Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 6: Features of irreversible complex systems

The features of SSCP waves for complex species involving a nonreversible electron transfer process are described. For quasi-reversible systems, with electron transfer rate constants, k0, between O(10−4) to O(10−6) m s−1, the shape of the SSCP wave is dependent on the value of k0, the deposition time and the electrode size. Under these conditions, the stability of a metal complex can be determined from the shift in half-wave deposition potential, Ed,1/2, as compared to the metal-only case. This is true even when the system is not fully labile, so long as the shape of the SSCP wave is not altered in the presence of the complexant, i.e., k0 remains the same. The experimental parameters, notably deposition time and electrode size, can be optimised to achieve this condition. For irreversible systems, k0 < O(10−7) m s−1, the SSCP wave shape is independent of k0, deposition time, and electrode size. Further decreasing k0 only leads to additional shift in the wave from E0. In such cases the contribution to the Ed,1/2 from the stability constant cannot be a priori deconvoluted from that due to k0 if its value changes due to the presence of ligand. In irreversible systems, direct reduction of electroactive complexes is not uncommon, nor is its appearance at potentials more positive, i.e., more reversibly, than that for the metal itself.