range depend strongly on solution pH. In neutral and alkaline solutions (pH≥6) the transformation between the limiting oxidation states, −1 and +5, takes place in a relatively narrow interval of electrode potentials, relative fractions of intermediate Br-containing species being low. On the contrary, for more acidic solutions the width of this potential interval is becoming broader, and numerous species in intermediate oxidation states are dominant there. Under such conditions, the relative fractions of these intermediate species depend essentially on the total Br-atom concentration in all oxidation states, ctot. Namely, tribromide anion, Br3−, and solute bromine, Br2, play the principal role for low ctot concentrations while the relative contribution of the liquid bromine, Br2liq, increases rapidly for higher ctot concentrations.