Iron redox couple

Fe(OH) (ferric hydroxide)(ferrous iron) redox couple ... 

Eh = 0.693 + 0.059pFe + -0.177pH Fe (OH)g (ferrosoferric hydroxide) Fe (ferrous iron) redox couple ... 

It is well established that iron reduction is coupled to soluble phosphorous release in soils dominated by iron redox couples (Figure 10.32) (see Chapter 9 for details). Although phosphorous itself is not normally involved in redox reactions, it does undergo reactions that have a pronounced effect on its reactivity. Most of this change in the reactivity of phosphorous in wetland soils and aquatic sediments is associated with the oxidation-reduction of iron and manganese. The reduction of ferric phosphate compounds results in the release of phosphorous, a major solubility mechanism in wetlands and aquatic systems. 

Oxidation-reduction reactions of iron and manganese are also involved in nutrient release in flooded soil and sediments. Fe(III) and Mn(IV) serve as electron acceptors for organic matter decomposition or turnover. Organic nitrogen mineralization results in the release of nutrients such as ammonium nitrogen. Iron reduction is also coupled to phosphorous release in soils dominated by iron redox couples. 

Analysis of practical A, B, C photogalvanic cells has shown that the concentration of Y and Z (the inorganic iron couple) helps induce the required homogeneous kinetics [12, 23, 24]. The concentrations of the iron redox couple aid the prevention of concentration polarization at the dark electrode, while trapping A at the illuminated electrode ([Fe " )] and inhibiting destruction of B or C in the electrolyte ([Fe )]. 

Park KS, Sehougaard SB, Goodenough JB (2007) Conducting-polymer/iron-redox-couple eomposite eathodes for lithium secondary batteries. Adv Mater 19(6) 848-851... 

Dehydrogenation, Ammoxidation, and Other Heterogeneous Catalysts. Cerium has minor uses in other commercial catalysts (41) where the element s role is probably related to Ce(III)/Ce(IV) chemistry. Styrene is made from ethylbenzene by an alkah-promoted iron oxide-based catalyst. The addition of a few percent of cerium oxide improves this catalyst s activity for styrene formation presumably because of a beneficial interaction between the Fe(II)/Fe(III) and Ce(III)/Ce(IV) redox couples. The ammoxidation of propjiene to produce acrylonitrile is carried out over catalyticaHy active complex molybdates. Cerium, a component of several patented compositions (42), functions as an oxygen and electron transfer through its redox couple. 

Three core oxidation states are known for protein-bound [Fe4-S4(S.Cys)4]3+ clusters as illustrated in Figure 2.9. Native proteins exhibit either the [Fe4-S4]2+ + or the [Fe4-S4]3+,2+ redox couple, with proteins involved in the latter couple being referred to historically as HiPIP (high-potential iron protein). The three oxidation states have not been traversed in one protein unless its tertiary structure is significantly perturbed. 

Ferric iron can act as an electron acceptor under the anaerobic conditions the azo dye is in. Like sulfate, it was found that addition of ferric iron to the reactor stimulates the azo dye reduction. Indeed, the reactions are dealing with the redox couple Fe (III)/Fe (II), which can act as an electron shuttle for transferring electrons from electron donor to the electron accepting azo dye. Meanwhile, reactions of both reduction of Fe (III) to Fe (II) and oxidation of Fe (II) to Fe (III) facilitate the electron transport from the substrate to azo dye, thus acting as an extracellular redox mediator [31]. 

To model the brine s chemistry, we need to estimate its oxidation state. We could use the ratio of sulfate to sulfide species to fix ao2 > but chemical analysis has not detected reduced sulfur in the brine, which is dominated by sulfate species. A less direct approach is to assume equilibrium with a mineral containing reduced iron or sulfur, or with a pair of minerals that form a redox couple. Equilibrium with hematite and magnetite, for example,... 

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. 

To trace a reaction path incorporating redox kinetics, we first set a model in redox disequilibrium by disabling one or more redox couples, then specify the reaction in question and the rate law by which it proceeds. To model the progress of Reaction 17.1, for example, we would disable the redox couple between vanadyl and vanadate species. In a model of the oxidation of Fe++ by manganite (MnOOH), we would likely disable the couples for both iron and manganese. 

Even neglecting the question of the precise steps that make up the overall reaction, our calculations are a considerable simplification of reality. The implicit assumption that iron in the fluid maintains redox equilibrium with the dissolved oxygen, as described in Chapter 7, is especially vulnerable. In reality, the ferrous iron added to solution by the dissolving pyrite must react with dissolved oxygen to produce ferric species, a process that may proceed slowly. To construct a more realistic model, we could treat the dissolution in two steps by disenabling the Fe++/Fe+++ redox couple. In the first step we would let pyrite dissolve, and in the second, let the ferrous species oxidize. 

