Other Redox Species

FIGURE 6.22. Current-time transients at -2.0V on /j-Si(lOO) in 2M KOH at 45°C measured for an oxide-free chemically etching electrode when 3mM ferricyanide was added to the solution at t = 0 measured after anodic oxidation at 0.0 V when, at t = 0, the potential was stepped back to -2.0 V in a solution containing 3mM ferricyanide. (Reprinted from Bressers et 1995, with permission from Elsevier Science.) 

The reduction of Fe(CN)6 in NH4F solutions, in contrast to that in KOH solutions, does not proceed on p-Si in the dark indicating that the reduction can only proceed with conduction band electrons. However, the presence of defects such as disloca- 

FIGURE 6.23. Current-potential curve of p-Si(lOO) in 2M KOH + 3mM FeCCN) at 45°C. The scan was from -2.0 to 0.0 V at a rate of 2mV/s. The arrows indicate the scan direction. (Reprinted from Bressers e.t a/. 1995, with permission from Elsevier Science.) 

Kooij et investigated the cathodic reaction of a number of one-electron oxi- 

FIGURE 6.24. Current-potential curves of stationary n-type (a) and p-type (b) crystalline silicon electrodes in 2mM MVCh, 0.1 M KCl aqueous solution. Measurements with the n-type electrode were made in the dark, whereas the p-type electrode was in the dark (dashed line) and under illumination (solid line). After Kooij. (Reprinted with permission.) 

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We represent the phase boundary separating this electrode and the solution containing the other redox species by a single vertical line a ... 

While desorption is operative to some extent at most Semico nductor/electrolyte interfaces, in some cases surface passivation can occur. Here charge transfer across the interface is used to establish covalent bonds with electrolyte species, which results in changes in surface composition. This is typified by the well-known oxide film growth on n-GaP and n-GaAs surfaces in aqueous solutions. In these cases, however, the passivation process can be competed with effectively by the use of high concentrations of other redox species such as the polychalcogenides. 

Another form of redox reference electrode is similar to the electrode of the first kind. In this case the inert metal (e.g., Pt, Au, or C) is used as the inner electrode and a stable and soluble redox couple is placed inside the inner reference electrode compartment. A normal liquid junction is used in this type of reference electrode. Unlike the electrode of the first kind, the redox reference electrode is relatively immune to changes in concentration inside the reference electrode compartment because it is the ratio of the reduced/oxidized form of redox couple that determines the potential and not the absolute concentrations. However, redox reference electrodes are sensitive to changes of concentration of oxygen and other redox species. 

As alluded to above, input data for total iron, Fe(II) and/or Fe(III) are accepted by the model, with solute modeling calculations done using whatever data are input. If either Fe(II) or Fe(III) are present, Fe(total) is ignored if Fe(II) only is present, speciation is done among Fe(II) complexes only, and likewise for Fe(III). To accomplish this, the reactions of the iron section have been extensively rewritten (10) and a procedure, named SPLIT IRON, has been added, which performs the mass balance calculations separately for Fe(II) and Fe(III) when they are input separately. An E value is calculated from the computed activities of Fe " and Fe " and may, by user option, be used to distribute other redox species in lieu of an input E value. If only Fe(total) is input, the input E value is used to distribute all redox species including Fe " " and Fe " if there is only Fe(total) input, and no input E value, all Fe calculations are bypassed. 

Other Redox Species. Reduction of ferricyanide in KOH solution takes place via hole injection into the valence band. The reaction path depends on whether an oxide film is present on the surface. On an oxide-free p-Si the reduction proceeds by hole injection as shown in Fig. 6.22. On an oxide-covered electrode, which is anodized at 0 V prior to the transient, the drop of current at about 3.5 min is due to the complete dissolution of the oxide film, resulting in the same current as that on the oxide-free surface. The lower current on the oxide-free surface is attributed by Bressers et al. to the reaction of silicon, which consumes a part of the injected holes by the reduction of ferricyanide. On the oxide-covered surface, silicon dissolution does not occur and all of the injected holes flow into the semiconductor. Figure 6.23 shows the dependence... 

The driving force for migration is established by the different electrochemical potentials (AU) that exist at the two interfaces of the oxide. In other words, the electrochemical potential at the outer interface is controlled by the dominant redox species present in the electrolyte (e.g. O2). 

