Use of electrochemical transient techniques to obtain thermodynamic and kinetic data on aqueous Fe(III)–1,6-dimethyl-4-hydroxy-3-pyridinecarboxylate and Fe(III)–4-hydroxy-2-methyl-3-pyridinecarboxylate complexes

Abstract

Voltammetric experiments were used to demonstrate the possibility to rapidly obtain stability constants, E° values and kinetic parameters of Fe(III) complexes with 1,6-dimethyl-4-hydroxy-3-pyridinecarboxylic acid (DQ716) at pH 2.3 and 4-hydroxy-2-methyl-3-pyridinecarboxylic acid (DQ2) at pH 3. Fe(III) diffusion coefficient (D_Fe= 5.5·10⁻⁶ cm²/s), heterogeneous electron transfer kinetic constant (k° = 2.7·10⁻⁴cm/s), symmetry coefficient (α= 0.57) and Fe(III)/Fe(II) standard reduction potential (E°= 0.53 V vs. SCE) were determined beforehand and used to obtain all the other results. Digital simulation together with potentiometric data were used to define the whole reaction system in terms of thermodynamic and kinetic parameters. In particular, E° and the dissociation kinetic constant, k_b, of the 1:1 (E°= 0.22 V vs. SCE, k_b= 0.032 s⁻¹), 1:2 (E°= 0.098 V vs. SCE; k_b= 0.22 s⁻¹) and 1:3 (E°≤–0.29 V vs. SCE, k_b= 157.9 s⁻¹) Fe(III)/DQ716 complexes, were estimated. Stability constants of the Fe(II) complexes were computed from these values. The voltammetric data were also interpreted with two independent formalisms: (1) solution of an equation system and (2) a curve fitting method based on the Koutecky–Levich equation. Both approaches allowed us to obtain the speciation of a Fe(III)/DQ716 solution at pH 2.3. Moreover, the second approach allowed the evaluation of the kinetic contributions, the stepwise stability constant of Fe(III)L₂ (7.65 ± 0.07), and to define the mathematical formalization of the experimental result which link some key-points of the voltammetric curve (inflection points and plateaux) to D_Fe, k°, α_j and E°. This approach was also successfully applied to obtain the speciation of a Fe(III)/DQ2 solution at pH 3.