Aqueous systems redox state

Only the obvious studies of aqueous plutonium photochemistry have been completed, and the results are summarized below. The course of discussion will follow the particular photochemical reactions that have been observed, beginning with the higher oxidation states. This discussion will consider primarily those studies of aqueous plutonium In perchloric acid media but will include one reaction in nitric acid media. Aqueous systems other than perchlorate may affect particular plutonium states by redox reactions and complex formation and could obscure photochemical changes. Detailed experimental studies of plutonium photochemistry in other aqueous systems should also be conducted. 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

Equation 8.173 estabhshes that in an electrolytic solution the relative proportions of oxidized and reduced species are controlled by the redox state of the system (we will see in section 8.19 that in actual fact the redox equilibrium among the various redox couples in natural, chemically complex, aqueous solutions is rarely attained). Imposing on equation 8.173 the value of the ruling redox potential (Eh)—i.e.,... 

Most chemists are familiar with chemistry in aqueous solutions. However, the common sense in aqueous solutions is not always valid in non-aqueous solutions. This is also true for electrochemical measurements. Thus, in this book, special emphasis is placed on showing which aspects of chemistry in non-aqueous solutions are different from chemistry in aqueous solutions. Emphasis is also placed on showing the differences between electrochemical measurements in non-aqueous systems and those in aqueous systems. The importance of electrochemistry in non-aqueous solutions is now widely recognized by non-electrochemical scientists - for example, organic and inorganic chemists often use cyclic voltammetry in aprotic solvents in order to determine redox properties, electronic states, and reactivities of electroactive species, including unstable intermediates. This book will therefore also be of use to such non-electrochemical scientists. 

Table 12 Redox sensitive elements in the PSE (bold = most common oxidation states in natural aqueous systems) (after Emsley 1992, Merkel and Sperling 1996, 1998)...

Control of the particle valence/conduction band oxidation/reduction potential is not only achieved through a judicious choice of particle component material band edge redox thermodynamics of a single material are also affected by solution pH, semiconductor doping level and particle size. The relevant properties of the actinide metal are its range of available valence states and, for aqueous systems, the pH dependence of the thermodynamics of inter-valence conversion. Consequently, any study of semiconductor-particle-induced valence control has to be conducted in close consultation with the thermodynamic potential-pH speciation diagrams of both the targeted actinide metal ion system and the semiconductor material. 

Redox initiation is often used in polymerizations in aqueous systems. Thus H2O2 and Fe + ion can be used to initiate the polymerization of acrylamide in water. Derive an expression for the steady-state rale of polymerization in this case ... 

The redox status of an aqueous system is described by the concentrations of the oxidized and reduced species of all system components. Redox systems, generally not at equilibrium as the result of kinetically slow redox reactions, are poorly characterized by intensity factors (Ej or pE) alone. Capacity factors, which reflect the total concentration of relevant species, are conservative parameters that can be meaningful guides to the redox status of aqueous systems. Oxidative capacity (OXC) is defined as a conservative quantity that incorporates a comprehensive chemical analysis of the redox couples of an aqueous system into a single descriptive parameter. OXC classifies aqueous systems in terms of well-defined geochemical and microbial parameters (e.g., oxic, sulfidic). Examples of model and actual groundwater systems are discussed to illustrate the concept. A redox titration model is another tool that is useful in describing a redox system as it approaches an equilibrium state. 

The calculations are typically carried out as follows. The whole system (CcO) is divided into an active site complex (QM system) and the surrounding medium— protein, membrane, and the external aqueous phase. For example, to explore the protonation states of the binuclear center and its ligands, the QM system is defined as shown in Figure 4.12. The protonation of state of His291, one of the ligands of Cub, at different redox states of the Fe j/Cug binuclear center is one of the sites of our interest the H20/0H" ligand to Cug or Fe a3 metals is another such site. 

Peiffer, S. (2000) Characterisation of the redox state of aqueous systems - towards a problem-oriented approach. In Redox - Fimdamentals, Processes and Measuring Techniques (Eds. Schiiring, X Schulz, H. D. Fischer, W. R. Bottcher, X Duijnesveld, W. H. M.), Springer Ver-lag, Berlin. 

In a purely aqueous system, simple chelation of copper by a histidine residue shifts the pKa of the imidazole group by about 3 pH units. On the other hand, in a more hydrophobic site, as in the inner mitochondrial membrane site of Complex IV, it would be interesting to see whether pKa shifts would be sufficient to access the imidazolate state in order to pick up and release proton as the redox state of the relevant center changed. Another point of consideration in this case relates to the superfamily of heme-copper respiratory oxidases, wherein there are ubiquinol oxidases in which ubiquinol replaces the binuclear copper center. This raises the question as to whether the binuclear (Cua)2 center might be examined as replacement for ubiquinol as a site of proton release on oxidation. ... 

One of the most widely studied electroactive polymer film systems is polyvinylfer-rocene (PVF). PVF films can be deposited by electroprecipitation from organic solvents (as discussed below) then studied as surface-immobilized films in aqueous media. Redox switching between the two ferrocene (Fc) states can be represented as a Pc+/o... 

The redox status of an aqueous system is described by the concentrations of the oxidized and reduced chemical species of all components in the chemical system (Scott Morgan 1990). Because of the slowness of oxidation-reduction reactions, natural rock-water systems are often not in redox equilibrium and thus the concept of a system Eh or pE becomes meaningless. Intensity factors such as Eh or pE are not very useful descriptors of the redox state of the system, but capacity factors which reflect the total concentration of redox-sensitive species may be better and more conservative measures of redox state. 

---