Ferric-ferrous couple

A redox cycle involving the ferric-ferrous couple may be the key mechanism in combustion catalysis by iron compounds. 

In the presence of a ferric/ferrous (Fe +/Fe2+) couple, and that of tetraammine copper (II) Cu(NH3)42+ ion, the measured Us values also shifted. The redox potential for the ferric/ferrous couple, both in equal concentrations, is +0.5 V (vs SCE) in a low pH region, and that for the tetraammine copper (II)/(HI) couple is 0 to —0.2 V, depending on the concentration of ammonia. In the presence of a vanadate/vanadite (V +/V2+) couple at pH < 3, whose formal redox potential is very highly negative (—0.5 V (SCE)), the Us shifted very little and was the onset potential for the cathodic current. 

Bellei M, Jakopitsch C, Battistuzzi G et al (2006) Redox thermodynamics of the ferric—ferrous couple of wild-type Synechocystis KatG and KatG(Y249F). Biochemistry 45 4678 1774... 

The FLUOBOR process uses the ferric/ferrous couple with oxidation of ferrous iron in the anode comparttnent of a diaphragm cell. Oxygen and Pb02 formation are completely avoided and a simple graphite anode can be used. 

Since the second suggested anode reaction only occurs at a very positive potential it is an unlikely contribution. LE is now small since the cathode and anode reactions occur at almost the same potential (the ferric/ ferrous couple is fairly reversible). 

Chu D, Tryk D, Gervasio D, Yeager EB. 1989. Examination of the ionomer/electrode interface using the ferric/ferrous redox couple. J Electroanal Chem 272 277-284. 

First, we read in the dataset of complexation reactions and specify that the initial mass balance calculations should include the sorbed as well as aqueous species. We disable the ferric-ferrous redox couple (since we are not interested in ferrous iron), and specify that the system contains 1 g of sorbing mineral. 

Nojeim et al (utilized an electrolytic cell model system, free of oxygen, buffered with phthalate to pH k.2 and designed to provide a redox potential between +300 and +650 mv to evaluate the effect of redox potential on the ionization and valence of four iron compounds El, FS, FOP, and SFEDTA, described previously (1+3.). Data obtained were used to predict ionization and valence trends in actual food systems of different redox potentials. Redox potential was found to have no significant effect on the ionization of any of the four compounds evaluated. However, in the case of El and FS lower potentials in the environment favored the reduced form of iron. This is to be expected since a greater difference between the +770 mv potential of the ferric to ferrous couple and the potential of its chemical environment would cause the reduction to be more spontaneous. 

In the presence of a ferric-ferrous redox reaction in the solution, the anodic hole-emitting GaAs dissolution of Reaction 22.53 can be coupled with the cathodic reduction of hydrated ferric ions, which injects holes in the valence band as follows ... 

The early research of RFB systems was mainly carried out in the United States and Japan. NASA built the first 1 kW true RFB system with an Fe/Cr redox couple in the 1970s [11]. In this system, an aqueous solution of ferric-ferrous is employed as the positive reactant redox couple, and the negative reactant is a solution of chromos-chromic couple, with hydrochloric acid as a supporting electrolyte in most cases. Because of the poor kinetics of the chromium redox reaction, a serious deterioration of RFBs was observed after a long period of time moreover, a relatively low open circuit potential was also obtained. These drawbacks limited its practical application. In the following years, several RFB systems were evaluated, but none of them was developed on a commercial scale until the bromine/polysulphide RFB and vanadium system was invented [4]. In this section, in addition to these two systems, we will also introduce new progress in tme RFB systems. 

For processes aimed at direct leaching of lead sulfide concentrates the most practical application is to make use of the ferric/ferrous iron couple. As with the chloride Systran, ferric fluosiUcate can be used as an effective leaching reagent to produce a solution of lead according to Equation 9.12 ... 

Here Kox and Kra are the disassociation constant for the oxidized and reduced complexes, respectively. With the use of a E value of 0.525 mV for Fe(ll) Fe(lll), the ratios of the equilibrium constants for EDTA are calculated to be 1.5 x 10 for Fe(lll)/Fe(ll), and 6.1 x 10 for the citrate couple. This calculation shows that the citrate ion stabilizes the ferric species more than 10 times more than the EDTA fer-ric/ferrous couple. 

In this process the reduction of the ferric components of the scale is coupled to oxidation of the base metal, both reactions yielding ferrous species readily soluble in the acid. For magnetite the processes are as shown in equations 11.1 and 11.2. 

Enterocytes in the proximal duodenum are responsible for absorption of iron. Incoming iron in the Fe " state is reduced to Fe " by a ferrireductase present on the surface of enterocytes. Vitamin C in food also favors reduction of ferric iron to ferrous iron. The transfer of iron from the apical surfaces of enterocytes into their interiors is performed by a proton-coupled divalent metal transporter (DMTl). This protein is not specific for iron, as it can transport a wide variety of divalent cations. 

Figure 8.3 A model of iron transport across the intestine. Reduction of ferric complexes to the ferrous form is achieved by the action of the brush border ferric reductase. The ferrous form is transported across the brush border membrane by the proton-coupled divalent cation transporter (DCT1) where it enters an unknown compartment in the cytosol. Ferrous iron is then transported across the basolateral membrane by IREG1, where the membrane-bound copper oxidase hephaestin (Hp) promotes release and binding of Fe3+ to circulating apotransferrin. Except for hephaestin the number of transmembrane domains for each protein is not shown in full. Reprinted from McKie et al., 2000. Copyright (2000), with permission from Elsevier Science.

The [2Fe-2S] dinuclear cluster itself is a classical example of such a system paramagnet in the reduced (1+) state it is made up of a high-spin ferric ion (S = 5/2) and a high-spin ferrous ion (S = 2), coupled antiparallel into a system spin S = 1/2. Under the simple model of Equations 11.15 and 11.16 the system g-values are (Gibson et al. 1966)... 

The one-electron reduced cluster, [3Fe-4S]° has a system spin 5 = 2, which is envisioned to be the result of parallel coupling (i.e., through double exchange) a ferric 5 = 5/2 and a ferrous 5 = 2 ion into a delocalized pair with 5 = 9/2, and subsequently coupling this structure antiparallel to the remaining 5 =5/2 iron into an overall system spin 5 = 2 (Papaefthymiou et al. 1987). 

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