Equilibrium redox species

Electrochemical methods may be classified into two broad classes, namely potentiometric metiiods and voltannnetric methods. The fonner involves the measurement of the potential of a working electrode iimnersed in a solution containing a redox species of interest with respect to a reference electrode. These are equilibrium experiments involving no current flow and provide themiodynamic infomiation only. The potential of the working electrode responds in a Nemstian maimer to the activity of the redox species, whilst that of the reference electrode remains constant. In contrast, m voltannnetric methods the system is perturbed... 

Emerson, S., Cranston, R., and Liss, P. (1979). Redox species in a reducing fjord Equilibrium and kinetic considerations, Deep-Sea Res. 26, 859-878. 

The modeler controls which redox reactions should be in equilibrium by interactively coupling or decoupling the redox pairs. For each coupled pair, the model uses the corresponding coupling reaction to eliminate redox species from the reactions in the database. For example, if the pair Fe+++-Fe++ is coupled, the model adds the coupling reaction to the reaction for hematite,... 

To appreciate that the concept of frustrated equilibrium does not apply when solutions of redox species are allowed to mix, so any equilibrium in this case is a true equilibrium. 

From equation (3.4.31), if the ratio n/nso is unity there will be no net current flow across the interface this condition is depicted in Fig. 3.13(a) for an n-type semiconductor. Under this equilibrium state surface electrons can undergo isoenergetic electron transfers to the redox species due to a built-in potential, equal to the difference of potential between Ecb and Eredox- Equilibrium can be perturbed, with a resulting observable transient current flow, by varying the concentrations of the redox species. The surface electron concentration ng is related to the bulk concentration no by the potential difference of the space charge layer as follows ... 

Fig. 2. Energy cycle for electron transfer under Franck-Condon conditions (<> ) in relation to electron transfer at thermal equilibrium (°E) for redox species in solution...

Fig. 4, Correlations between electron transfer energy levels in metal electrode and in redox species. At equilibrium (Fy = °Fre for anodic (rja> 0) and cathodic (tjc<0) polarisation (Fy =°Fredox — e0 > ). The arrows indicate the direction of electron transfer...

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. 

Figure 3.37 illustrates the Nernst diffusion layer in terms of concentration-distance profiles for a solution containing species O. As pointed out previously, the concentration of redox species in equilibrium at the electrode-solution interface is determined by the Nernst equation. Figure 3.37A illustrates the concentration-distance profile for O under the condition that its surface concentration has not been perturbed. Either the cell is at open circuit, or a potential has been applied that is sufficiently positive of Eq R not to alter measurably the surface concentrations of the 0,R couple. 

Reference to step 4, above as applied to Eq. 3.25a indicates that the boundary line separating Fe3+(aq) from Fe2, (aq) in the pE-pH diagram will be a horizontal one at pE = 13, the maximum value observed in soil systems (see Section 2.2). Thus Fe3+ (aq) is not a likely redox species and can be excluded from the diagram. Equation 3.25e implies that equilibrium between ferric hydroxide and ferrihydrite is possible only at (HzO) t 0.1, regardless of pE or pH. If (HjO) > 0.1, ferrihydrile can also be excluded from the pE-pH diagram. This leaves Hqs. 3.25b, 3.25d, and 3.251 to define (lie stability fields... 

The two-electron reduction of Compound I to Fe(III) and the one electron reduction of Compound I to Compound II and Compound II to Fe(III) have been estimated by different methods. These methods will be described in the next section, but they differ in the strategy to estimate the redox potential. Two of them rely on spectral determination of equilibrium between redox species [63-65], whereas a third one proposes the use of catalytic measurements [53]. The values obtained with the different methods are shown in Table 4.4. It should be noted these are not standard values. As expected, one of the more oxidant enzymes is the versatile peroxidase, which is able to catalyze the oxidation of Mn(II) to Mn (III) ( 0,=1.5 V). MPO has the highest two-electron redox potential, supporting the fact that only this enzyme is able to catalyze the oxidation of chloride to hypochlorite at neutral pH [72, 73], whereas eosinophil peroxidase performs better at acidic pH [74]. 

