Negative redox potentials

The implications of these mechanistic studies for our understanding of environmental iron sequestration by siderophores is as follows. The hydroxyl containing aqua ferric ions will tend to form ferri-siderophore complexes more rapidly than the hexaaqua ion and ferrous ion will be sequestered more rapidly than the ferric ion. However, once in a siderophore binding site the ferrous ion will be air oxidized to the ferric ion, due to the negative redox potentials (see Section III.D). This also means that Fe dissolution from rocks will be influenced by mineral composition (other donors in the first coordination shell) as well as surface reductases in contact with the rock, and of course surface area (4,13). 

The extremely negative redox potential of Os(OEP)(l-MeIm)2 [29f] is therefore with confidence attributed to an internal effect which is in our opinion the additional 7r-donor effect invoked for the imidazole moiety in Sect. 5.4. Obviously this ligand induces an additional electron density at the Os11 ion which is not transmitted to the porphyrin ring because the a-band of [29f] falls between [29e] and [29i] which both have higher redox potentials than [29f itself. 

For the prevention of nuisance therefore there are two possibilities. First, the formation or release of odorous chemical species can be discouraged. In practice this usually means the prevention of reducing conditions (negative redox potential) and possibly the prior removal of certain key compounds. Second, the time of contact between the sludge/ slurry and the air can be reduced, for example by ploughing in or sub-surface injection, and the act of spreading can be timed to coincide with favourable atmospheric conditions. These two approaches can of course be used in combination. 

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...

The consequences are obvious. The redox reaction with reduction of D has at equilibrium a much lower Fermi energy, that means a more positive redox potential, the redox reaction with oxidation of D has a much higher Fermi energy, that is a more negative redox potential than in the ground state. This is schematically demonstrated in Fig. 1. 

The one-electron electrochemical reduction of NP (57) is a reversible process in aqueous solution, provided the measurements are performed at pH > 8 (—0.123 V vs. NHE) (57a,57b). Different chemical reductants such as sodium in liquid ammonia, tetrahydroborate, ascorbic acid, quinol, dithionite, superoxide or thiolates are also known to generate the [Fen(CN)5NO]3 ion (48,57). However, care must be taken in the products analysis, because the negative redox potentials of some of these reductants allow for further nitrosyl reduction (57a). Also, the reduced product is unstable toward cyanide... 

The counterpart of anodically electrocatalyzed oxidation by redox oxides, namely the cathodic reduction of organic substrates by surface-coup led redox system with sufficiently negative redox potential, is almost unknown. Beck reports that specially prepared TiO coating on Ti-electrodes can be reduced cathodically and that the electrogenerated Ti(III) and Ti(II) species do in fact reduce nitrobenzene to aniline (207). 

Figure 5. Plot of e vs. pH for the system Fe-Mn-S-H20 (top) and Fe-Mn-S0C02-H20 (bottom) at 100°C. and 600 bars Total activities Fe and Mn = 10, S — 10 CO = lOr1 Negative redox potentials obtained by some organic compounds owing to their decomposition into hydrogen...

Increased reducing power, i.e., increased antioxidant activity or more negative redox potential, is also one of the symptoms of the Maillard reaction (see Chapter 1). This has considerable significance, because, as far as foods are concerned, one of the main ways in which they deteriorate chemically is through oxidation, particularly of unsaturated fats and oils, leading to oxidative rancidity. The topic has considerable physiological significance as well (see Chapter 8). 

Figure 17.6 Electron transport chain showing redox potentials at each step. Note that electrons travel from components with a very negative redox potential (e.g., -0.32 V for NAD+/NADH) to one with a very high one (0.82 V for Oz). With each step, the AEq increases. The overall AE 0 is 1.14 V. (Reproduced by permission from Hall DO, Rao KK, Cammack R. Science Prog Oxford 62 285-317, 1975.)...

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