ELECTRON-TRANSFER KINETICS OF FE(CN)63-/4- ON LASER-ACTIVATED AND CN- -MODIFIED PT ELECTRODES

The heterogeneous electron transfer rate constant k-degrees for Fe(CN)6(3-/4-) was measured by fast voltammetry at Pt microdisk electrodes with radii from 10 to 50-mu-m. The observed k-degrees was dependent on electrode pretreatment, but in many cases exceeded the previously reported maximum value of 0.24 cm/s for 1 M KCl electrolyte. When the electrode was polished with alumina in a slurry made with 10 mM KCN, and the 1 M KCl also contained 3 mM KCN, k-degrees was invariant with scan rate and electrode radius for the range of 200-1000 V/s and 10-50-mu-m, with a value of 0.55+/-0.07 cm/s. If KCN was absent from the electrolyte, k-degrees decreased with time to below 0.10 cm/s. Laser activation of the Pt electrodes in situ produced reproducibly high k-degrees values of approx. 0.5 cm/s which varied slightly with laser power density up to 75 MW/cm2. Above this level k-degrees increased up to approx. 1.2 cm/s, but this increase is attributed to a laser-induced increase in microscopic surface area. The observations support a mechanism for electrode passivation based on Fe(CN)6(3-/4-) decomposition to a Prussian blue like film. This process can be prevented by initial saturation of the Pt surface with CN-, resulting in a high stable k-degrees.