When the amount of mercury present is quite large, the metal ion cannot penetrate the electrode completely, so, initially, the situation is equivalent to working with a pure mercury electrode; this gives rise to the typical plateau of PSA curves, the potential of which can be calculated by applying the Nernst equation to the HgI4 2–/Hg system. Once the entire mercury surface is depleted or if the amount deposited is small enough to penetrate the gold electrode, the redox process may be limited by the kinetics of transfer from the bulk electrode to the interface.
This phenomenon, together with slower transfer kinetics than those of the chemical oxidation reaction, accounts for the appearance of kinetic curves after the typical plateau (or even from the beginning of stripping if the amount of deposited mercury is rather small).
This interpretation is supported by the fact that only the copperized graphite electrode gives rise to the expected plateau: that for copper followed by that for mercury.
As the copper has already been stripped by the time the mercury is, the latter does not need to diffuse through the electrode, so it is only reoxidized in a step controlled by the chemical process and diffusion of the oxidant.
The shape of the kinetic curves obtained, with two distinct slopes, can be explained by assuming the electrode potential to be determined first by the mercury concentration at the electrode surface, which in turn depends on its rate of diffusion through the gold. Once all the mercury has been oxidized, the slope changes again and becomes steeper as it reaches the plateau yielded by the blank