Redox potential, hydrated electron

Fig. 4-17. Electronic electrode in equilibrium of electron transfer OX = hydrated oxidant particles RED = hydrated reductant partides FWEDQx, s) = Fermi level of redox electrons in hydrated redox partides in solution S p. = electrochemical potential of electrons.

Figure 5-64 shows the band edge potential for compound semiconductor electrodes in aqueous solutions, in which the standard redox potentials (the Fermi levels) of some hydrated redox particles are also shown on the right hand side. In studying reaction kinetics of redox electron transfer at semiconductor electrodes, it is important to find the relationship between the band edge level (the band edge potential) and the Fermi level of redox electrons (the redox potential) as is described in Chap. 8. 

TABLE 8-1. Preference for the conduction band mechanism (CB) and the valence band mechanism (VB) in outer sphere electron transfer reactions of hydrated redox particles at semiconductor electrodes (SC) Eo = standard redox potential referred to NHE c, = band gap of semiconductors. [From Memming, 1983.]... 

Fig. 8-90. Normalized cathodic cur> rent of redox reactions of hydrated redox particles as a function of standard redox potential at n-type electrodes of zinc oxide / (n, cqx) = normalized cathodic reaction current n, = concentration of interfacial electrons Cqx = concentration of oxidant particles au = arbitrary unit. [From Morrison, 1969,1980.]...

Upon ejection from an ion or molecule by photoionization or high energy radiolysis, the electron can be captured in the solvent to form an anionic species. This species is called the solvated electron and has properties reminiscent of molecular anions redox potential of —2.75eV and diffusion coefficient of 4.5 x 10-9 m2 s-1 (Hart and Anbar [17]) in water. Reactions between this very strong reductant and an oxidising agent are usually very fast. The agreement between experimental results and the Smoluchowski theoretical rate coefficients [3] is often close and within experimental error. For instance, the rate coefficient for reaction of the solvated (hydrated) electron in water with nitrobenzene has a value 3.3 x 10+1° dm3 mol-1 s-1. 

The hydrated electron, if the major reducing species in water. A number of its properties are important either in understanding or measuring its kinetic behavior in radiolysis. Such properties are the molar extinction coefficient, the charge, the equilibrium constant for interconversion with H atoms, the hydration energy, the redox potential, the reaction radius, and the diffusion constant. Measured or estimated values for these quantities can be found in the literature. The rate constants for the reaction of Bag with other products of water radiolysis are in many cases diffusion controlled. These rate constants for reactions between the transient species in aqueous radiolysis are essential for testing the "diffusion from spurs" model of aqueous radiation chemistry. 

The hydration energy of the electron and the redox potential of the reaction... 

The hydrated electron has an extensive chemistry, and it is clear that e q is a thermodynamic entity. Its redox potential is defined by the following cell ... 

The reactivity of hydroxide ion (and that of other oxyanions) is interpreted in terms of two unifying principles (a) the redox potential of the YO /YO- (Y = H, R, HO, RO, and O) couple (in a specific reaction) is controlled by the solvation energy of the YO anion and the bond energy of the R-OY product (RX - - YO R-OY - - X ), and (b) the nucleophilic displacement and addition reactions of YO occur via an inner-sphere single-electron shift. The electron is the ultimate base and one-electron reductant which, upon introduction into a solvent, is transiently solvated before it is leveled (reacts) to give the conjugate base (anion reductant) of the solvent. Thus, in water the hydrated electron... 

Scherer and Willig (65) have studied the rate enhancement, due to cations and protons, of electron transfer from the surface of an organic insulator crystal, such as perylene, to oxidized ions, such as [Fe(CN)g] and fMo(CN)g] ", in solution. In an electrochemical method such as this, the saturation current directly renders the rate constant for electron transfer at the crystal surface. Furthermore, electron transfer on [Fe(CN)6l or [Mo(CN)g] can be studied in the absence of reduced forms, whereas the salt effect can be measured up to the solubility limit. They found that for the same concentration of added electrolyte, rate constants increased with the increased charge of the cation. Up to s 1M rate enhancement was of the order Li < Na < Cs but at salt concentrations >3.5 M a reversal that could be explained by different hydrations of the cations took place. They also found a good linear correlation in the shift to higher redox potentials (simultaneously increasing rate constants) with higher salt concentrations. 

