Reduction-oxidation potentials redox

The cytochrome (by virtue of its ability to accept and donate electrons during its function in electron transport) can exist in either the oxidised or the reduced state. In reduced-minus-oxidised difference spectra, it has absorption maxima at 426, 530 and 558 nm, typical of many b-type cytochromes. The ease with which the cytochrome can accept and donate electrons is expressed by its redox (reduction-oxidation) potential, which is measured in millivolts. Unlike most mammalian b cytochromes, which have much higher midpoint potentials, that of the cytochrome of the NADPH oxidase is -245 mV. Be-... 

The midpoint potential of a half-reaction E, is the value when the concentrations of oxidized and reduced species are equal, [Aox] = [Aredl- In biological systems the standard redox potential of a compound is the reduction/oxidation potential measured under standard conditions, defined at pH = 7.0 versus the hydrogen electrode. On this scale, the potential of 02/water is +815 mV, and the potential of water/H2 is 414 mV. A characteristic of redox reactions involving hydrogen transfer is that the redox potential changes with pH. The oxidation of hydrogen H2 = 2H + 2e is an m = 2 reaction, for which the potential is —414 mV at pH 7, changing by 59.2 mV per pH unit at 30°C. 

The reduction-oxidation potential (typically expressed in volts) of a compound or molecular entity measured with an inert metallic electrode under standard conditions against a standard reference half-cell. Any oxidation-reduction reaction, or redox reaction, can be divided into two half-reactions, one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. In biological systems the standard redox potential is defined at pH 7.0 versus the hydrogen electrode and partial pressure of dihydrogen of 1 bar. 

The importance of pH as a master variable controlling chemical reactions in soils has been stressed in previous chapters. However, soils subjected to fluctuations in water content come under the influence of another master variable the reduction-oxidation (or redox) potential Under conditions of water saturation, the lack of molecular oxygen can result in a sequence of redox reactions that changes the soil pH. In this sense the redox state of the soil exerts control over the pH. The nature of redox reactions will be discussed in this chapter, as these reactions profoundly influence metal ion solubility and the chemical form of ions and molecules dissolved in soil solution. The reader is referred to section 1.2f in Chapter 1 for a review of the basic chemical principles necessary for the understanding of redox reactions. 

Before we discuss redox titration curves based on reduction-oxidation potentials, we need to learn how to calculate equilibrium constants for redox reactions from the half-reaction potentials. The reaction equilibrium constant is used in calculating equilibrium concentrations at the equivalence point, in order to calculate the equivalence point potential. Recall from Chapter 12 that since a cell voltage is zero at reaction equilibrium, the difference between the two half-reaction potentials is zero (or the two potentials are equal), and the Nemst equations for the halfreactions can be equated. When the equations are combined, the log term is that of the equilibrium constant expression for the reaction (see Equation 12.20), and a numerical value can be calculated for the equilibrium constant. This is a consequence of the relationship between the free energy and the equilibrium constant of a reaction. Recall from Equation 6.10 that AG° = —RT In K. Since AG° = —nFE° for the reaction, then... 

The oxidizing or reducing power of a substance is indicated by its redox (reduction-oxidation) potential. The redox potential is related to the potential of the... 

Interpretation of electrochemical data is usually based on comparison of experimental values acquired from systematic research of similar molecules. The best situation occurs if the studied compounds form homologous series, where only one substituent is systematically changed. Then the group of substances can be treated using the linear free-energy relationship (LFER) approach [8] to analyze the role of substitution and the influence of structural modifications on redox properties [9, 10]. According to the LFER approach, reduction (oxidation) potentials of compounds belonging to the mentioned series are proportional to the experimentally determined Hammett-type constant a, which is characteristic for each substituent (special tables are available [8, 9, 11]). Recently, a theoretical treatment of substituent constants was published [12]. Eor characterization of coordination compounds, special electrochemical parameters were introduced [13]. 

Generations of chemists and even alchemists have noticed how different chemicals react in water. Now we have a single number for the tendency of each chemical to react, called its reduction-oxidation potential. This is a mouthful even for chemists, so we call it redox potential. ... 

For the majority of electrochemical applications, the analyte participates in a reduction-oxidation (or redox) reaction as consequence of an electric potential, measured between the electrodes. 

