Redox, processes

A large number of other semiconductors and redox systems have been investigated. A selection is given in Table 3 including references. 

Estimated by comparison with similar redox systems, c.b., conduction band mechanism. 

The question also arises whether quantitative information can be obtained from the experimentally determined exchange currents (Z or il). This problem will be treated in Section 4.5. 

The position of a system within one of these series is established by its redox potential E (see p. 18). The redox potential has a sign it can be more negative or more positive than a reference potential arbitrarily set at zero (the normal potential of the system [2 H /H2]). In addition, E depends on the concentrations of the reactants and on the reaction conditions (see p.l8). In redox series (4), the systems are arranged according to their increasing redox potentials. Spontaneous electron transfers are only possible if the redox potential of the donor is more negative than that of the acceptor (see p.l8). 

In redox reactions, protons (H ) are often transferred along with electrons (e ), or protons may be released. The combinations of electrons and protons that occur in redox processes are summed up in the term reduction equivalents. For example, the combination 1 e /l corresponds to a hydrogen atom, while 2 e and 2 together produce a hydrogen molecule. However, this does not mean that atomic or molecular hydrogen is actually transferred from one molecule to the 

In the cell, redox reactions are catalyzed by enzymes, which work together with soluble or bound redox cofactors. 

Some of these factors contain metal ions as redox-active components. In these cases, it is usually single electrons that are transferred, with the metal ion changing its valency. Unpaired electrons often occur in this process, but these are located in d orbitals (see p.2) and are therefore less dangerous than single electrons in non-metal atoms ( free radicals see below). 

We can only show here a few examples from the many organic redox systems that are found. In the complete reduction of the flavin coenzymes FMN and FAD (see p.l04), 

All oxidation reactions are coupled to reduction reactions. In many cases redox reactions can also involve or be affected by changes in the surrounding environment, such as changes in the pH or temperature (i.e., endothermic or exothermic reactions). Many elements in the subsurface can exist in various oxidation states, some examples include elements like carbon, nitrogen, oxygen, sulfur, iron, cobalt, vanadium, and nickel. 

In aqueous solutions, the general form of a redox reaction equation is given as 

The voltage of the general reaction presented in Fq. 2.34 is given by the Nemst equation  

In a redox reaction, the energy released in a reaction due to movement of charged particles gives rise to a potential difference. The maximum potential difference is called the electromotive force (FMF), E, and the maximum electric work, W, is the product of charge q in Coulombs (C), and the potential NE in volts or FMF  

Note that the FMF (or A ) is determined by the nature of the reactants and electrolytes, not by the size of the system or amounts of material in it. The change in Gibbs free energy, AG, is the negative value of maximum electric work. 

Redox (reduction-oxidation) reactions play a key role In many chemical and biochemical processes. 

Voltaic cells convert chemical energy to electrical energy and electrolytic cells convert electrical energy to chemical energy. 

Whilst thermolyses have aspects in common with photolyses, redox reactions are closely related to radiation reactions, in that they involve electron-transfer reactions. Electron-transfers, like proton transfers, can be extremely fast processes, and are therefore often key steps in biochemical systems. Probably the best understood example is that of photosynthesis, but there are many others not involving initial light absorption. The basic process is [2.19], but this 

Examples of many of these types of reaction are given throughout this book. We end on a note of caution. The detection of radicals, for example by ESR spectroscopy, does not in itself establish that they are important intermediates in the system under study. The technique is very sensitive, and the radicals detected may be formed by a side reaction of no particular significance. Further study is then required to establish their significance. 

Together with acid-base reactions, where a proton transfer occurs (pH-dependent dissolution/ precipitation, sorption, complexation) redox reactions play an important role for all interaction processes in aqueous systems. Redox reactions consist of two partial reactions, oxidation and reduction, and can be characterized by oxygen or electron transfer. Many redox reactions in natural aqueous systems can actually not be described by thermodynamic equilibrium equations, since they have slow kinetics. If a redox reaction is considered as a transfer of electrons, the following general reaction can be derived  

Inserting an inert but highly conductive metal electrode into an aqueous solution allows electrons to transfer both from the electrode to the solution and vice versa. A potential difference (voltage) builds up, which can be determined in a current-less measurement. Per definition, this potential is measured relative to the standard hydrogen electrode with P(H2) = 100 kPa, pH = 0, temperature = 20°C and a potential of 

In the aqueous solution, the potential is measured as an integral over all existing redox species (mixed potential). 