Alkali metal cations and alkaline earth metal cations are not reduced by the hydrated electron as the redox potential of the or couple is lower than the redox potential of the hydrated... 

Radiolysis has been used successfully in order to synthesize various noble (such as silver, gold and platinum) and non-noble (such as nickel and iron) metal nanoparticles in aqueous solution and also in other solvents such as alcohols. Due to their relatively low redox potential compared to that of the bulk, metal clusters can be oxygen-sensitive. However, the deoxygenation (by bubbling with an inert gas such as argon or nitrogen) of the solutions prior to irradiation and their study under inert atmosphere prevent their oxidation. Moreover, since water radiolysis leads to the formation of protons in addition to that of hydrated electrons, radio-induced acidification of the medium may lead to non-noble metal clusters corrosion. Therefore, to avoid the oxidation by protons, the solutions can be prepared in slightly basic medium. 

While H" exists as a hydrated species in water, c does not. As we shall see, pe is related to the equilibrium redox potential (volts, hydrogen scale). The electron, as discussed here and used as a component in our equilibrium calculations, is different from the solvated electron, which is a transient reactant in photolyzed solutions. 

The electron in H O becomes fully hydrated in ps time to become a discrete chemical species with a known charge (—1), ionic conductivity (190 cm 0 and diffusion coefficient (4.9 X 10 cm s )- From estimates of its thermodynamic quantities, the standard redox potential of [e ] is ca. —2.87 V, making it a powerful reducing agent. Because of its intense and broad optical absorption spectrum, (A = 710 nm ma. = cm ) extending from the UV into the ir and its relatively long... 

There is abundant evidence that 8-oxo-G is ubiquitous in cellular DNA as discussed in detail by Cadet et al. in Chapter 3 [82, 83], The major pathway of formation of this important marker of oxidative DNA damage involves the one-electron oxidation of 8-HO-G radicals derived from either the hydration of the G +/G(-H) radicals or the addition of OH radicals to the C8 position of G [62, 84], In DNA, the redox potential of 8-oxo-G at pH 7, E7 = 0.74 V versus NHE, indicates that it is more easily oxidized than any of the four natural nucleobases [85], The primary... 

An example of different iron-coordination environments, which alter the chemical properties of iron, is the difference in the redox potentials of hydrated Fe- and the electron-transport protein cytochrome c (Table 1.4). The coordina-... 

There is no absolute rule governing the reactivity of anions towards the hydrated electron. In the case of oxyanions, the availability of a vacant orbital on the central atom is an important factor. Sulphate, carbonate, perchlorate and phosphate have rate coefficients <10 Imole" sec. The halides, pseudohalides and hydroxide ion are similarly unreactive because they have no vacant orbitals for electron accommodation. There seems to be no direct correlation between reactivity and redox potential of the solute. Many anions react at diffusion controlled rates as shown in Table 2. 

Whereas much of the underlying mechanisms for the effects of radiation on materials were outlined using steady state radiation sources, the advent of pulse radiolysis on the heels of flash photolysis opened a window into direct observation of the intermediates. One of the early discoveries utilizing pulse radiolysis was the spectrophotometric detection of the hydrated electron by Boag and Hart (35,36). Since then thousands of rate constants, absorption spectra, one-electron redox potentials and radical yields have been collected using the pulse radiolysis technique. The Radiation Chemistry Data Center at the University of Notre Dame accumulates this information and posts it (at www.rcdc.nd.edu/) for the scientific community to use. They cover the reactions of the primary radicals of water and many organic radicals and inorganic intermediates. 

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