Jp. Pinheiro et al., VOLTAMMETRY OF LABILE HETEROGENEOUS COMPLEXES WITH LOW DIFFUSION-COEFFICIENTS, Journal of electroanalytical chemistry [1992], 402(1-2), 1996, pp. 47-55
The heterogeneity of natural organic ligands in the presence of a meta
l ion affects its concentration profile in the diffusion layer. In fac
t, owing to heterogeneity, the ratio of complexed metal to free metal
increases in the diffusion layer with proximity to the electrode surfa
ce. The diffusing species are then decelerated because the diffusion c
oefficient of the macromolecular complex is lower than that of the met
al ion, and so the total amount of reduced metal is decreased compared
with that obtained in a homogeneous system with the same bulk speciat
ion. Therefore, the voltammetric current, which is directly related to
the concentration profile in the diffusion layer, is affected by hete
rogeneity. In the work reported in this paper the concentration profil
e in the diffusion layer was determined on the basis of the diffusion
process and a heterogeneity model describing the concentration of the
different complexing sites and their complexing affinity for the metal
. The mean stability parameters for the complexation of natural organi
c ligands with a metal ion were determined from this profile, simulati
ng the value that should be determined from voltammetric currents. The
difference between the simulated value and the mean stability paramet
er related to bulk speciation was analysed.