Dynamic metal speciation analysis by stripping chronopotentiometry at scanned deposition potential (SSCP)

In the broad context of stripping voltammetric techniques, the depletive mode of stripping chronopotentiometry, SCP, has fundamental advantages for metal speciation analysis. Complete voltammetric potential-current curves are inherently rich in information content: as measurements are made from the foot of the wave to the limiting deposition current region the relevant part of the stability distribution and corresponding parts of the rate constant distribution are scanned. Conventional DC steady-state voltammetry lacks the necessary sensitivity for measurements at environmentally relevant concentrations, however analogous curves, denoted as SSCP waves, can be constructed by plotting the magnitude of the SCP stripping peak as a function of deposition potential. Analogous to the Deford-Hume expression for voltammetric waves, speciation parameters derive from the change in halfwave deposition potential that occurs on complexation, and the magnitude of the limiting plateau. The distinctive features of SSCP include (i) an effective getting around part of the Nernstian extension of the reoxidation process, leading to (ii) greater resolution than conventional stripping voltammetries, (iii) a certain insensitivity to electrochemical irreversibility, especially at a microelectrode, (iv) practically freedom from induced metal ion adsorption interferences, (v) no requirement for excess ligand during stripping, and (vi) ability to provide a certain unambiguous measure of any chemical heterogeneity in the metal speciation. In case of kinetic currents, i.e. systems with limited association/dissociation rates, invoking the Koutecky-Koryta approximation allows a rigorous expression to be obtained for the full SSCP wave. These features are illustrated by practical examples.