Concentration Profiles at the Electrode Surface
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18-2-2020
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Concentration Profiles at the Electrode Surface
In an unstirred solution, mass transport of the analyte to the electrode surface occurs by diffusion alone.¹ Fick’s Law for mass transfer diffusion relates the distance from the electrode (x), time (t), and the reactant concentration (CA) to the diffusion coefficient (DA).
During a reduction, current increases until it reaches a peak: when all M+ exposed to the surface of an electrode has been reduced to M. At this point additional M+ to be reduced can travel by diffusion alone to the surface of the electrode, and as the concentration of M increases, the distance M+ has to travel also increases. During this process the current which has peaked, begins to decline as smaller and smaller amounts of M+ approach the electrode. It is not practical to obtain limiting currents Ipa, and Ipc in a system in which the electrode has not been stirred because the currents continually decrease with time.¹
In a stirred solution, a Nernst diffusion layer ~10-2 cm thick, lies adjacent to the electrode surface. Beyond this region is a laminar flow region, followed by a turbulent flow region which contains the bulk solution.¹ Because diffusion is limited to the narrow Nernst diffusion region, the reacting analytes cannot diffuse into the bulk solution, and therefore Nernstian equilibrium is maintained and diffusion-controlled currents can be obtained. In this case, Fick’s Law for mass transfer diffusion can be simplified to give the peak current
Here, (n) is equal to the number of electrons gained in the reduction, (S) is the surface area of the working electrode in cm², (DA) is the diffusion coefficient, (v) is the sweep rate, and (CA) is the molar concentration of A in the bulk solution.
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