Munoz 1994). Furthermore the electrode is highly susceptible to contamination effects. While contaminations of a platinum electrode can be disposed of managed, thermodynamic disequilibrium and low concentrations can not. Therefore redox measurements should be aborted after 1 hour if no steady value is reached. The statement derived from the measurement in that case is, that the water is redox species are not in thermodynamical redox equilibrium with the platinum electrode. 

The equilibrium redox potential can be calculated from the following Nemst equation  

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U, and model calculations suggest that nonnally has values in the neighbourhood of 1 eV (10 J moD ) for the simplest redox processes. 

In our simple model, the expression in A2.4.135 corresponds to the activation energy for a redox process in which only the interaction between the central ion and the ligands in the primary solvation shell is considered, and this only in the fonn of the totally synnnetrical vibration. In reality, the rate of the electron transfer reaction is also infiuenced by the motion of molecules in the outer solvation shell, as well as by other... 

The measurement of the current for a redox process as a fiinction of an applied potential yields a voltaimnogram characteristic of the analyte of interest. The particular features, such as peak potentials, halfwave potentials, relative peak/wave height of a voltaimnogram give qualitative infonnation about the analyte electrochemistry within the sample being studied, whilst quantitative data can also be detennined. There is a wealth of voltaimnetric teclmiques, which are linked to the fonn of potential program and mode of current measurement adopted. Potential-step and potential-sweep... 

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]. 

Redox doping Red oxide Redox indicators Redox polymers REDOX process Redox reactions Red PDC [80-22-8]... 

The clearest manifestation of molybdenum in biology is its presence in over 20 enzymes which participate in a wide variety of redox processes (44—46). Some of the Mo enzymes and their occurrence are as follows ... 

In the reprocessing environment there are many mthenium compounds, some of which are gaseous. Some reprocessing approaches, notably the REDOX process, require a mthenium removal step in the off-gas system. The PUREX process maintains mthenium in one of its nonvolatile states. 

S. Lawroski and M. Levenson, The Redox Process—H Solvent Extraction Reprocessing Methodfor Irradiated Cranium, TlD-7534, US AEG, Oak Ridge, Term., 1957. 

Historically, the Redox process was used to achieve the same purification as in the Purex process (97,129). The reagents were hexone (methyl isobutyl ketone) as the solvent, dichromate as an oxidant, and A1(N02)3 as the salting agent. The chief disadvantages of hexone are its flammability and its solubihty in water. However, because A1(N03)3 collects in the highly radioactive waste, thereby impeding the latter s further processing, the Redox process was abandoned in favor of the Purex process. 

The Lo-Cat process, Hcensed by US Filter Company, and Dow/Shell s SulFerox process are additional Hquid redox processes. These processes have replaced the vanadium oxidizing agents used in the Stretford process with iron. Organic chelating compounds are used to provide water-soluble organometaHic complexes in the solution. As in the case of Stretford units, the solution is regenerated by contact with air. 

NKK s Bio-SR process is another iron-based redox process which instead of chelates, uses Thiobacillusferroidans )2iQ. - 2i to regenerate the solution (9). This process absorbs hydrogen sulfide from a gas stream into a ferric sulfate solution. The solution reacts with the hydrogen sulfide to produce elemental sulfur and ferrous sulfate. The sulfur is separated via mechanical means, such as filtering. The solution is regenerated to the active ferric form by the bacteria. 

These redox processes are usually appHcable for small sulfur capacities. The sulfur is typically produced as a slurry, and can be upgraded to cake or molten sulfur. At low pressures, the redox processes can replace the amine Claus and tail gas cleanup processes with a single step, yet obtain sulfur recoveries of 99%. At higher pressures, the redox processes experience sulfur plugging and foaming problems. 

The advantages of titanium complexes over other metallic complexes is high selectivity, which can be readily adjusted by proper selection of ligands. Moreover, they are relative iaert to redox processes. The most common synthesis of chiral titanium complexes iavolves displacement of chloride or alkoxide groups on titanium with a chiral ligand, L ... 