In contrast to the relative ease of reduction, oxidation of fullerenes requires more severe conditions [113, 114]. Not only does the resonance stabilization raise the level of the corresponding oxidation potential (1.26 V versus Fc/Fc ), but also the reversibility of the underlying redox process is affected [115]. 

Based on correlations between energy level positions and electrochemical redox potentials, it has been estabHshed that polymethine dyes with reduction potentials less than —1.0 V (vs SCE) can provide good spectral sensitization (95). On the other hand, dyes with oxidation potentials lower than +0.2 V ate strong desensitizets. 

Redox Potential the equilibrium electrode potential of a reversible reduction-oxidation reaction, e.g. Cu /Cu, Fe /Fe, Cl /Cr. 

The more positive the potential, the greater the electron-pulling power of the reduction half-reaction and, therefore, the more strongly oxidizing the redox couple (the stronger the tendency for the half-reaction to occur as a reduction). 

The coordination of redox-active ligands such as 1,2-bis-dithiolates, to the M03Q7 cluster unit, results in oxidation-active complexes in sharp contrast with the electrochemical behavior found for the [Mo3S7Br6] di-anion for which no oxidation process is observed by cyclic voltammetry in acetonitrile within the allowed solvent window [38]. The oxidation potentials are easily accessible and this property can be used to obtain a new family of single-component molecular conductors as will be presented in the next section. Upon reduction, [M03S7 (dithiolate)3] type-11 complexes transform into [Mo3S4(dithiolate)3] type-I dianions, as represented in Eq. (7). 

The reactions discussed above show that arsenic(fV) is of redox amphoteric character and a stronger reducing agent than arsenic(in), but at the same time it is a stronger oxidant than arsenic(V). Partners of the oxidation-reduction reactions of arsenic(fV) known so far can be seen in Table 13. It follows from the redox amphoteric character that the oxidation potentials of couples involving arsenic species are in the order... 

Related Polymer Systems and Synthetic Methods. Figure 12A shows a hypothetical synthesis of poly (p-phenylene methide) (PPM) from polybenzyl by redox-induced elimination. In principle, it should be possible to accomplish this experimentally under similar chemical and electrochemical redox conditions as those used here for the related polythiophenes. The electronic properties of PPM have recently been theoretically calculated by Boudreaux et al (16), including bandgap (1.17 eV) bandwidth (0.44 eV) ionization potential (4.2 eV) electron affinity (3.03 eV) oxidation potential (-0.20 vs SCE) reduction potential (-1.37 eV vs SCE). PPM has recently been synthesized and doped to a semiconductor (24). 

The formal potential of a reduction-oxidation electrode is defined as the equilibrium potential at the unit concentration ratio of the oxidized and reduced forms of the given redox system (the actual concentrations of these two forms should not be too low). If, in addition to the concentrations of the reduced and oxidized forms, the Nernst equation also contains the concentration of some other species, then this concentration must equal unity. This is mostly the concentration of hydrogen ions. If the concentration of some species appearing in the Nernst equation is not equal to unity, then it must be precisely specified and the term apparent formal potential is then employed to designate the potential of this electrode. 

Considering the case when the experiment at three different concentrations of redox ratios is being carried out, with C0o constant and the concentration of the reductant varying as C°R, C°R> and C°R. The oxidation potential is given by... 

Reduction-oxidation is one of the most important processes controlling solubility and speciation of trace elements in soils, especially for those elements with changeable values, such as Cr, As and Se. Within normal ranges of redox potentials and pH commonly found in soils, the two most important oxidation states for Cr are Cr(III) and Cr(VI). Cr(III) is the most stable form of chromium and less soluble and nontoxic, but Cr(VI) is mobile, soluble and toxic. The main aqueous species of Cr(III) are Cr3+, Cr(OH)2+, Cr(OH)3° and Cr(OH)4" and the major aqueous species of Cr(VI)... 

The reactions that are more favored thermodynamically tend to be also favored kineti-cally. Semiconductor electrodes can be stabilized by using this effect. For this purpose, redox couples in the electrolyte are established with the redox potential more negative than the oxidative decomposition potential, or more positive than reductive decomposition potential in such a manner that the electrolyte redox reaction occurs preferentially compared to the electrode decomposition reaction. 

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