Metal redox potentials

A depth of this reaction correlates with the electron donor ability of Red and the stability degree of M +(Ox ) complex. The complexation causes anodic shift of metal redox potentials, which reaches almost 100 mV for transition metal cations (Maletin et al. 1979, 1980, 1983). 

The effect of substitution is in the order meso > /S > phenyl, as expected from the shapes of HOMO and LUMO and the distances from the porphyrin n system. The ring substituent effects on the metal redox potential are smaller than those on the ring redox for comparison, gEMP = 18 mV (Nim/I1), 38 mV (Fe111 1), 54mV (Mn111/n) for MCT PP) in CH2C12. 

As a result of the high ionic charge to radius ratio of titanium(IV), normal salts of titanium(IV) are difficult to prepare from aqueous solutions these often yield basic, hydrolyzed species. A tris-catechol species, [Ti(cat)3], prepared by Raymond etal. is one exception it is stable in aqueous solution up to pH 12. The catechol ligand is so stabilizing to Ti that the Ti ATi reduction potential is shifted from the value of -1-0.1V cited as the standard potential in acid in Scheme 1 to a value for [Ti(cat)3] of -1.14 V vs. NHE, affording a powerful example of ligand tuning of metal redox potential. 

STANDARD REDOX POTENTIALS OF SOME COMMON METALS... 

In an aquo-complex, loss of protons from the coordinated water molecules can occur, as with hydrated non-transition metal ions (p. 45). To prevent proton loss by aquo complexes, therefore, acid must usually be added. It is for these conditions that redox potentials in Chapter 4 are usually quoted. Thus, in acid solutions, we have... 

Electron donor molecules are oxidized in solution easily. Eor example, for TTE is 0.33V vs SCE in acetonitrile. Similarly, electron acceptors such as TCNQ are reduced easily. TCNQ exhibits a reduction wave at — 0.06V vs SCE in acetonitrile. The redox potentials can be adjusted by derivatizing the donor and acceptor molecules, and this tuning of HOMO and LUMO levels can be used to tailor charge-transfer and conductivity properties of the material. Knowledge of HOMO and LUMO levels can also be used to choose materials for efficient charge injection from metallic electrodes. 

A.uxilia driers do not show catalytic activity themselves, but appear to enhance the activity of the active drier metals. It has been suggested that the auxihary metals improve the solubiUty of the active drier metal, can alter the redox potential of the metal, or function through the formation of complexes with the primary drier. Auxihary driers include barium, zirconium, calcium, bismuth, zinc, potassium, strontium, andhthium. 

It follows from the electrochemical mechanism of corrosion that the rates of the anodic and cathodic reactions are interdependent, and that either or both may control the rate of the corrosion reaction. It is also evident from thermodynamic considerations (Tables 1.9 and 1.10) that for a species in solution to act as an electron acceptor its redox potential must be more positive than that of the M /M equilibrium or of any other equilibrium involving an oxidised form of the metal. 

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. 

It is usual to choose a container metal for fused salts sufficiently noble for the displacement reaction (2.16) to be negligible, and the most important aspects of corrosion are, as in aqueous solutions, those which involve reducible impurities, although in a salt melt there is also the additional possibility of a reducible anion (see above). All such factors can be described as controlling the oxidising power of the melt, which can be defined in terms of a redox potential just as in aqueous solutions The redox potential is expressed by relationships of the form... 

The redox potential of a melt is a measure of its aggressiveness towards metals, and a metal in contact with the melt will react with it until its potential becomes the same as the redox potential of the melt d. The potential of the metal will depend on the activity of its ions in solution in the melt... 

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