Complexation modulated redox behavior of transition metal systems (review)

Abstract

Ligand effect is a favorable factor in modulation of redox potential of transition metal ion oxidation-reduction systems. Coordination promoted redox action of transition metals can be an efficient approach to design of new redox systems with specific applications. The current review is devoted to the complexation effect of selected ligands on the redox potential of iron, cobalt and copper redox couples and application of such systems in analytical estimations. Indirect estimation of non redox systems by a non redox reaction over a platinum electrode has been referred to as a pseudo indicator action. Application of coordination modulated redox potentials in the natural attenuation of toxic environmental contaminants is also presented.1 The text was submitted by the author in English.Complexation effect on redox potential. Redox systems with tailor made potentials are desired for applications in artificial photosynthetic reactions [1], fuel cell catalysts solar energy conversions [2], hydrogen production and bio-memitic chemistry [3]. Complexation with an appropriate ligand can modulate the redox potential of transition metals redox couples beyond the natural range [4,5]. The complexation effect on redox potential can be explained by specific stabilization of transition metal oxidation states in a redox couple. The complexation effect on redox potential can be rationalized using the thermochemical cycle (Scheme 1) [7,8]. The ligand effect is the promising phenomenon in the design and development of redox potentials with desired applications [6].According to Eq. (3) the ratio of formation constants β III /β II modulates the redox potential of a redox couple in complexes. With the proper adjustment of the ratio the redox potential can be tuned to any desired value. Thus the electrochemical behavior of transition metals ions is also modified by the complexation effect which leads to a different reduction potential in the complexed state compared to the free state [Eq. (3)]. The complexation effect on redox potentials helps to understand mechanisms of the natural redox processes [9],Scheme 1. Thermochemical cycle for complexation effect on redox potential. M 3+ aqua M 2+ aqua ΔG 1 e − ΔG 3 nL ΔG 2 nL M 3+ complex M 2+ complex ΔG 4 e − (3) E complex = E aqua − RT nF ln β III β II . ΔG 1 + ΔG 2 = ΔG 3 + ΔG 4 , ΔG 1 = −nFE aqua ; ΔG 2 = −RTln β II , ΔG 3 = −RTln β III ; ΔG 4 = −nFE complex , ΔG 4 = (ΔG 1 + ΔG 2 ) − ΔG 3 , −nFE complex = −nFE aqua + RT nF ln β III β II ,

A Novel and Facile Method to Characterize the Suitability of Metallic Iron for Water Treatment

“…In the presence of Phen, Fe 2+ is complexed to form very stable Fe-Phen (Equation (3)) which blocks the Fe 2+ oxidation by dissolved oxygen (Equation (4)). In the presence of EDTA, Fe 2+ oxidation to Fe 3+ (Equation (4)) is rather accelerated because (Fe-EDTA) 3+ is more stable than (Fe-EDTA) 2+ [53,55,58]. Resulting Fe 3+ is then complexed by EDTA (Equation (5)).…”

“…Nine (9) commercial Fe 0 SW were used. The selection of Phen was justified by the extreme stability of the Fe 2 -Phen complex [53,55]: (1) avoiding any further oxidation of Fe 2 from Fe 0 dissolution and (2) non-addressing any Fe 3 species already available in the system [52]. Parallel experiments using EDTA were performed and the results are comparatively discussed.…”

Steel Wool for Water Treatment: Intrinsic Reactivity and Defluoridation Efficiency

“…3+ which tend to polymerize and precipitate as Fe(OH) 3 but under the conditions of this study H 2 O is replaced by EDTA that form very stable complexes with Fe 3+ (Equation (5)) [64,65]. In other words, Fe(OH) 3 precipitation will not occur before the solution is saturated ([Fe] = 112 mg L −1 ).…”

Characterizing the Suitability of Granular Fe0 for the Water Treatment Industry

“…The suitability of the EDTA test for mimicking real-world situations suffers from the inherent evidence that aqueous Fe III -EDTA complexes are very stable and induce: (i) accelerated Fe 0 dissolution, and (ii) dissolution of FeCPs [44][45][46][47]. Thus, the EDTA test does not only characterize the extent of iron corrosion as this is accelerated by the consumption of Fe II species after Le Chatelier's Principle.…”

“…In EDTA solutions, Fe II species from Equation (2) is rapidly oxidized by dissolved oxygen (O 2 ) (Equation (3)) to Fe III as Fe III -EDTA complexes are far more stable than Fe II -EDTA [39,44,47]. The same extreme stability of Fe III -EDTA also explains the dissolution of Fe III -oxides.…”