Anomalous voltammetry at copper in alkaline solution

In alkaline solution a passivating layer is formed at a copper metal surface during an anodic sweep to potentials beyond approx. +0.5V (vs SCE). This layer requires maintenance of oxidising conditions, and on the return scan an anodic current can be seen during the reduction scan between +0.5 and 0V (ie while scanning from the positive limit in a negative direction) as the passivating layer is lost at a potential that still allows oxidation to occur. There is also a curve-crossing phenomenon such that scan reversal before the potential at which substantial passivation becomes evident produces a higher anodic current in the reverse (reductive) direction than during the forward anodic scan before reversal. In addition, although increase in scan rate in mildly alkaline (0.1M NaOH) conditions produces the expected increases in current for both anodic and cathodic processes (although with different scan-rate dependences depending upon the nature of each peak), when this experiment is repeated in stronger alkali (1M NaOH), the anodic and cathodic peak currents in the voltammogram are independent of scan rate between 10 mVsec^-1 and 80 mVsec^-1. All that is observed is some change in shape of the peaks.