Systematic approach to the quantitative voltammetric analysis of the FeIII/FeII component of the [α2-Fe(OH2)P2W17O61]7-/8- reduction process in buffered and unbuffered aqueous media

The one-electron reduction of [alpha(2)-Fe-III(OH2)P2W17O61](7-) at a glassy carbon electrode was investigated using cyclic and rotating-disk-electrode voltammetry in buffered and unbuffered aqueous solutions over the pH range 3.45-7.50 with an ionic strength of approximately 0.6 M maintained. The behavior is well-described by a square-scheme mechanism P + e(-) <-> Q (E-1(0/) = -0.275 V, k(1)(0/) = 0.008 cm s(-1), and alpha(1) = 1/2), PH+ + e(-) <-> QH(+) (E2(0/) = -0.036 V, k(2)(0/) = 0.014 cm s(-1), and alpha(2) = 1/2), PH+ <-> P + H+ (K-p = 3.02 x 10(-6) M), and QH(+) <-> Q + H+ (K-Q = 2.35 x 10(-10) M), where P, Q, PH+, and QH(+) correspond to [alpha(2)-Fe-III (OH)P2W17O61](8-), [alpha Fe-2-(III)(OH)P2W17O61](9-), [alpha(2)-Fe-III(OH2)P2W17O61](7-), and [alpha(2)-Fe-III(OH2)P2W17O61](8-), respectively; E-1(0') and E-2(0') are the formal potentials, k(1)(0') and k(2)(0'), are the formal (standard) rate constants, and K-p and K-Q are the acid dissociation constants for the relevant reactions. The analysis for the buffered media is based on the approach of Laviron who demonstrated that a square scheme with fully reversible protonations, reversible or quasi reversible electron transfers with the assumption that (alpha(1) = alpha(2), can be well-described by the behavior of a simple redox couple, ox + e- <-> red, whose formal potential, E-app(0'), and standard rate constant, k(app)(0'), are straightforwardly derived functions of pH, as are the values of E-1(0'), k(1)(0'), E-2(0'), K-2(0'), and K-p (only three of the four thermodynamic parameters in a square scheme can be specified). It was assumed that alpha(app) = 1/2, and the simulation program DigiSim was used to determine the values of E-app(0') and k(app)(0'), which are required to describe the cyclic voltammograms obtained in buffered media in the pH range from 3.45 to 7.52 (buffer-related reactions which effect general acid-base catalysis are included in the simulations). DigiSim simulations of cyclic voltammograms obtained in unbuffered media yielded the values of E-1(0') and k(1)(0'), K-Q was then directly computed from thermodynamic constraints. These simulations included additional reactions between the redox species and H2O. The value of the diffusion coefficient of the [alpha(2)-Fe-III(OH2)P2W17O61](7-), 2.92 x 10(-6) cm(2) s(-1), was determined using DigiSim simulations of voltammograms at a rotating disk electrode in buffered and unbuffered media at pH 3.45. The diffusion coefficients of all redox species were assumed to be identical. When the pH i greater than 6, instability of P (i.e., [alpha(2)-Fe-III(OH)P2W17O61](8-)) led to the loss of the reactant and precluded lengthy experimentation.