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The electrochemical production of ferrate ions from iron anodes in alkaline solutions Tuffrey, Nigel Edwin


An investigation into the electrochemical production of ferrate ions by anodic dissolution of iron anodes in strong alkaline electrolytes has been carried out. The stability of ferrate ions in alkaline solutions and the dependence of the decomposition reaction on temperature, hydroxyl ion concentration and ferrate ion concentration were determined. The stability of ferrate ions is favoured by low temperatures and ferrate ion concentrations, and high hydroxyl ion concentrations. Particulate ferric hydroxide is catalytic to the ferrate decomposition reaction. An electrochemical cell was designed to measure the rate of ferrate production. Using this cell and other techniques the efficiency of ferrate production as a function of the quantity of charge passed, the superficial current density and the hydroxyl ion concentration wane determined. The efficiency of ferrate formation is low and declines gradually to zero as increasing quantities of charge are passed. The efficiency of ferrate formation is independent of the superficial current density but declines rapidly as the hydroxyl ion concentration is reduced. A limited quantity of ferrate can be produced from an anode. There are indications that periodic current reversal can prevent the steady decline in the ferrate production rate. The oxide or hydroxide layer which formed on the anode during electrolysis could not be positively identified. This layer was x-ray amorphous, thickened slowly and an outer layer dissolved or spalled as the layer grew. The anode behaviour was examined using potentiodynamic, potentiostatic and constant current techniques. Transient anode behaviour was observed during the period of ferrate formation. It is proposed that the rate of ferrate production and the decline in the efficiency of ferrate formation with increasing quantities of charge passed can be explained, by: 1) the involvement of the oxide or hydroxide layer on the anode in the ferrate formation mechanism, and 2) changes which occur in the chemical or physical properties of the anode layer during electrolysis.

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