Voltammetric lability of metal complexes at spherical microelectrodes withvarious radii

The size of a microelectrode can have a dramatic impact on the relative imp ortance of the diffusional and kinetic contributions to the voltammetric cu rrent of an electroactive metal ion in a complexing medium. Decreasing the radius enhances the diffusional contribution and, as a consequence, the com plex system tends to move away from labile behaviour (where an equilibrium relationship holds). Therefore, sufficiently small microelectrodes (either or not combined with short measuring times) should be able to sense free me tal concentrations directly for not too fast association/dissociation kinet ics. The particular case of steady-state spherical (or hemispherical) diffu sion under ligand excess (pseudo-first-order kinetics) is solved analytical ly. The ensuing lability criterion is shown to be in accordance with a geom etrical derivation based on an analysis of the random walk of the free meta l ions produced by the dissociation of the complex. It is shown that, for a generated metal ion, the probability of reaching the microelectrode surfac e can be quite different from the planar case. Alternatively, the classical reaction layer concept can be used in the derivation of the lability crite rion for spherical geometry as is shown in this work. All treatments show q uantitatively how the lability of metal complexes is reduced with decreasin g dimension of microelectrode. (C) 2001 Elsevier Science B.V. All rights re served.