Hydrodynamic Voltammetry at a Rocking Disc Electrode: Theory versus Experiment

• Experiments confirm well behaved limiting current with Levich-type equation quantitatively agreeing with uniformly planar diffusion.
• Theory developed to link the case of rotating disc voltammetry with the more complex rocking disc voltammetry.
• Close agreement of rocking disc voltammetry and rotating disc voltammetry when comparing the average angular movement for both methods.

Rocking disc electrode voltammetry (RoDE) is introduced as an experimentally convenient and versatile alternative to rotating disc voltammetry. A 1.6 mm diameter disc electrode is employed with an overall rocking angle of θ = 90 degree applied over a frequency range of 0.83 Hz to 25 Hz. For a set of known aqueous redox systems (the oxidation of Fe(CN)64− in 1 M KCl, the reduction of Ru(NH3)63+ in 0.1 M KCl, the oxidation of hydroquinone in 0.1 M pH 7 phosphate buffer, the oxidation of I− in 0.125 M H2SO4, and the reduction of H+ in 1 M KCl) the mass transport controlled limiting current Ilim is demonstrated to follow in good approximation the Levich-type expression Ilim=0.111 nFAcD2/3v−1/6Θf with n, the number of electrons transferred per molecule diffusing to the electrode surface, F, the Faraday constant, A, the geometric area, c, the concentration of the active redox species, D, the diffusion coefficient, v, the kinematic viscosity, θ is the overall rocking angle in degree, and f, the rocking rate in Hz. Quantitative theory is developed based on a two-dimensional (2D) axisymmetric laminar flow model accounting for the conservation of mass, momentum and species along with the kinematic analysis of a “four-bar mechanism” to obtain the rocking motion.

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