Environment (aqueous) Lower the redox potential of the solution, i.e. lower Increase the potential of the M /M equilibrium, i.e. increase, Lower a by raising pH, remove dissolved O2 or other oxidising species Increase / + by removing complexants (e.g. CN ions) from solution... 

Environment Increase redox potential of solution Addition of anodic inhibitors Passivation of stainless steel by additions of O2, HNO3 or other oxidising species to a reducing acid Additions of chromates, nitrates, benzoates, etc. to neutral solutions in contact with Fe inhibitive primers for metals, e.g. red lead, zinc chromate, zinc phosphate... 

In every reaction in which the oxidation number of an element in one reactant (or more than one) goes up, an element in some reactant (or more than one) must go down in oxidation number. An increase in oxidation number is called an oxidation. A decrease in oxidation number is called a reduction. The term redox (the first letters of reduction and oxidation) is often used as a synonym for oxidation-reduction. The total change in oxidation number (change in each atom times number of atoms) must be the same in the oxidation as in the reduction, because the number of electrons transferred from one species must be the same as the number transferred to the other. The species that causes another to be reduced is called the reducing agent in the process, it is oxidized. The species that causes the oxidation is called the oxidizing agent in the process, it is reduced. 

A second complication is that we would like to decouple zero-valent sulfur from the element s other redox states, since Reaction 17.28 produces native sulfur, but the database does not include such a coupling reaction. Situations of this nature are not uncommon, occurring when an element in a certain oxidation state is stable as a solid, but no corresponding aqueous species occurs under geochemical conditions. To work the problem, we invent a ficticious zero-valent species S(aq) with an arbitrarily low stability. Setting log K for the reaction... 

Most of the reactions that involve significant fractionation of Se and Cr isotopes appear to be far from chemical or isotopic equihbrium at earth-surface temperatures. Redox disequilibrium is common among dissolved Se species. Dissolved Se(IV) and solid Se(0) are often observed in oxic waters despite their chemical instability (Tokunaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGrath 1998). In one study of shallow groundwater, Se species were found to be out of equilibrium with other redox couples such as Fe(III)/Fe(II) (White and Dubrovsky 1994). The kinetics of abiotic Se(VI) reduction, like those of sulfate, are quite slow. In the laboratory, conversion of Se(VI) to Se(IV) requires one hour of heating to ca. 100°C in a 4 M HCl medium. 

In a similar feshion, fi. n-pH diagrams can be constructed for other redox half reactions. Some examples are given in Figure 7.8. These diagrams suggest that in oxic seawater = +0.4 V and pH = 8), the stable form of iron is Fe(OH)3, nitrogen is stable as nitrate, sulfur as sulfete, and carbon as bicarbonate, if each of these species reaches redox equilibrivun. 

Np through Lr are all prepared artificially by bombardment with neutrons and/or light element ions (He-4, B-10, B-11, C-12,0-16,0-18, Ca-48, Fe-56). Some routes are presented in Table 18.1. The elements have been separated from the targets and other product species by redox reactions, ion exchange, and solvent extraction. In a typical separation, a sulfonic acid ion exchange resin is placed in a column, the tripositive ions of Am through Lr are poured into the column where they are taken up, then the column is eluted with a solution of ammonium a-hydroxybutyrate. As elution proceeds, the An+ ions come off in this order Lr-Md-Fm-Es-Cf-Bk-Cm-Am. They are detected by the distinctive energies of their radioactive emissions. 

M(n+1)+. For oxygenation involving oxometal species, M(" 2)+o, the regeneration mode in the catalytic cycle is substrate oxidation via equation 6. Occasional side reactions with M-based or other redox active species can lead to the intermediate oxidation state of the catalyst, Equations 9-12 are routes to in the... 

Depending on the water composition other radical species are formed, such as carbonate and chloride radicals. This imposes net oxidizing conditions at the water—fuel interface because the generated oxidants, molecular oxygen and hydrogen peroxide, predominate under a radiation, and other radical species like OH- or CQf- are more active than the generated reductants, mainly molecular hydrogen. This is why we propose that the spent fuel-water interface is a dynamic redox system, independently of the conditions imposed on the near field (Merino et al. 2001). 

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