Redox Potential Eh

Redox reactions always involve a transfer of electrons. Reactions such as Equation (3.38) which show both the donor (Fe2+) and the acceptor (Cu2+) of electrons can always in theory form the basis of an electrochemical cell, and can be called cell reactions. They can always be broken down into their separate half-cell reactions . 

A complete description of the conventions and calculations involved is too lengthy for inclusion here. The point is, however, that the half-cell potential, or voltage, of reactions such as (3.44) and (3.45) is directly proportional to the ratio of the product and reactant activities in the reaction. Thus, under standard conditions, 

This half-cell voltage (0.651 V) is called the redox potential of the fluid, Eh, thus both Ei and fo2 are quantities which are proportional to the ratio of reduced to oxidized 

The extent of reducing ion species present can be collectively measured as the Eh. Oxidation involves the loss of electrons, while the process of reduction can be viewed as the gaining of electrons. In order to maintain electrical neutrality overall, it is clear that the oxidation of one species must be accompanied by the reduction of another somewhere in the system hence, the concept of redox - the simultaneous occurrence of reduction and oxidation. In practice, a chemically inert electrode such as platinum or gold is used, which can transfer electrons to or from the environment. The potential developed is measured relative to a standard cell which itself is calibrated relative to the standard hydrogen electrode. Although measurement of Eh can be problematic, relatively stable measurements can be obtained using permanently installed electrodes of platinum or gold whereas spot measurements should be avoided due to unstable and 

This means that spot measurements, e.g. in dipwells or directly in the soil, cannot be compared with results from permanently installed electrodes and that the absolute values from such spot measurements are probably of little use. Experience has shown that redox electrodes can be left in the soil for at least 5 years without major problems. 

For most applications platinum has become the preferred material for redox electrodes. However, there is no evidence to suggest that it should actually give better results than gold when measuring Eh in soil with permanently installed electrodes. It is therefore considered that gold electrodes are a good alternative to platinum due to their lower price and lower tendency to form oxides and possibly also sulfides. 

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The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

In general the processes are initiated by changes of pH and the redox potential (Eh) of the water. 

Environmental conditions determine in large part the chemical reactions that will occur when waste is injected. For example, precipitation-dissolution reactions are strongly controlled by pH. Thus, iron oxides, which may be dissolved in acidic wastes, may precipitate when injection-zone mixing increases the pH of the waste. Similarly, redox potential (Eh) exerts a strong control on the type of microbiological degradation of wastes. 

The kinetics of transformation of Mn and Fe among soil fractions is related to the redox potential (Eh changes) as well as the content of reductants or oxidants in the soil. Generally, a drop in Eh is observed within a few days of the waterlogging of a soil. In a coastal saline silty-clay soil, Eh was reduced to 210 mV after four days of submergence (Bandyopadhyay and Bandyopadhyay, 1984). The decrease in Eh mobilized Mn, and the maximal soluble Mn concentration was found after 14 days of submergence in a sandy-loam soil (Sadana and Takkar, 1988). 

The water samples were analysed for pH, electrical conductivity (EC), temperature (T), dissolved oxygen (DO), redox potential (Eh), reduced iron (Fe2+) and alkalinity (dissolved inorganic carbon, DIC, as HC03") in the field. A few special water samples were collected to determine background levels of CD4, SF6 and Kr in the headspace gas. SWL and T were recorded hourly in 3 shallow (Sites H, I, R) and 3 deep (Sites J, V, W) open bores using automatic data loggers, which were downloaded biannually. 

Acidity (pH) and redox-potential (Eh) of melted snow samples were measured by conventional methods using a Yokogawa pH 81 ionometer. The main cations and anions in the melted snow fraction were determined by titrimetric analysis. In group 1 (Institute of Mineralogy UB RAS), AAS (Fe, Mn, Cu, Zn, Ni, Co, Pb, Cd) and titrimetric analysis (Ca, Mg, S, Cl, C) were performed to determine elements in melted snow and solid residue upon filtrating 1 liter snow sample through a 0.5 micrometer pore size filter. 

Reaction Typical Redox Potential (Eh Volt) Electron Acceptor End product... 

