Oxidation-enhanced diffusion decrease with increasing concentration

Tarnishing is enhanced if the electrochemical potential is increased either by oxidizing species other than oxygen in the solution or by an externally applied potential the rate is decreased if the potential is decreased. The rate of formation and final thickness of the tarnish increases with an increase in the zinc content of the brass, an increase in the complex-ion concentration in the solution and an increase in temperature. The effect of the zinc concentration in the alloy has been attributed to the selective depletion of zinc at the surface, creating a surface with enhanced reactivity for the formation of Cu20. The selective depletion of zinc is readily related to the greater electrochemical activity of zinc relative to copper, but is confined to a few atom layers at the surface because of the slow rate of solid-state diffusion at practical service temperatures (Ref 146). The effect of the zinc content of the brass also has been attributed to the reaction (Ref 142, 147) ... 

Response to Catechols in the Presence of Ascorbic Acid. In addition to the enhanced response for most catechol compounds, the voltammetric signals due to species in solution that are not complexed by the polymer are often diminished. Because the solvent-swollen polymer occupies space near the electrode surface, it effectively decreases the concentration of uncomplexed solution species. Furthermore, the polymer hinders diffusion of all species to the electrode surface. In the case of catechols, the increase in concentration in the film offsets this effect, but for species that do not bind with the polymer (e.g. ascorbic acid), the rate of mass transport (and subsequently the oxidation current monitored) is attenuated. This effect can be very useful when determining catechol in biological samples. 

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