Theoretical Analysis of System’s Composition Changes in the Course of Electrolysis of Acidic Chloride Aqueous Solution

Abstract: Variations of the indicator electrode potential under zero current condition, E, and of the (quasi)equilibrium composition of the aqueous solution (which initially contained 0.5 M chloride anion) in the anode chamber of a model electrolytic cell were calculated, provided that the pH of the solution is kept constant (pH 0) and that the same total number of Cl atoms in all chlorine compounds (including the gas phase above it) is maintained. Theoretical analysis is carried out for five different hypotheses regarding the possible depth of electrolysis and the nature of the occurring processes. The hypotheses are: (a) no formation of chlorine compounds with oxidation states above +1, i.e., the electrolysis produced molecular chlorine Cl₂ as a solute and in the gas space above the solution (Formula Presented.) trichloride anion (Formula Presented.) as well as dissolved HClO and ClO^– (in addition to the retaining amount of Cl^–); (b) in addition to the compounds indicated for case (a), chlorine compounds are formed in solution in the +3 oxidation state, i.e. HClO₂ and (Formula Presented.) (c) in addition to the compounds listed for case (b), chlorine compounds are formed in the +4 oxidation state, i.e. dissolved ClO₂ and (Formula Presented.) in the gas phase; (d) the process proceeds with formation of both the chlorate ion (Formula Presented.) and the chlorine compounds of lower oxidation states in solution and in gas phase indicated above for case (c) (Cl^–, (Formula Presented.) Cl₂, (Formula Presented.) HClO, ClO^–, HClO₂, (Formula Presented.) ClO₂, and (Formula Presented.) (e) in addition to the components indicated for variant (d), perchlorate anion (Formula Presented.) can also be formed. All electrochemical and chemical reactions involving the chlorine-containing species which are taken into account within the framework of each of the options (a), (b), (c), (d) or (e) are assumed to be in (quasi)equilibrium state. Predictions for all these five hypotheses on the relationship between the redox-charge of the system, Q (or the average oxidation state of chlorine atoms, x), and the indicator electrode potential, E, as well as on the dependence of the system’s composition (concentrations of all compounds) on either x or E are derived. Approaches for the experimental determining of the variant for the evolution of the Cl-containing anolyte composition in the course of electrolysis are proposed.