Redox couple, Fe

Another problem is that the Nernst equation is a function of activities, not concentrations. As a result, cell potentials may show significant matrix effects. This problem is compounded when the analyte participates in additional equilibria. For example, the standard-state potential for the Fe "/Fe " redox couple is +0.767 V in 1 M 1TC104, H-0.70 V in 1 M ITCl, and -H0.53 in 10 M ITCl. The shift toward more negative potentials with an increasing concentration of ITCl is due to chloride s ability to form stronger complexes with Fe " than with Fe ". This problem can be minimized by replacing the standard-state potential with a matrix-dependent formal potential. Most tables of standard-state potentials also include a list of selected formal potentials (see Appendix 3D). 

Again for the titration of Ce(IV) with Fe(II) we shall now consider constant-potential amperometry at one Pt indicator electrode and do so on the basis of the voltammetric curves in Fig. 3.71. One can make a choice from three potentials eu e2 and e3, where the curves are virtually horizontal. Fig. 3.74 shows the current changes concerned during titration at e1 there is no deflection at all as it concerns Fe(III) and Fe(II) only at e2 and e3 there is a deflection at A = 1 but only to an extent determined by the ratio of the it values of the Ce and Fe redox couples. The establishment of the deflection point is easiest at e2 as it simply agrees with the intersection with the zero-current abscissa as being the equivalence point in fact, no deflection is needed in order to determine this intersection point, but if there is a deflection, the amperometric method is not useful compared with the non-faradaic potentiometric titration unless the concentration of analyte is too low. 

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. 

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. 

Calhoun and Voth also utilized molecular dynamic simulations using the Anderson-Newns Hamiltonian to determine the free energy profile for an adiabatic electron transfer involving an Fe /Fe redox couple at an electrolyte/Pt(lll) metal interface. This treatment expands upon their earlier simulation by including, in particular, the influence of the motion of the redox ions and the counterions at the interface. 

There is no redox couple in solution at the start of the ferrous-ceric titration because the solution contains only Fe ". The oxidation of ferrous to ferric occurs as soon as an aliquot of ceric ions enter the solution to effect the redox reaction shown in equation (4.1). The bulk of the initially present ferrous ions remain, with the ferric products of the redox reaction residing in the same solution, i.e. a Fe " ", Fe + redox couple is formed. This couple has the electrode potential Epf + pg2+. 

The standard electrode potentials, E°(V) for some chelates of the Fe /Fe redox couple areas follows o-phenanthroline, 1.20 2,2 -bipyridyl 1.096 water, 0.77 cyanide, 0.10 oxalate, -0.01 and 8-hydroquinone, -0.15 (Latimer, 1952). In the case of bipyridyl... 

Ferredoxins and Rieske proteins employ a (Fe )2/Fe Fe redox couple for biological electron transfer reactions. Within the protein, the two iron atoms are rendered inequivalent, even in the hilly oxidized (Fe )2 state, by the surrounding protein environment Within a synthetic cluster, however, both iron atoms are typically equivalent, as may be expected from the symmetry of the overall complex. Table 4 shows reduction potentials for selected analog clusters. 

Sagle LB, Zimmermann J, Matsuda S, Dawson PE, Romesberg FE. Redox-coupled dynamics and folding in cytochrome c. J. Am. Chem. Soc. 2006 128 7909-7915. 

In the absence of substrate at MU-modified Au electrode oxidation and reduction was measured which corresponded to the Fe /Fe redox couple of the heme site with E° = -120 mV [120]. The heterogeneous electron transfer rate constant is in the order of = 15 s. The oxidation peak current increases proportionally to sulfite concentrations between 10 and 100 /aM. The current increase is approaching saturation at concentrations higher than 3 mM. 

Spectroscopic data obtained from spectroelectrochemical experiments require careful and case-specific analysis. The Fe /Fe redox couple has a unique role in diflferent iron-containing proteins. It is hypothesised that the mammalian iron-transport protein transferrin uses the Fe /Fe redox couple as a switch that controls the time and site-specific release of iron, while other iron-containing proteins, such as myoglobin, are able to hold on to iron in both oxidation states. Therefore, it is very important to evaluate the protein and its interaction with both the oxidised and reduced states of iron and accordingly develop a data-analysis model. The spectroelectrochemical response of an iron binding protein can be ideal Nernstian, non-Nernstian resulting from coupled... 

The Lever s model has also been extended to sandwich and half-sandwich complexes with jr-cyclopentadienyl or jt-arene ligands [66, 67, 69[. The l parameter for the TT-ligands has been defined [69] on the basis of the low spin Fe" /" redox couple, by Eqs. (21) or (22), for homolep-tic sandwich [Fe(7r-I. )21 or mixed sandwich [Fe(7r-Li)(7r-L2)[ complexes, respectively. 

Metal-complexing ligands, including humic and fulvic acids, by preferentially com-plexing with Fe or Fe " actually shift the redox potential of soil solutions. This fact can be illustrated simply by considering the Nernst equation for the Fe /Fe redox couple (see Table 7.1) ... 

In biological systems the properties of redox partners are modified by their surroundings, their proximity to water, or their protein carriers to which they are boimd. This is shown in the cases of various cytochromes, all of which involve the Fe +/Fe + redox couple, but they have a range of reduction potentials. 

FIGURE 4.3 Electrochemical cell with Hj/H redox couple or standard hydrogen electrode (SHE) in the left cell (L) and Fe /Fe redox couples in the right cell (R). The left cell is maintained at 1 atm pressure of Hj and 1 M activity. In this cell platinum electrode in Hj/H half cell is anode and platinum electrode in Fe +/Fe cell is cathode. 

Assume that the right cell R contains a solution with Fe /Fe redox couple and a platinum electrode. This cell represents the reduction of an oxidant or electron acceptor, and in... 

Since activity of H+ unity and = 1 atm, and for many dilute solutions, we assume that the activities of reactants and products are equal to their respective concentration expressed as [M], the equilibrium relationship for Fe /Fe redox couple can be written as... 

Fe is the active metal for high-temperature WGS reaction. Hence, we introduced a variety of metal dopants (M = Cr, Mn, Co, Ni, Cu, Zn and Ce) for iron oxide (spinel lattice) and screened their effectiveness for high-temperature WGS reaction [1]. The idea was to examine if ferrite formation can occur with dopants and promote the Fe Fe redox couple. The substitution of Fe sites in the ferrite strucmre with other transition/non-transition/inner transition metal atoms leads to the crystallization of an inverse (or mixed) spinel. The stoichiometry of an inverse spinel can be represented as A(i a)Ba[AaB(2 a)]04, where 8 is the degree of inversion, while A and B represent typical divalent and trivalent cations, respectively. The catalysts were synthesized by coprecipitation method using nitrates as precursors. The synthesized catalysts were evaluated for ultra high temperature WGS reaction in the temperature region 400-550 °C and GHSV 60,000 h- ... 

In this section we look at ways in which Nature carries out redox chemistry with reference to blue copper proteins, iron-sulfur proteins and cytochromes. The redox steps in Photosystem II were outlined in the discussion accompanying equation 22.54. We have already discussed two topics of prime importance to electron transfer in Nature. The first is the way in which the reduction potential of a metal redox couple such as Fe +/Fe + can be tuned by altering the ligands coordinated to the metal centre. Look back at the values of for Fe +/Fe + redox couples listed in Table 8.1. The second is the discussion of Marcus-Hush theory in Section 26.5 this theory applies to electron transfer in bioinorganic systems where communication between redox active metal centres may be over relatively long distances as we shall illustrate in the following examples. 

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