Mechanism of anodic oxidation of chlorate to perchlorate on platinum electrodes

Abstract

The kinetics and mechanism of the anodic oxidation of chlorate to perchlorate on platinum electrodes have been investigated. The current efficiency for perchlorate formation and the electrode potential have been determined as a function of current density for various solution compositions, flow rates and pHs at 50 ° C. The results have been compared with theoretical relations between the ratio of the current efficiencies for perchlorate and oxygen formation, the electrode potential, the concentration of chlorate at the electrode surface and the current density for various possible mechanisms. It is concluded that the formation of an adsorbed hydroxyl radical is the first step in the overall electrode reaction. The mechanism proposed for the C104 perchlorate and oxygen formation is:The ratio between the current efficiency for perchlorate and that for oxygen formation is determined by the rate of the electrode reaction involving C103 and that for the combination reaction of (OH)a d. List of symbols

Sodium molybdate—a possible alternate additive for sodium dichromate in the electrolytic production of sodium chlorate

“…Meanwhile, dichromate hydrolyses to form HCrO 4 -in equilibrium with H + and CrO 4 2-. This buffering system effectively maintains the pH of the solution during electrolysis at about 6, which gives the highest reaction efficiency of chlorate formation [2][3][4]. However, addition of dichromate brings about several disadvantages; the greatest concern is its carcinogenicity.…”

“…Sodium dichromate is added to prevent cathodic reduction of hypochlorite and chlorate species by formation of a thin film of chromium hydroxide on the cathodic surface; this is one of the key factors in keeping the current efficiency >96% [1,5,6]. Meanwhile, dichromate hydrolyses to form HCrO 4 -in equilibrium with H + and CrO 4 2-. This buffering system effectively maintains the pH of the solution during electrolysis at about 6, which gives the highest reaction efficiency of chlorate formation [2][3][4].…”

“…Dichromate in the chlorate liquor must be less than 0.5% during sodium chlorate crystallization, otherwise the crystal product will exhibit a characteristic yellowish colour. To achieve this, the pH of the crystallizer feed liquor must be lowered, to reduce CrO 4 2-co-crystallization with sulphate during chlorate crystallization [7]. The overpotential of HER on mild steel is also quite high (~500 mV) in the presence of dichromate, which directly affects the specific power consumption.…”

“…In the modern sodium chlorate industry the few remaining opportunities to improve cathode performance are reduction in hydrogen evolution overpotential and elimination of dichromate [1][2][3][4]. Sodium dichromate is added to prevent cathodic reduction of hypochlorite and chlorate species by formation of a thin film of chromium hydroxide on the cathodic surface; this is one of the key factors in keeping the current efficiency >96% [1,5,6].…”

Physical properties of aqueous solutions containing NaClO3 and/or NaClO4

“…Mass transfer of chlorate to the anode strongly affects the current efficiency for perchlorate formation [2]. To correlate experimental data on the mass transfer coefficient for chlorate in solutions of different compositions, the density and the dynamic viscosity for aqueous NaC103 and NaC104 solutions are required.…”