Redox potentials effect

Rossetti R, Nakahara S and Brus L E 1983 Quantum size effects In the redox potentials, resonance Raman spectra and electronic spectra of CdS crystallites In aqueous solution J. Chem. Phys. 79 1086... 

The data in Tables 4.2 and 4.3 refer to ions in aqueous acid solution for cations, this means effectively [MlHjO), ]" species. However, we have already seen that the hydrated cations of elements such as aluminium or iron undergo hydrolysis when the pH is increased (p. 46). We may then assume (correctly), that the redox potential of the system... 

Other important effects of ligand and pH changes on redox potentials will be given under the appropriate element. 

Stannate(II) ions are powerful reducing agents. Since, for tin, the stability of oxidation state -b4 is greater than that of oxidation state -b2, tin(II) always has reducing properties, but these are greater in alkaline conditions than in acid (an example of the effect of pH on the redox potential, p. 101). 

An effective method for localizing causes of redox potentials is to plot the total backbone and side chain contributions to ( ) per residue for homologous proteins as functions of the residue number using a consensus sequence, with insertions treated by summing the contribution of the entire insertion as one residue. The results for homologous proteins should be examined for differences in the contributions to ( ) per residue that correlate with observed redox potential differences. These differences can then be correlated with any other sequence-redox potential data for proteins that lack crystal or NMR structures. In addition, any sequences of homologous proteins that lack both redox potentials and structures should be examined, because residues important in defining the redox potential are likely to have semi-sequence conservation of a few key amino acid types. 

AK Churg, A Warshel. Control of the redox potential of cytochrome c and microscopic dielectric effects m proteins. Biochemistry 25 1675, 1986. 

The effects of concentration, velocity and temperature are complex and it will become evident that these factors can frequently outweigh the thermodynamic and kinetic considerations detailed in Section 1.4. Thus it has been demonstrated in Chapter 1 that an increase in hydrogen ion concentration will raise the redox potential of the aqueous solution with a consequent increase in rate. On the other hand, an increase in the rate of the cathodic process may cause a decrease in rate when the metal shows an active/passive transition. However, in complex environmental situations these considerations do not always apply, particularly when the metals are subjected to certain conditions of high velocity and temperature. 

In the case of CaCl2 and NaCl, the order corresponds with the corrosion behaviour expected from cathodic polarisation curves . The order of aggressiveness of chlorides can also be explained on the basis of redox potentials of the melts, calculated on thermodynamic grounds from the free energies of formation of the appropriate oxides and chlorides . The order of aggressiveness of nitrates is complicated by passivity effects , while that of alkalis in contact with air is... 

A further important feature of HMPA is its stabilizing effect on the Redox potential of [Fe(CO)4]2 by ion solvation. In less polar solvents, electron-transfer reactions take place and [Fe(CO)4]2 is oxidized to [HFe3(CO)iThis redox reaction is suppressed in HMPA. 

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes, on the kinetics of heterogeneous electron-transfer reactions, and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media upon the redox process. 

Similar effects are observed in the iron complexes of Eqs. (9.13) and (9.14). The charge on the negatively charged ligands dominates the redox potential, and we observe stabilization of the iron(iii) state. The complexes are high-spin in both the oxidation states. The importance of the low-spin configuration (as in our discussion of the cobalt complexes) is seen with the complex ions [Fe(CN)6] and [Fe(CN)6] (Fq. 9.15), both of which are low-spin. 

The conspicuous separation between the cathodic and anodic peak potentials was initially interpreted in terms of the simple theory for redox polymers as a kinetic effect of slow heterogeneous charge transfer the thermodynamic redox potential of the whole systems was calculated from the mean value between Ep and Ep ... 

This concept implies low interaction energies between the segments consequently, the essential determinant of the position of the redox potentials must be the structure of the so-called effectively conjugated segments (ECS). 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

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