Electrocatalytic responses at mediator modified electrodes with several cyclic step and cyclic sweep potential techniques. Application to the oxidation of ascorbate at a ferrocene-monolayer modified gold electrode

Analytical expressions for the current--potential and charge-potential curves of an electrocatalytic process taking place at redox mediator modified electrodes corresponding to the application of cyclic staircase or cyclic sweep potentials are presented. Simple equations for cyclic voltsmmetry and cyclic volteoulometry curves have been also deduced for the usual experimental case corresponding to high catalytic rate constants, showing that whereas the current reach an stationary behavior, the charge always depend on time. The value of the rate constant of the chemical step can be easily determined from the plateau of the voltammogram or from the linear anodic region of the charge potendal curves. The theoretical predictions have been tested with the electeocatalytic oxidation of ascorbate mediated by a mixed 6-(ferroeenyl)hexanethiol-butanethiol monolayer at a disk gold electrode, with an excellent agreement between theory and experiments. A catalytic rate constant 3440 [M.sup.-1] [s.sup.-1] has been obtained, which is higher than those previously reported for this system with longer alkanethiol chains.