Electron in aqueous solution

Note that the word proton refers to the nucleus of a hydrogen atom — an H ion that has been removed from the acid molecule. It does not refer to a proton removed from the nucleus of another atom, such as oxygen or sulfur, that may be present in the acid molecule. As mentioned previously, ions share electrons with any species (ion or molecule) that has a lone pair of electrons. In aqueous solution, the proton bonds with a water molecule to form the hydronium ion. Unlike the Arrhenius theory, however, the Brqnsted-Lowry theory is not restricted to aqueous solutions. For example, the lone pair of electrons on an ammonia molecule can bond with H+, and liquid ammonia can act as a base. 

The value of xh o is important for estimating theoretically the energy levels of hydrated ions and redox electrons in aqueous solutions. 

The greatest research effort on radiation sensitizers has focused on organic compounds however, platinum complexes conform to the hypotheses for radiation sensitizers since they are electron affinic and react preferentially with the hydrated electron in aqueous solution. Early studies of cisplatin in combination with radiation therapy suggested a synergistic effect in antitumor activity (50,51). Much of the initial data were obtained using cells in tissue culture (52), these data indicated that the potential of cisplatin to inhibit repair of radiation-induced damage to DNA could be an important contributor to the enhanced tumor cell killing seen in vivo by the combination of these two modes of treatment. 

Note that this is also equivalent to setting the standard free energies of formation, AfG°, of the proton and the electron in aqueous solution equal to zero. 

Coyle et al. [94] and Dainton and Logan [113] have studied the reactions of hydrated electrons in aqueous solution of two electron scavengers, one of which was usually N20. Since the rate coefficient for the reaction... 

The following review will summarize and systematize the available knowledge on the chemical reactivity of solvated electrons and the products of their reactions. Since most of the work was carried out with solvated electrons in aqueous solutions, we shall confine ourselves mainly to hydrated electrons. We do not intend to discuss the interaction of solvated electrons with their solvents since this will be covered in other chapters. 

Mezyk SP (1995) Rate constant determination for the reaction of sulfhydryl species with the hydrated electron in aqueous solution. J Phys Chem 99 13970-13975 Mezyk SP, Bartels DM (1995) Direct EPR measurement of Arrhenius parameters for the reactions of H atoms with H2O2 and D atoms with D2O2 in aqueous solution. J Chem Soc Faraday Trans 91 3127-3132... 

The chemical potential of the proton and of the electron in aqueous solution in the Standard State is, by convention, equal to the value 0. 

These concepts are developed in greater detail in Chap. 7 of W. Stumm and J. J. Morgan, Aquatic Chemistry, Wiley, New York, 1981. As with any single-ion activity, it is always possible to relate the electron activity to a chemical potential formally (see, e.g., Eq. s2.22) and thus define an equivalent electrochemical emf (see Eq. s2.23). Accordingly, the (electro)chemical potential of an electron in aqueous solution is related to the pE value by the equation (see Special Topic 2) ... 

The bare proton has an exceedingly small diameter compared with other cations, and hence has a high polarising ability, and readily forms a bond with an atom possessing a lone pair of electrons. In aqueous solution the proton exists as the H30+ ion. The existence of the H30+ ion in the gas phase has been shown by mass spectrometry [4], and its existence in crystalline nitric acid has been shown by NMR [5], Its existence in aqueous acid solution may be inferred from a comparison of the thermodynamic properties of HC1 and LiCl [6]. The heat of hydration of HC1 is 136 kcal mole"1 greater than that of LiCl, showing that a strong chemical bond is formed between the proton and the solvent, whereas the molar heat capacity, molar volume and activity coefficients are similar,... 

Assel M, Laenen R, Laubereau A. (1998) Dynamics of excited solvated electrons in aqueous solution monitored with femtosecond-time and polarization resolution. /Phys Chem A 102 2256-2262. 

Mezyk SP. (1995) Rate constant determination for the reaction of sulfliydryl species with the hydrated electron in aqueous solution. J Phys Chem 99 13970-13975. 

In the oxidation state V, Pa shows distinct differences from U and the following elements. Hydrolysis of Pa(V) in aqueous solutions can only be prevented by the presence of concentrated acids (e.g. 8 M HCl) or of complexing agents such as F. In contrast to Pa(V), U(V), Np(V) and Pu(V) form dioxocations MOj in which oxygen is strongly bound by the metal. Obviously, the formation of these dioxocations depends on the availability of a sufficient number of f electrons. In aqueous solution, UO2 exists in small amounts in equilibrium with U + and UO. NpOj is quite stable, whereas PuOj and AmOj disproportionate easily. 

The metallotexaphyrins have also been found to be easy to reduce and capable of capturing hydrated electrons in aqueous solution. This has made them of potential interest as X-ray radiation therapy (XRT) enhancement agents. While a discussion of this and other applications of metallotexaphyrins is deferred to Chapter 10 of this book, it is worth mentioning that preliminary results have been obtained that are highly encouraging. This, in turn, is stimulating on-going efforts to prepare additional texaphyrin and texaphyrin-like macrocycles. ... 

The traditional approach for characterizing redox processes in groundwater is based on conventions and methods developed in classical physical chemistry (Sillen, 1952). Back and Barnes (1965) used platinum electrode measurements to determine the Eh of groundwater samples. This approach was systematized by Stumm and Morgan (1981), who suggested that the theoretical activity of electrons in aqueous solution (pe)> could be used by direct analogy to hydrogen ion activity (pH) as a master variable to describe redox processes. In this treatment, the Pe of a water sample is a hnear function of Eh (Pe = 16.9Eh at 25 °C). 

A wealth of information on the reduction of metal ions in aqueous solutions has been obtained and a compilation was published in 1988 [20], However, alkali or alkaline earth metal ions such as Li Na or cannot be reduced by the hydrated electron in aqueous solution but can form an ion pair with the solvated electron in polar liquids. Among the various reactions of the solvated electron, the reduction of halogenated hydrocarbons is often used in radiation chemistry to produce well-defined radicals because of the selective cleavage of the carbon-halogen bond by the attack ofthe solvated electron. This reaction produces the halide ion and a carbon-centered radical, and is of great interest for environmental problems related to the destruction of halogenated organic contaminants in water and soil [21,22]. 

From this definition, it is clear that this concept is based on a solute-solvent interaction that dictates how easily a compound can liberate a hydrogen atom without its electron, in aqueous solution. Due to the solvent dependence that is included in the definition, this is not the best starting point to uniquely classify a substance as an acid, according to our purposes. 

During water radiolysis, radicals such as H, OH", H+, H2O2, H2 and electrons in aqueous solution (e-q) are produced. The concentration of species created in this process is given by the G factor. This factor represents the number of molecules, or radicals, produced per 100 eV of radiation energy absorbed. The normally accepted values of product yields in a Co-60 gamma irradiated aqueous solution, with both neutral pH and ambient temperature, are presented in Table 22.1 (Appleby and Schwarz 1969). 

The alkali metals are a group of very reactive metals. The first three members of the group are lithium, sodium and potassium. Their atomic and physical properties are summarized in Table 3.20. The electrode potentials are a measure of reducing strength (Chapter 19). The more negative the value, the greater the tendency for the atom to lose an electron (in aqueous solution). 

Kinetics of reduction by solvated electrons in aqueous solution [13]... 

One year ago, employing femtosecond spectroscopy technique we have obtained unique informations on the dynamics of both trapping and solvation of electron in aqueous solutions (Gauduel et al. 1986 Migus et al. 1987). Direct photoionization of water molecules by laser light does not seem possible since water is transparent in the UV spectral region up to 190 nm. However, recent... 

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