Redox titrations involve determination of equilibrium between the enzyme and a redox agent of known redox potential. The method requires a redox agent with redox potential close to the protein of interest, to ensure reversibility. The protein is exposed to different concentrations of the redox agent, and once equilibrium is attained, the half cell potential is measured with electrodes and the oxidation-reduction state of the proteins is measured by some physical technique, usually UV-Vis spectrophotometry. The concentration of the oxidized and reduced forms is determined at isosbestic points, and thus spectral characterization of redox species (ferric enzyme,... 

While the redox titration method is potentiometric, the spectroelectrochemistry method is potentiostatic [99]. In this method, the protein solution is introduced into an optically transparent thin layer electrochemical cell. The potential of the transparent electrode is held constant until the ratio of the oxidized to reduced forms of the protein attains equilibrium, according to the Nemst equation. The oxidation-reduction state of the protein is determined by directly measuring the spectra through the tranparent electrode. In this method, as in the redox titration method, the spectral characterization of redox species is required. A series of potentials are sequentially potentiostated so that different oxidized/reduced ratios are obtained. The data is then adjusted to the Nemst equation in order to calculate the standard redox potential of the proteic species. Errors in redox potentials estimated with this method may be in the order of 3 mV. 

In multiscan cyclic voltammetry measurements, e.g., of a redox film-coated electrode, the isopotential point is the potential of the same current value for voltam-mograms of different cycles. That is, by analogy to, e.g., the isosbestic point in absorption spectroscopy of two species remaining in equilibrium, this is the potential at which voltammograms of two redox species being in equilibrium in the film cross each other. A pair of such redox species can have several isopotential points in their multicyclic voltammograms. 

Mass transport overpotential (or concentration polarization (overpotential)) tjc — is a departure of the - electrode potential (or cell potential), E, from the - equilibrium electrode potential (or zero-current potential), Eeq, caused by a -> faradaic current flow and the associated surface concentration, Cox(x = 0), deviation from the bulk concentration, Cox(°°)> of the reacting redox species, Ox... 

Munoz 1994). Furthermore the electrode is highly susceptible to contamination effects. While contaminations of a platinum electrode can be disposed of managed, thermodynamic disequilibrium and low concentrations can not. Therefore redox measurements should be aborted after 1 hour if no steady value is reached. The statement derived from the measurement in that case is, that the water is redox species are not in thermodynamical redox equilibrium with the platinum electrode. 

One approach to determine the reliability of geochemical codes is to take well-defined input data and compare the output from several different codes. For comparison of speciation results, Nordstrom et al. (1979) compiled a seawater test case and a river-water test case, i.e., seawater and river-water analyses that were used as input to 14 different codes. TTie results were compared and contrasted, demonstrating that the thermodynamic databases, the number of ion pairs and complexes, the form of the activity coefficients, the assumptions made for redox species, and the assumptions made for equilibrium solubilities of mineral phases were prominent factors in the results. Additional arsenic, selenium, and uranium redox test cases were designed for testing of... 

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. 

Figure 2-21 presents a graph of environmental pe and Eh ranges as a function of the dominant redox species in the environment. This graph must be regarded as approximate strictly, the pe (or Eh) of an environment is an equilibrium notion and is therefore not rigorously applicable in natural waters where chemical transformations may be actively occurring and have not... 

A.QUATIC oxidation-reduction (redox) processes control the distribution of many major and minor elements in natural environments (1). Equilibrium redox calculations can be used to indicate the boundary conditions toward which a natural system must be proceeding. Real systems are frequently far from equilibrium because photosynthesis traps the energy of the sun in the form of energy-rich chemical bonds and thus creates nonequilibrium chemical species. The return to equilibrium (even when mediated by bacteria)... 

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