COPPER DISSOLUTION IN PERSULPHATE ENVIRONMENTS AT CATHODIC POTENTIALS
Abstract
The paper studies the dissolution of a copper electrode at cathodic potentials in a solution containing persulphate ions as an oxidant of S2O82-. The investigation of the nature of interference of partial electrode processes which was carried out earlier for a number of metals in solutions with oxygen-containing oxidants (O2, H2O2, NO2-, Cr2O72-, etc.) shows that the crucial role in this phenomenon is played by highly active intermediates of different nature, the hydroxyl ions OH-, in particular. They are generated in the reaction zone (at the metal/solution interphase) during the reduction of the oxidant. As a result, the activation energy of the process of the metal ionization decreases. Moreover, under particular conditions it is accompanied by the transfer of the free electrochemical energy from the cathodic process to the process of the metal oxidation. As a consequence, this transfer demonstrates an apparent violation of the laws of electrochemical thermodynamics since the metal gains an ability to oxidise in the immune area of electrode potentials. The specific feature of persulphate ions is that their reduction is accompanied with the formation of sulphate ions instead of hydroxyl ions. The study was carried out with a stationary copper electrode in deoxygenated solutions of i) 0.2 M Na2SO4 + 0.01 M (NH4)2S2O8 + 0.001 M H2SO4 and ii) 0.2 M Na2SO4 + 0.001 M H2SO4 for comparison. Both solutions were stirred at a constant rate. The chosen potential range was between 0.20 and 0.30 V where the cathodic potentials range was between 0.00 and 0.30 V. The concentration of Cu2+ ions which were formed in the work space of the cell after 30 minutes of cathodic polarization served as a measure of metal dissolution. It was revealed that for this time interval, a Cu2+ concentration of 0.5 – 0.6 μM was achieved in the solution with the (NH4)2S2O8 oxidant. However, in the solution without the oxidant the concentration of Cu2+ was up to 0.1 μM under the same polarization conditions. In accordance with these concentrations the average rates of copper dissolution were 1.2 – 1.5 and ~ 0.1 μA/cm2. Therefore, in the cathodic range of potentials the metal ionization is conjugated with the reduction of persulphate ions. The effectiveness of conjugation is low. Between 55 and 250 ions of S2O82- are required to form one Cu2+ ion. The interpretation of the phenomenon is based on the idea of a stage reduction of persulphate with the formation of highly active radical as one of intermediates of (SO4-)*. The latter provides the energy transfer from the cathodic reaction to the process of copper atom ionization.
ACKNOVLEDGMENTS
The results of the research were obtained using the equipment of VSU's Shared Scientific Equipment Centre.
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References
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