Redox buffering

Most of the reversible redox buffers are highly coloured 

For fuller details, see Swartz and Wilson (1971). Nightingale (1958) has discussed quantitative aspects. 

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In the controlled (constant) potential method the procedure starts and continues to work with the limiting current iu but as the ion concentration and hence its i, decreases exponentially with time, the course of the electrolysis slows down quickly and its completion lags behind therefore, one often prefers the application of a constant current. Suppose that we want to oxidize Fe(II) we consider Fig. 3.78 and apply across a Pt electrode (WE) and an auxiliary electrode (AE) an anodic current, -1, of nearly the half-wave current this means that the anodic potential (vs. an RE) starts at nearly the half-wave potential, Ei, of Fe(II) - Fe(III) (= 0.770 V), but increases with time, while the anodic wave height diminishes linearly and halfway to completion the electrolysis falls below - / after that moment the potential will suddenly increase until it attains the decomposition potential (nearly 2.4 V) of H20 -> 02. The way to prevent this from happening is to add previously a small amount of a so-called redox buffer, i.e., a reversible oxidant such as Ce(IV) with a standard... 

The result of the entire procedure, being a 100% conversion of Fe(II) to Fe(III), thus represents a so-called coulometric titration with internal generation of course, it seems possible to titrate Fe(II) with Ce(IV) generated externally from Ce(III), but in this way one would unnecessarily remove the solution of the 100% conversion problem hence the above titration with internal generation in the presence of a redox buffer as an intermediary oxidant represents an extremely reliable method, unless occasional circumstances are prohibitive for the remainder internal generation offers the advantage of no dilution of the analyte solution. 

Table 7.1 Comparison of the yields of carbon-containing compounds obtained from an atmosphere of CH4, NH3, H2O and H2 using spark discharges with those obtained under hydrothermal conditions from a mixture of HCN, HCHO and NH3 at 423 K and 10 atm in the presence of pyrite-pyrrhotite-magnetite redox buffer (Holm and Andersson, 1995)...

The pE-range 2 is representative of many ground and soil waters where 02 has been consumed (by degradation of organic matter), but SO is not yet reduced. In this range soluble Fe(II) and Mn(II) are present their concentration is redox-buffered because of the presence of solid Fe(III) and Mn(IIUV) oxides. 

NO 3-Reducing. Fig. 9.15 shows data on groundwater below agricultural areas. The sharp decrease of 02 and NO3 at the redox cline indicate that the kinetics of the reduction processes are fast compared to the downward water transport rate. Postma et al., 1991 suggest that pyrite, present in small amounts is the main electron donor for NO3 reduction (note the increase of SOJ immediately below the oxic anoxic boundary). Since NO3 cannot kinetically interact sufficiently fast with pyrite a more involved mechanism must mediate the electron transfer. Based on the mechanism for pyrite oxidation discussed in Chapter 9.4 one could postulate a pyrite oxidation by Fe(III) that forms surface complexes with the disulfide of the pyrite (Fig. 9.1, formula VI) subsequent to the oxidation of the pyrite, the Fe(II) formed is oxidized direct or indirect (microbial mediation) by NO3. For the role of Fe(II)/Fe(III) as a redox buffer in groundwater see Grenthe et al. (1992). 

Glutathione, which is synthesized by two ATP molecules requiring synthetase enzymes (y glutamate cysteine synthetase and glutathione synthetase), is present at a concentration of about 2 mM in a red cell. To be an effective redox buffer , the ratio of GSH to GSSG must be kept high. This is achieved by the reduction of GSH by NADPH and glutathione reductase ... 

Sorbents Redox buffering > Aggregation Plant nutrient Pedogenesis Soil classifications... 

Schwertmann, 1993). Such soils are characterized by a hydraulic conductivity somewhere in the profile which is too low to cope with the high rainfall, so that all pores will be filled with water for certain periods of time (see above). In this case, the oxygen supply is limited by the low level of O2 dissolved in the soil water (46 mg O2 at 25 °C) and reduction of Mn-oxides, nitrate and Fe oxides sets in. Soils containing Fe oxides are, therefore, redox-buffered (poised). The redox titration curve (Fig. 16.14) of a soil with 23 g kg Fe as Fe oxides shows buffering at two different pe -1- pH levels, one at ca. 11 and another at ca. 9, which indicate the presence of a more reducible (e. g. ferrihydrite) and a less reducible (e. g. goethite) Fe oxide, respectively, in accordance with their different solubilities (see Chap. 9). 

Heron, G. Christensen, T.H. (1995) Impact of sediment-bound iron on redox buffering in a landfill leachate polluted aquifer (Vejen, Denmark). Environ. Sci. Techn. 29 187-192... 

As exemplarily shown in the case of charybdotoxin, a 37-residue peptide with three intramolecular disulfide bonds,[70] operating in redox buffer was crucial for efficient formation of the correct disulfide bonds.[71] When the reduced peptide was oxidized in 0.1 M NHtOAc buffer (pH 8.0) at 0.11 mM concentration in the presence of redox reagents (peptide/GSH/GSSG ratio of 1 60 6), the main product was the native peptide contaminated... 

The redox buffering capacity of the bentonite material is provided by the Fe(II)-containing accessoiy minerals, particularly Fe(II)-carbonates and pyrite. The key reactions are the following ... 

Equations 16-9 and 16-10 are analogous to the Henderson-Hasselbalch equation of acid-base buffers. Prior to the equivalence point, the redox titration is buffered to a potential near E+ = formal potential for Fc 1 Fe2+ by the presence of Fe 1 and Fe2+. After the equivalence point, the reaction is buffered to a potential near E+ = formal potential for Ce4+ Ce3+. [R. de Levie Redox Buffer Strength, J. Chem. Ed. 1999, 76, 574.]... 

It is important not to confuse the reactions of Eq. 17-42 as they occur in an aerobic cell with the tightly coupled pair of redox reactions in the homolactate fermentation (Fig. 10-3 Eq. 17-19). Tire reactions of steps a and c of Eq. 17-42 are essentially at equilibrium, but the reaction of step b may be relatively slow. Furthermore, pyruvate is utilized in many other metabolic pathways and ATP is hydrolyzed and converted to ADP through innumerable processes taking place within the cell. Reduced NAD does not cycle between the two enzymes in a stoichiometric way and the "reducing equivalents" of NADH formed are, in large measure, transferred to the mitochondria. The proper view of the reactions of Eq. 17-42 is that the redox pairs represent a kind of redox buffer system that poises the NAD+/NADH couple at a ratio appropriate for its metabolic function. 

Hydrogen can be incorporated into silicates in the form of water, H2 molecules, Hatoms, H+, OH", and other ways. Since oxygen is one component of a silicate, both the oxygen and hydrogen potentials (mo2,Hh) must be defined in order to fix the thermodynamic state of the hydrogen containing silicates. Furthermore, the proton activity must be defined by an additional external (electrode) or internal redox buffer (e.g., Fe2+/Fe3+). 

With the methods already described above, disulfide-bridged antiparallel and parallel two-stranded peptides can be prepared. The question arises which peptides are stable under benign conditions. The best way to test for specificity of interaction for the parallel or antiparallel orientation is to place the peptides in a redox buffer with various additives to determine if there is specificity and what interactions are driving the specificity. 

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