Redox couple, definition

An oxidation half-reaction is a conceptual way of reporting an oxidation the electrons are never actually free. In an equation for an oxidation ha If-reaction, the electrons released always appear on the right of the arrow. Their state is not given, because they are in transit and do not have a definite physical state. The reduced and oxidized species in a half-reaction jointly form a redox couple. In this example, the redox couple consists of Zn2+ and Zn, and is denoted Zn2+/Zn. A redox couple has the form Ox/Red, where Ox is the oxidized form of the species and Red is the reduced form. 

ECb. Evb. Ef. ancl Eg are, respectively, the energies of the conduction band, of the valence band, of the Fermi level, and of the band gap. R and O stand for the reduced and oxidized species, respectively, of a redox couple in the electrolyte. Note, that the redox system is characterized by its standard potential referred to the normal hydrogen electrode (NHE) as a reference point, E°(nhe) (V) (right scale in Fig. 10.6a), while for solids the vacuum level is commonly used as a reference point, E(vac) (eV) (left scale in Fig. 10.6a). Note, that the energy and the potential-scale differ by the Faraday constant, F, E(vac) = F x E°(nhe). where F = 96 484.56 C/mol = 1.60219 10"19 C per electron, which is by definition 1e. The values of the two scales differ by about 4.5 eV, i.e., E(vac) = eE°(NHE) -4-5 eV, which corresponds to the energy required to bring an electron from the hydrogen electrode to the vacuum level. 

As a bottom-line definition, the CV will look like that shown in Figure 6.13 only if (i) the ratio of activities of the oxidized and reduced forms of the redox couple satisfies the Nemst equation for the potential... 

Reducing powers of redox couples 300 Reduction potential(s) definition of 300 table, 301... 

Figure 30(a) concerns the EE mechanism for the reaction O + 2 e = R. The solid curve represents the standard free energy profile pertaining to the standard potential E° of the redox couple O/R. In this case, the energy levels of the initial and the final state are equal by definition. Well... 

The definition of Eh, and thus Pe, is given by the Nemst equation, in which the Eh of a solution is related to concentrations of aqueous redox couples at chemical equilibrium and the voltage of a standard hydrogen electrode ( ). For example, when concentrations of aqueous Fe and Fe " " are at equilibrium. Eh is defined as... 

We are now in a position to relate the electronic energy levels of the solution and the electrode on the same scale. It follows from the definition of absolute electrode potential and its value for the SHE, given in eq. 1A.14, that the solution Fermi level qr of a redox couple 0,R is related to its electrode potential Uq r (SHE) on the SHE scale by... 

Platinum electrodes in conjunction with a reference electrode can be used to measure E values in environmental samples. However, these values cannot be considered definitive since they may represent a composite response of several redox couples and the actual response of the electrode can be limiting. A more direct approach is to measure the proportion of the oxidized and reduced components in a system and calculate Fh using the Nemst equation. Redox indicators [listed below with °(W)] may also be used as probes to assess redox status. Comparable to acid-base indicators, the color of these compounds changes when oxidized or reduced. With the exception of resomfin which is pink, the oxidized form of these compounds is blue while the reduced counterpart is yellow or colorless. 

In general, a UV-Vis transmission experiment offers the fastest and most direct method of estimating the optical bulk band gap and should be a priority for any newly synthesized material. A diffuse reflectance or absorption configuration can be used if the sample is not transmissive. If a diffuse reflectance experiment is not available, then photocurrent spectroscopy (as described in Chapter Efficiency definitions in the field of PEC ) with extremely facile redox couples can be performed, though errors in this method may arise from poor charge carrier mobilities or lifetimes and from slow kinetics at the sample-electrolyte interface. 

The universal definition of the standard potential of a redox couple Red/Ox is as follows the standard potential is the value of emf of an electrochemical cell, in which diffusion potential and thermo-emf are eliminated. This cell consists of an electrode, on which the Red/Ox equilibria establish under standard conditions, and a SHE. 

For solutions in protic solvents (water is the most important one) the universal reference electrode for which, under standard conditions, the standard electrode potential is zero by definition all temperatures is the hydrogen electrode (HVH2). In Table 4.7 are the absolute electrode potentials for hydrogen, which can be interpreted in the following way. A redox couple more negative than 0.414 V should liberate hydrogen from water and a couple more negative than 0.828 V should liberate H2 from 1 mol OEI solution. 

By definition, the potential of a redox couple vs SHE is the emf of the Active electrochemical cell, whereby the working electrode is in the half-cell involving the redox couple in question. The counter-electrode is the standard hydrogen electrode at the same temperature. The terminals are made of the same metals and the sum of the possible ionic junction voltages are considered to be equal to zero. In this case we therefore have E,she =... 

Fig. 3.3 Definition of the electrolyte window Eg for (a) liquid and (b) solid electrolytes. The EpA and/or Epc of solid electrolytes may lie in either a band of one-electron stales as in (a) ot a multielectron redox couple as in (b) for liquid electrodes, they lie in a mixed-valence redox couple...

By definition, both redox potentials in Eq. (3) have to be referred to a common reference potential, in this case that of the standard hydrogen electrode (SHE). Obviously, some thermodynamic assumptions have to be introduced in order to evaluate [ Eo2/r2]she. which denotes the formal redox potential of a redox couple in the organic phase versus the SHE in water. 

In the fabrication of chemically modified electrodes (CMEs), one deliberately seeks, in some hopefully rational fashion, to immobilize a chemical on an electrode surface, so that the electrode thereafter displays the chemical, electrochemical, optical, and other properties of the immobilized molecule (s) (Ref. 1 and references therein). Ordinarily, electrocatalysis at a CME features a mediation of electron-transfer reactions, by the immobilized redox couple, between the electrode and some substrate that would otherwise undergo a slow electrochemical reaction at a naked electrode [1]. However, in the following, electrocatalysis will be taken in the broad sense usually attached to this term in most applications of chemically modified electrodes [2] and which can be defined as the dependence of the electrode reaction rate on the nature of the electrode material . In this section, we are not concerned about cases in which the electrode material is catalytic per se. In other words, intentional modification of the electrode composition or surface is necessary. In line with this choice, examples of electrocatal-yses triggered by deposited adatoms will be included. Selection was made of and emphasis put on examples, old and recent, that illustrate the above definitions, but the recent literature will be mostly retained. 

C. Region containing an important potential parameter called the potential at half-maximum current ( 1/2). The reaction rate is fastest at j/2, which can be understood by noting that the slope of the voltammogram is at its maximum [49]. 1/2 is indicative of the formal potential of the dominating redox couple. As there may be a distribution of redox couples in the biofilm, it is appropriate to refer to the distribution as an apparent redox couple. We note that this j/2 is different from the definition presented in Table 5.2 because that 1/2 was derived for diffusion-based electron transfer such as that of the ferricyanide in Case study 5.2. 

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