Quasi-Reversible and Irreversible Voltammograms at Microelectrodes

When the kinetics of the reaction occuring in cyclic voltammetry is limited, the shape of the voltammogram depends strongly on the kinetic parameter: the standard rate constant This Demonstration shows a simulation of cyclic voltammetry with a microelectrode in steady state and under quasi-reversible and irreversible kinetics.



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At the microelectrode, the redox reaction occurs: . With the size of the microelectrode, the steady state is quickly attained. If the standard rate constant ( of the redox reaction on the microelectrode is limited, we find a quasi-reversible or irreversible voltammogram. One can use a dimensionless parameter for the classification:
According to [1], the quasi-reversible and irreversible steady-state voltammograms satisfy the equation , where the limiting current of the microelectrode is given by ; is defined as , and is defined as .
(in ) are the diffusion coefficients of the oxidant and reducer, (in m) is the radius of the microelectrode, (in V) is the standard potential of the redox reaction, (in V) is the applied potential, is the gas constant ( J/K mol), is the Faraday constant ( C/mol), (in mol/L) is the concentration of oxidant, (in K) is the temperature, is the number of electrons transferred, is the standard rate constant (in m/s), and is the symmetry factor ( in this case).
In this Demonstration, the kinetic parameter controls the shape of the voltammograms; when decreases, the irreversibility increases. As tends to infinitity, we have the reversible voltammogram as shown in the Demonstration "Reversible Steady-State Voltammograms at Microelectrodes".
[1] K. B. Oldham and C. G. Zoski, "Comparison of Voltammetric Steady States at Hemispherical and Disc Microelectrodes," J. Electroanal. Chem., 256(1–2), 1988 p. 11.
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