Redox buffer capacity

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 ... 

The redox buffering capacities of seawater illustrated by the ranges of pe in which electron acceptors are stable (ordinate) plotted against the concentration of the electron acceptor in seawater. Although O2 and NO3 reduction dominate the redox reactions and the range of pe in seawater and sediments of the ocean, the most abundant electron acceptors are SO4 and CO2 and they occupy a relatively small range of pe. 

Copper casse is specific to white wines. They are not as well protected from oxidation and reduction phenomena as red wines, where phenols have a redox buffer capacity. Furthermore, the colloidal cupric derivative contains proteins, while red wines have a low protein content due to combination reactions with phenols. 

The dosage liqueur can be acidified with citric acid, if necessary. It also contains the quantity of sulfur dioxide required to eliminate any dissolved oxygen, and may be supplemented with ascorbic acid (50 mg/1). This offsets the sudden oxidative effect of disgorging the redox potential may increase by 150 mV, or even more, depending on the redox buffer capacity of the wine. 

Neretnieks (1985) came to similar conclusions from a different conceptual standpoint with regard to the redox buffer capacity of granites in the deep Scandinavian Shield and regarded it unlikely that oxidizing conditions in recharge waters could penetrate more than a few metres or tens of metres into granitic rock over a million year time period. 

These measurements indicate that it is not possible to identify a single value of pe surrounding the O2/H2S interface in the environment. Redox couples do not respond to the pe of the environment with the same lability as hydrogen ion donors and acceptors. There is no clear electron buffer capacity other than the most general states of "oxygen containing" or "H2S containing." The reason for the vast differences in pec in the oxic waters is the slow oxidation kinetics of the reduced forms of the redox couples. The reduced species for which the kinetics of oxidation by O2 has been most widely studied is Mn. This oxidation reaction... 

The slope of the tangent to the curve at the inflection point where oc = is thus inversely proportional to the number of electrons n. The E-oc curves are similar to the titration curves of weak acids or bases (pH-or). For neutralization curves, the slope dpH/doc characterizes the buffering capacity of the solution for redox potential curves, the differential dE/da characterizes the redox capacity of the system. If oc — for a buffer, then changes in pH produced by changes in a are the smallest possible. If a = in a redox system, then the potential changes produced by changes in oc are also minimal (the system is well poised ). 

At the molecular level, the buffering capacity of the cellular solution may block the pollutant in its course. Pollutants that generate acids or bases may be neutralized by acid-base buffers. Excess calcium or other cations may complex fluoride, and redox systems may buffer S02, 03, or PAN, or the free radicals they generate. On another level, enzyme structure determines whether the pollutant will penetrate and react with an active site, and the functioning of an enzyme, apparently through effects on its structure, also modifies its susceptibility to the pollutant (6). Moreover, inhibition of a susceptible enzyme may not affect a pathway the enzyme affected may not be rate-limiting in a particular pathway, and considerably greater inhibition must occur before it is. 

Not all iron oxides are available for reduction. Some iron minerals are solid crystals or even entire iron grains, which makes them resistant to microbial reduction (Lovley, 1991 Postma, 1993 Heron et al., 1994b). Other iron oxides or hydroxides are amorphous and readily reducible. Over time, even some crystalline minerals such as goethite and hematite may be reduced in the complex environment in leachate (Heron and Christensen, 1995). This indicates that the importance of iron as a redox buffer controlling the size of plumes is not given just by the amount of iron oxides present. The composition and microbial availability of iron for reduction are key parameters. Methods for the actual quantification of the microbial iron reduction capacity have, however, not been developed. 

The concept of pH buffer capacity as a measure of resistance to pH change can be applied to our thinking about the buffering of environmental systems with respect to their concentrations of other substances, including electrons (as defined by redox potential, cf. Nightingale 1958) or contaminant trace metals (cf. Pankow 1991). 

The units of OXC and RDC, as defined, are eq/L or meq/L. This makes OXC comparable to acidity and alkalinity, which are also given in eq/L or meq/L (Chap. 5). The pH buffer capacity measures the resistance of a system to pH change at a given pH, upon addition of a strong acid or base (Chap. 5). The concept of redox capacity, as proposed by Nightingale (1958) and defined above, is... 

The capacity factor in redox is referred to as poise and is defined as the change in added equivalents of reductant or oxidant to bring about a one unit change in pe (or Eh change of 59 mv). The concept is similar to that of buffer capacity for pH (Stumm Morgan 1996). However, poise in soils has been less studied than pH buffering. 

Absorbance of the partly oxidized TF+-citrate solution is measured at 527 nm on a U 7V1S spectrophotometer. Released oxygen on a whole-plant basis is determined by extrapolation of the measured absorbance to a standard calibration curve. These hydroponic studies using artificial redox buffer may be useful in screening related response of wetland plants to reduced soil conditions. However, the conditions do not truly mimic the conditions in wetland soils. Oxidation of other reductants such as DOC, ferrous iron, and ammonium can also be used to estimate oxygen transport capacity of wetland plants (Reddy et al., 1990 Burgoon and Reddy, 1996). 

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