Simultaneous electrochemical detection of ozone and free chlorine with a boron-doped diamond electrode

O₃ and free chlorine play significant roles in disinfection and organic degradation. There are numerous reports about their mixed-use, yet detection of the residual concentrations is not easily accomplished, whilst the interactions between them remain unclear. Herein, we develop a detection method using a boron-doped diamond (BDD) electrode to achieve the simultaneous determination of O₃ and free chlorine, which benefits from the unique property of the wide potential window of BDD electrodes. It is indicated that O₃ can always be accurately determined at 0.35 V vs. Ag/AgCl in an acidic solution (pH = 4-5), whether or not free chlorine is present in the solution, whereas free chlorine can be precisely monitored at -0.78 V vs. Ag/AgCl only after the O₃ is completely depleted. Furthermore, in a basic solution (pH = 9-10), the reduction peak of O₃ at 0.57 V vs. Ag/AgCl promptly disappears accompanied by a decrease in the peak current of free chlorine at 1.41 V. All the phenomena observed in the acidic and basic solutions are concurrently confirmed in a quasi-neutral solution. Based on these complementary measurements, a mechanism is proposed, in which the O₃ reduction results in partial oxidation of the BDD surface, hindering the reduction of free chlorine in the acidic mixture; whereas O₃ reacts quickly with free chlorine in the basic solution, which causes the co-consumption of both of them. It is hoped these results will give us a guide as to how better utilize mixtures and more precisely simultaneously determine O₃ and free chlorine in the mixture.