In REACT, we prepare the calculation by disenabling the redox couple between trivalent and pentavalent arsenic (arsenite and arsenate, respectively). As well, we disenable the couples for ferric iron and cupric copper, since we will not consider either ferrous or cupric species. We load dataset FeOH+.dat , which contains the reactions from the Dzombak and Morel (1990) surface complexation model, including those for which binding constants have only been estimated. The procedure is... 

These two forms of iron are called a redox couple, which is usually expressed by placing a double arrow in the equation, to indicate an equilibrium ... 

Most electrodes are metallic. Sometimes the metal of an electrode can also be one component part of a redox couple. Good examples include metallic iron, copper, zinc, lead or tin. A tin electrode forms a couple when in contact with tin(IV) ions, etc. Such electrodes are called redox electrodes (or non-passive). In effect, a redox electrode has two roles first, it acts as a reagent and, secondly, it measures the energy of the redox couple of which it forms one part when connected to a voltmeter. 

Interestingly, the sulfur-linked bis-crown ligand [8] shows an unprecedented cathodic potential shift upon addition of K+ cations to the electrochemical solution (Table 3). It is believed to be a conformational process that causes the anomalous shift of the ferrocene/ferrocenium redox couple and not a through-space or through-bond interaction, as these effects would produce the expected anodic potential shift of the ferrocene redox couple. The origin of the effect may be a redirection of the lone pairs of the sulfur donor atoms towards the iron centre upon complexation. This would increase the electron density... 

A clear avenue of future research is to explore the S-Fe redox couple in biologic systems. Bacterial sulfate reduction and DIR may be spatially decoupled, dependent upon the distribution of poorly crystalline ferric hydroxides and sulfate (e.g., Canfield et al. 1993 Thamdrup and Canfield 1996), or may be closely associated in low-suUate environments. Production of FIjS from bacterial sulfate reduction may quickly react with Fefll) to form iron sulfides (e.g., Sorensen and Jeorgensen 1987 Thamdrup et al. 1994). In addition to these reactions, Fe(III) reduchon may be coupled to oxidation of reduced S (e.g., Thamdrup and Canfield 1996), where the net result is that S and Fe may be cycled extensively before they find themselves in the inventory of sedimentary rocks (e.g., Canfield et al. 1993). Investigation of both S and Fe isotope fractionations produced during biochemical cycling of these elements will be an important future avenue of research that will bear on our understanding of the isotopic variations of these elements in both modem and ancient environments. 

Table XVI shows a selection of stability constants and redox potentials for iron(II) and iron(III) complexes. This Table covers a wide range of the latter, showing how the relative stabilities of the iron(II) and iron(III) complexes are refiected in. B (Fe /Fe ) values. A more detailed illustration is provided by the complexes of a series of linear hexadentate hydroxypyridinonate and catecholate ligands, where again high stabilities for the respective iron(III) complexes are refiected in markedly negative redox potentials (213). The combination of the high stabilities of iron(III) complexes of hydrox5rpyridinones, as of hydroxamates, catecholates, and siderophores, and the low stabilities of their iron(II) analogues is also apparent in Fig. 8. Here redox potentials for hydroxypyranonate and hydroxypyridinonate complexes of iron are placed in the overall context of redox potentials for iron(III)/iron(II) couples. The -(Fe /Fe ) range for e.g., water, cyanide, edta, 2,2 -bipyridyl, and (substituted) 1,10-phenanthrolines is...

Next, we remember that represents a redox couple, i.e. Fe (aq) in contact with solid Fe metal. Because the iron electrode is a solid in its normal standard state, we say that the denominator within the bracket is unity. We will see the reason for this choice of value in the next section. 

We also need to note that if a redox electrode made of iron is not clean but is rusty, then another redox couple is possible, i.e. the couple ... 

To add to the complexity yet further, we note that the solution obtained from the iron-containing ore could itself undergo reaction. Ferrous ion can be readily oxidized by oxygen from the air, so a solution that originally contained just Fe could also contain Fe- " " if left standing for some time, where the oxidation process depletes the amount of Fe + in solution. In generating Fe " ", such aerial oxidation has also caused yet another redox couple to form in this case, for the Fe- ", Fe " couple is 0.77 V. 

A redox couple that is wholly in solution can be analysed without recourse to a redox electrode - indeed, in the example given here, analysis with an iron rod would complicate the situation since the Fe " ", Fe " " system itself obeys the Nernst equation (equation (3.8)). 

The multinuclear tetrahedral iron clusters have the potential for additional formal oxidation states. Because not all of these states have been found in proteins or model compounds, it is possible that some oxidation states may be unstable. For a given Fe S protein only one redox couple is used the other possible states appear to be excluded by restrictions of the protein structure. This selection rule is illustrated with two 4Fe 4S low-molecular-weight electron transfer proteins ferredoxin and high-potential iron protein (HiPIP). The 4Fe 4S clusters in both proteins were shown by X-ray crystallography to be virtually identical. However, the redox potential and oxidation states for the two proteins are vastly... 

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