We have found that the parallel reduction of some metal ions (Cd²⁺, Cu²⁺, Eu³⁺) and hydrogen ions at the dropping (DME) and the hanging (HMDE) mercury electrodes reveals interesting non-additivity effects. In the potential range of hydrogen evolution, the current is different from the sum of individual currents for the metal and hydrogen ions, recorded in separate experiments. This effect was diagnosed in double potential step chronocoulometric (DPSC) experiments as the deviations of the anodic charge of the metal from its limiting value, if the anodic oxidation was preceded by the parallel cathodic reduction of the metal and hydrogen ions. The deviations of the anodic charge mean either its anomalous decrease (for relatively small hydrogen evolution rate) or its anomalous increase (for relatively high hydrogen reduction current, at sufficiently negative potentials). In order to explain the origin of these effects we have conducted comparative experiments with the mercury film electrode, deposited on silver. It is noteworthy that for the film Hg electrode those anomalies were not observed. The preliminary explanation of non-additivity of the currents at the DME and HMDE assumes the role of convection induced on the surface of these electrodes. This additional convective transport changes the spatial distribution of the Cd²⁺ ions and Cd(Hg) – i.e. on both sides of the Hg|water interface. This affects the anodic current of Cd(Hg) oxidation. These effects manifest themselves differently if both Ox and Red species are on the solution side (as for Eu³⁺/Eu²⁺). Various numerical simulations semiquantitatively confirm the above mechanism.

Fig. 1. The pulse chronocoulometric Q-E characteristics of Cd²⁺/Cd(Hg) system at dropping mercury electrode, recorded as the faradaic charge of the cadmium amalgam oxidation at the constant potential (E₂ = 0.0 V vs. SCE, t₂ = 100 ms ) after prior electroreduction of Cd²⁺ ions (t₁ = 100 ms) at the potential E₁ increasing linearly towards negative values. In the presence of H⁺ ions, which start to reduce at -1.50 V, the anodic charge of Cd(Hg) oxidation exhibits substantial deviation from the diffusion limited value.

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