Voltammetric currents for any ligand-to-metal concentration ratio in fullylabile metal-macromolecular complexation. Easy computations, analytical properties of the currents and a graphical method to estimate the stability constant

In order to enable a wider use of voltammetric methods in speciation analys is, it is convenient not to be restricted by ligand excess conditions. This work assumes labile ideal complexation of a metal ion by a ligand, planar electrode, no electrodic adsorption and equal diffusion coefficients for th e complex and the ligand, but very different from the metal ion. It is show n that the system of non-linear equations describing the diffusion of the s pecies in a potentiostatic experiment for any ligand to metal ratio can be reduced to only one ordinary differential equation by means of a change of variable. Standard numerical methods can then be used in the computation of the solution with a great saving of computational time and resources in co mparison with other existing: methods. Some properties of the currents are also proved: (i) Cottrellian behaviour for any current in normal pulse pola rography (NPP) and for limiting currents in reverse pulse polarography (RPP ), (ii) the dependence of the normalised limiting current (phi) on just thr ee parameters, and (iii) the equality of limiting NPP and RPP currents. The normalised current for high stability constant values depends on just two parameters, one of which is the ratio of total metal/total ligand concentra tions, and can be found from an implicit algebraic equation. A new represen tation for the normalised limiting currents is suggested: the iso-phi diagr am, which for each ratio of diffusion coefficients, epsilon, describes the currents for any stability constant in a unique drawing. A new graphical pr ocedure arising from this diagram is suggested and then applied to data cor responding to Zn/poly(methacrylic) acid at pH 6 and fixed ionic strength. ( C) 1999 Elsevier Science S.A. All rights reserved.