Redox equilibria, establishment

Commonly, light enhances the rate of many reactions, probably because of the increased population of radicals and/or other favorable changes in the structure of melanin molecules. Thus, photoinduced changes have been reported in redox equilibria established between melanin and nitroxides (Section IV) (579) these may result from a photochemical effect on redox potentials of either one or both com-... 

Figure 29.4 shows an example, the energy diagram of a cell where n-type cadmium sulfide CdS is used as a photoanode, a metal that is corrosion resistant and catalytically active is used as the (dark) cathode, and an alkaline solution with S and S2 ions between which the redox equilibrium S + 2e 2S exists is used as the electrolyte. In this system, equilibrium is practically established, not only at the metal-solution interface but also at the semiconductor-solution interface. Hence, in the dark, the electrochemical potentials of the electrons in all three phases are identical. 

The response of a reversible reaction (2.146) depends on two dimensionless adsorption parameters, Pr and po. When pR = po the adsorbed species accomplish instantaneously a redox equilibrium after application of each potential pulse, thus no current remains to be sampled at the end of the potential pulses. The only current measured is due to the flux of the dissolved forms of both reactant and product of the reaction. For these reasons, the response of a reversible reaction of an adsorbed redox couple is identical to the response of the simple reaction of a dissolved redox couple (2.157), provided Pr = po- As a consequence, the real net peak current depends linearly on /J, and the peak potential is independent of the frequency. If the adsorption strength of the product decreases, i.e., the ratio increases, the net peak current starts to increase (Fig. 2.73). Under these conditions, the establishment of equilibrium between the adsorbed redox forms is prevented by the mass transfer of the product from the electrode surface. Thus, the redox reaction of adsorbed species contributes to the overall response, causing an increase of the current. In the hmiting case, when ]8o —0, the reaction (2.146) simplifies to reaction (2.144). 

It is often difficult to measure stability constants directly for Fe + complexes of natural and model siderophores, since their very high stabilities mean there is extremely little free Fe + in equilibrium with the complex. It is therefore common to use the link between redox potentials and stability constants in the two oxidation states involved in estimating values for logAiFeinL- The correlation of logAlpeniL with redox potential, established over a range of 10 in can thus... 

The redox equilibrium Hg2+ + 2e Hg(/) is established rapidly at the surface of the Hg electrode, so the Nernst equation for the cell can be written in the form... 

Redox equilibrium is not achieved in natural waters, and no single pe can usually be derived from an analytical data set including several redox couples. The direct measurement of p thus is usually not meaningful because only certain electrochemically reversible redox couples can establish the potential at an electrode (4, 35). However, p is a useful concept that indicates the direction of redox reactions and defines the predominant redox conditions. Defining pe on the basis of the more abundant redox species like Mn(II) and Fe(II) gives the possibility of predicting the equilibrium redox state of other trace elements. The presence of suitable reductants (or oxidants) that enable an expedient electron transfer is, however, essential in establishing redox equilibria between trace elements and major redox couples. Slow reaction rates will in many cases lead to nonequilibrium situations with respect to the redox state of trace elements. 

The EPR-nondetectable ions in laccase function as a cooperative 2-e" unit (5). With cytochrome oxidase redox titrations based on the heme absorption bands (2) indicate the presence of a high and a low potential site (380 and 220 mV, respectively). On the other hand, the quasi equilibrium established in the rapid initial transfer of electrons from reduced cytochrome c to the primary electron acceptor in the oxidase, cytochrome a, indicates a potential of 285 mV for this site (18). 

In a chemically inert atmosphere, EDL is formed via establishing redox equilibriums of electrolyte ions [94,108-110], for example ... 

The foregoing example illustrates how equilibrium constants for overall cell reactions can be determined electrochemically. Although the example dealt with redox equilibrium, related procedures can be used to measure the solubility product constants of sparingly soluble ionic compounds or the ionization constants of weak acids and bases. Suppose that the solubility product constant of AgCl is to be determined by means of an electrochemical cell. One half-cell contains solid AgCl and Ag metal in equilibrium with a known concentration of CP (aq) (established with 0.00100 M NaCl, for example) so that an unknown but definite concentration of Kg aq) is present. A silver electrode is used so that the half-cell reaction involved is either the reduction of Ag (aq) or the oxidation of Ag. This is, in effect, an Ag" Ag half-cell whose potential is to be determined. The second half-cell can be any whose potential is accurately known, and its choice is a matter of convenience. In the following example, the second half-cell is a standard H30" H2 half-cell. 

Under these circumstances, the following redox equilibrium is established in solution ... 

In the absence of electronic equilibrium established by a redox electrolyte, the potential difference across the semiconductor/electrolyte jimction can be controlled in a three electrode cell with a reference electrode. The variation of the depletion layer capacitance with potential, U, is described by the Mott-Schottky equation [1] ... 

There are, fortunately, some instances in which measured Eh values can be interpreted in a quantitative sense. Nordstrom et al. (1979), for example, showed that Eh measurements in acid mine drainage accurately reflect the aFe+++/aFe++ ratio. They further noted a number of other studies establishing agreement between measured and Nernst Eh values for various couples. Nonetheless, it is clearly dangerous for a geochemical modeler to assume a priori that a sample is in internal redox equilibrium or that an Eh measurement reflects a sample s redox state. 

Rabinowitch discovered this redox equilibrium was the most light-sensitive known -thionine dyes absorb at 500-700 nm, comparable with values of maximal solar radiation - and as a result would possess a high level of PG sensitivity. As explained in the later sections, the thionine-iron (a dye and reductant, respectively) redox cell yielded a low energy conversion efficiency (<1 %), but the results were somewhat promising due to the ability to manipulate the dye and/or reductant - and subsequently established much research into optimization of the necessary components within the system [9, 15]. 

An important point must be noted at this stage The observation that electronic or redox equilibrium is established over a given experimental... 

A chemical equilibrium (Chapter 7) known as a redox equilibrium is established when a piece of metal is placed in an aqueous solution of its cations. The forward reaction involves some of the atoms of the metal entering the solution as hydrated metal ions, leaving behind a layer of electrons on the surface of the metal. This oxidation process (at an anode) can be represented by... 

Since the electrolyte is inert, redox equilibrium according to Eq. (19) can be established only between the metal electrode and the polymer redox sites. Equation (19) can thus be written in the following appropriate form ... 

The redox equilibrium is established when the rate of electron gain equals the rate of electron loss. 

In half-cells that do not contain a metal, electrical contact with the solution is made by using platinum wire or platinum foil as an electrode. The redox equilibrium is established at the surface of the platinum. The platinum electrode is inert so plays no part in the reaction. 

When fission of an active metal occurs in a molten solution of its fluoride, the fission fragments must originate in energy states and ionization levels very far from tho.se normally encountered. These fragments, however, quickly lose energy through collisions in the melt and come to equilibrium as common chemical entities. The valence states which they ultimately assume are determined by the necessity for cation-anion equivalence in the melt and the requirement that redox equilibrium be established among components of the melt and constituents of the metallic container. 

---