Hydrodynamic voltammetry at a rocking disc electrode: theory versus experiment

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)₆^4- 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 I_lim is demonstrated to follow in good approximation the Levich-type expression I_lim=0.111 nFAcD^2/3v^-1/6√Θf 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.