Complexation Effect on Redox Potential of Iron(III)−Iron(II) Couple: A Simple Potentiometric Experiment

Abstract

When a mixture of two or more reducing agents is titrated, a curve with multiple inflection points results provided the reduction potentials of the species are sufficiently different (1). This is comparable to the titration of two acids with different dissociation constants or of two ions forming precipitates of different solubilities with same reagent (2). The effect of complexation on redox potentials is well documented (3-19) and some articles to this effect are published in this Journal (3, 4). Our experiment is a simple potentiometric titration aimed to invoke the underlying concept of the complexation effect on redox potential of a redox couple. Potentiometric titrations have been widely discussed in this Journal (20-24); however, no method is reported for potentiometric titration of metal ions on the basis of the ligand effect. We choose iron(III)-iron(II) couple because it occurs in many natural redox systems. In this experiment, three iron(II) octahedral complexes, iron(II) aqua, [Fe(OH 2 ) 6 ] 2þ ; iron(II) EDTA, [Fe EDTA] 2-; and iron(II) phenanthroline [Fe(o-phen) 3 ] 2þ , are combined into mixtures of reducing agents and are oxidized to the corresponding iron(III) complexes. Multiple inflections in the titration curve depict the different redox potential of iron(III)-iron(II) couple in these complexes that can be attributed to the complexation effect. This experiment illustrates the effect of the stabilities of iron(III)-iron(II) complexes with EDTA, water, and 1-10 phenanthroline ligands on redox potential.

Electroanalytical, kinetic, and mechanistic investigations of coordination-inspired electron transfer between Fe(II)/Cu(II)

“…A long-standing challenge related to electron-transfer processes in metalloproteins is to unravel the basic cause for the metal ion center in electron transfer proteins that allows them to mediate a huge number of electron transfer reactions with high sensitivity and accuracy [9,10]. XRD and other structural studies in blue copper proteins have established that this ability of metalloproteins can be partly attributed to a large variation in the redox potential of metal ion center (190-780 mV) due to the nature and complexation geometry specific to the redox tuning properties of ligands [11,12].In continuation to our work on coordination-inspired redox systems [13,14], we investigated redox titration between Fe(II) and Cu(II) in the presence of 2,9-dimethyl-1, …”

Complexation of Oxovanadium(IV) and Dioxouranium(VI) with Synthesized 1,2-(Diimino-4′-antipyrinyl)-1,2-diphenylethane Schiff Base: A Thermodynamic, Kinetic, and Bioactivity Investigation

“…The calculated stability constant values of metal-DE complexes are 5.01 × 10 3 and 2.82 × 10 3 (log = 3.70 and 3.45) for vanadyl and uranyl complexes, respectively. The relatively higher stability constant of the vanadyl complex than uranyl complex can be attributed to the preorganization energies needed for the metal ion to get into the planar Schiff base ligand (DE) [27,33,34].…”

Thermo-Kinetic Investigation of Comparative Ligand Effect on Cysteine Iron Redox Reaction

“…12 As an extension to our previous studies of complexation effect on iron redox couple. 13,14 We attempted to investigate the coordination coupled electron transfer between cysteine and iron (III) complexes to highlight the ligand effect on the thermokinetic aspects of iron (III) cysteine redox reaction in acid aqueous system. A comparative propensity of two closely related N,N-bidentate pi (π) acceptor ligands for the cysteine iron electron transfer was kinetically investigated and plausibly explained using quantum chemical descriptors.…”