Abstract
The aim of this study was to gain a better understanding of the feasibility of partial replacement of dichromate, Cr(VI), with phosphate buffer, focusing on the cathode reaction selectivity for hydrogen evolution on mild steel and Fe-Mo cathodes in undivided cell for chlorate production. To evaluate the ability of phosphate and Cr(VI) additions to hinder hypochlorite and chlorate reduction, overall current efficiency (CE) measurements in laboratory cell for chlorate production on stationary electrodes were performed. The concentration of hypochlorite was determined by a conventional potentiometric titration method using 0.01 mol dm-3 As2O3 solution as a titrant. The chlorate concentration was determined by excess of 1.0 mol dm-3 As2O3 solution and excess of arsenic oxide was titrated with 0.1 mol dm-3 KBrO3 solution in a strong acidic solution. Cathodic hypochlorite and chlorate reduction were suppressed efficiently by addition of 3 g dm-3 dichromate at both cathodes, except that Fe-Mo cathode exhibited higher catalytic activity for hydrogen evolution reaction (HER). The overvoltage for the HER was around 0.17 V lower on Fe-Mo cathode than on mild steel at the current density of 3 kA m-2. It was found that a dichromate content as low as 0.1 g dm-3 is sufficient for complete suppression of cathodic hypochlorite and chlorate reduction onto Fe-Mo catalyst in phosphate buffering system (3 g dm-3 Na2HPO4 + NaH2PO4). The overall current efficiency was practically the same as in the case of the presence of 3 g dm-3 dichromate buffer (98 %). However, for the mild steel cathode, the overall current efficiency for the chlorate production was somewhat lower in the above mentioned mixed phosphate + dichromate buffering system (95%) than in the pure dichromate buffering solution (97.5%).
Highlights
The aim of this study was to gain a better understanding of the feasibility of partial replacement of dichromate, Cr(VI), with phosphate buffer, focusing on the cathode reaction selectivity for hydrogen evolution on mild steel and Fe–Mo cathodes in undivided cell for chlorate production
The concentration of hypochlorite was determined by a conventional potentiometric titration method using 0.01 mol dm–3 As2O3 solution as a titrant
The chlorate concentration was determined by excess of 1.0 mol dm–3 As2O3 solution and excess of arsenic oxide was titrated with 0.1 mol dm–3 KBrO3 solution in a strong acidic solution
Summary
The aim of this study was to gain a better understanding of the feasibility of partial replacement of dichromate, Cr(VI), with phosphate buffer, focusing on the cathode reaction selectivity for hydrogen evolution on mild steel and Fe–Mo cathodes in undivided cell for chlorate production. Cathodic hypochlorite and chlorate reduction were suppressed efficiently by addition of 3 g dm–3 dichromate at both cathodes, except that Fe–Mo cathode exhibited higher catalytic activity for hydrogen evolution reaction (HER). For the mild steel cathode, the overall current efficiency for the chlorate production was somewhat lower in the above mentioned mixed phosphate + dichromate buffering system (95%) than in the pure dichromate buffering solution (97.5%). The second and almost unavoidable path represents another Foerster reaction of further anodic oxidation of hypochlorite to the final state: This is known as the reaction of electrochemical chlorate formation [2] and represents the corresponding current losses. In addition to the hydrogen evolution reaction, two other reduction processes could theoretically take place at the cathode and would lead to further current losses: ClO− + H2O + 2e− → Cl− + 2OH−
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