The nickel oxide modification obtained electrochemicaHy in KOH electrolyte contained potassium ion and its nickel oxidation level are higher than that of NiO 5. Conclusions regarding the transitions between the reduced and oxidized products within the two series are that the redox process was not reversible and although the oxidized phases of the P- and the y-nickel hydroxides differ in energy contents, differences in analyses and x-ray patterns are not significant. 

Polymerization Initiator. Some unsaturated monomers can be polymerized through the aid of free radicals generated, as transient intermediates, in the course of a redox reaction. The electron-transfer step during the redox process causes the scission of an intermediate to produce an active free radical. The ceric ion, Ce" ", is a strong one-electron oxidizing agent that can readily initiate the redox polymerization of, for example, vinyl monomers in aqueous media at near ambient temperatures (40). The reaction scheme is... 

Cytochromes c (Cyt c) can be defined as electron- transfer proteins having one or several haem c groups, bound to the protein by one or, more commonly two, thioether bonds. Cyt c possesses a wide range of properties and function in a large number of different redox processes. 

A redox process also occurs in the reaction of selenium diimides with bis(amino)stannylenes. Eor example, the cyclic stannylene McaSi)//-N Bu)2Su reacts in a 1 1 molar ratio with BuN=Sc=N Bu to give a spirocyclic tin complex, which reacts with a second equivalent of the stannylene to generate a Sn-Sn bond [d(Sn-Sn) = 2.85 A, /( Sn- Sn) = 13,865 Hz)] (Scheme 10.6). ... 

Although redox processes are sometimes observed in metathetical reactions with metal halides, the pyramidal dianion [Te(NtBu)3] has a rich coordination chemistry (Scheme 10.8). For example, the reaction... 

Outer-sphere. Here, electron transfer from one reactant to the other is effected without changing the coordination sphere of either. This is likely to be the ea.se if both reactants are coordinatively. saturated and can safely be assumed to be so if the rate of the redox process is faster than the rates observed for substitution (ligand tran.sfer) reactions of the species in question. A good example is the reaction. 

Redox processes, which of necessity entail a change in the occupancy of the 4f shell, vary in a very irregular manner across the series. Quantitative data from direct measurements are... 

Figure 30.3 Variation with atomic number of some properties of La and the lanthanides A, the third ionization energy (fa) B, the sum of the first three ionization energies ( /) C, the enthalpy of hydration of the gaseous trivalent ions (—A/Zhyd)- The irregular variations in I3 and /, which refer to redox processes, should be contrasted with the smooth variation in A/Zhyd, for which the 4f configuration of Ln is unaltered.

Nitroedianeundergoes base-catrilyzed addidon to to give3-hydroxy-l,3-dihydrofu]leryl ketoxime by way of a unique intramolecidar redox process, which is not observed in normal electron deficient alkenes fEq. S.77. " FSee Secdon 4.3 Michael addidon of nitroalkanes. ... 

The presupposition is that parallel electrochemical reactions (i.e., ion or electron transfer) occur across the phase boundary, if the measured ions and interfering ions are both present in the solution. A redox process in which electrons pass the phase boundary is also considered an interfering electrochemical reaction. 

Standard potentials are determined with full consideration of activity effects, and are really limiting values. They are rarely, if ever, observed directly in a potentiometric measurement. In practice, measured potentials determined under defined concentration conditions (formal potentials) are very useful for predicting the possibilities of redox processes. Further details are given in Section 10.90. 

Until recent years only a relatively few studies had been reported on the amphiphilic polyelectrolytes. However, several years ago attention began to be directed to the microphase structure as a reaction medium that modifies photophysics and photochemistry [50 — 64], redox processes [65 — 67], and chemical reactions [68, 69]. Since then the number of reports on amphiphilic polyelectrolyte systems have increased sharply. 

Certain polymerizations (e.g.. S, see 3.3.6.1) can be initiated simply by applying heal the initiating radicals are derived from reactions involving only the monomer. More commonly, the initiators are azo-compounds or peroxides that are decomposed to radicals through the application of heal, light, or a redox process. 

Without the addition of corrosion inhibitors, acid cleaning or pickling processes to remove oxides and scales would result in severe corrosion of exposed metal surfaces. Acid corrosion is an electrochemical or redox process, and raising cleaning temperatures or acid strength (lowering the pH) increases the hydrogen ion concentration and consequently the rate of corrosion. 

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