Equation (6) links, in a simple way, the thermodynamically important stability constants Kox and /Cred of a complex in different oxidation states with experimentally measurable redox potentials EH and EHa. Therefore it provides an easy way to obtain the ratio of KoxIKted, which is a theoretically useful parameter known as the binding enhancement factor (BEF). We propose that a better description for this ratio would be the reaction coupling efficiency (RCE) since binding by so-called molecular switches may be reduced or enhanced, depending upon the particular system involved. Equation (6) also allows the calculation of Kox if Kted is known or vice versa. 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

Equation 8.173 estabhshes that in an electrolytic solution the relative proportions of oxidized and reduced species are controlled by the redox state of the system (we will see in section 8.19 that in actual fact the redox equilibrium among the various redox couples in natural, chemically complex, aqueous solutions is rarely attained). Imposing on equation 8.173 the value of the ruling redox potential (Eh)—i.e.,... 

It is to be expected that reductive dissolution of Fe oxides becomes faster as the electron activity increases, i.e. the lower the redox potential (Eh) of the aqueous system, the faster the dissolution. Fischer (1987) dissolved goethite at pH 3 and RT in an Eh range of between -0.3 and -rO.l V and found that the dissolution rate. Ink, decreased linearly from about 5 to 1 mg Ee " L min (r = 0.96). Organic and inorganic additives that shift the redox potential in a negative direction, accelerate dissolution of iron oxides (Frenier Growcock, 1984). 

Pourbaix diagrams (Pourbaix, 1963) indicate graphically the conditions of redox potential (Eh) and pH under which different types of corrosion behaviour may be expected. These plots of potential vs. pH indicate the phase and species in equilibrium with iron under various conditions (see Chap. 8). The solid phases indicated are those that are thermodynamically the most stable owing to kinetic factors other phases may be present during the initial stages of the corrosion process. What the different regions show, however, are the predominant oxidation states to be expected. 

The aquatic solution chemistry of the actinides is also influenced by pH and redox potential (Eh). The approximate ranges of pH and Eh for natural waters are shown in Figure 15.19. The pH varies from 4 to 9.5 and Eh from —300 to +500 mV. In these... 

Redox potential (Eh) and pH measurements were made daily for the four week duration of the experiment. The power was turned off for a few minutes, and each probe was read in sequence, vertically downwards from anode to cathode. Eh values plotted in Figure 2 represent in situ mV readings with respect to the potential of the standard hydrogen electrode. The data show that the Eh conditions at the anode remained constant at 750 mV for the duration ofthe experiment. Eh at the rest ofthe column showed a sharp decrease from positive mV to approximately -750 mV after 4 days of electroosmosis (Figure 2). 

Table I. Cation Exchange capacity (CEC), Redox Potential (Eh), and pH in soil after 672 hrs...

Effects of Groundwater Composition and Eh. Radionuclide sorption on geologic solids is dependent on the chemical composition of the groundwater solution and the redox potential (Eh) of the solid-groundwater system. Aquifers at various depths in the Columbia Plateau formation have -been observed to have significant differences in composition. To accurately model radionuclide migration, it is necessary to understand the effects of chemical components and Eh on sorption and solubility of key radionuclides. An additional benefit of this work is to better understand the mechanisms of sorption and desorption of the radionuclides. 

The effect of solution pH on the concentration of uranium is shown in Figure 5. To produce the results shown in this figure, the temperature of the model solution was held constant at 25°C and uraninite was equilibrated with the solution at various redox potentials (Ehs). Figure 5 shows that the amount of uranium that... 

The conversion from pE to the measured redox potential EH follows from ... 

Structural lability due to intermolecular association and molecular conformation changes in response to changes in pH, redox potential (Eh), electrolyte concentration, and functional-group binding... 

The factors that affect the ability of microorganisms to decompose organic material include type of organic material, temperature, pH, and redox potential (Eh). Fungi and actinomycetes are primarily responsible for the initial decomposition of organic waste. After that, bacteria are able to produce protease, a proteolytic enzyme which breaks protein down into simple compounds such as amino acids. The amino acids are absorbed by the microorganisms and ammonia is released by the following reactions ... 

TABLE 8.4. Oxidized and Reduced Forms of Certain Elements in Soils and the Redox Potentials (Eh) at which Change in Forms Commonly Occurs0... 

Wetlands, at times, may be a poor environment for the formation of metal oxides and/or oxyhydroxides because of the typically low redox potential (Eh). Optimizing the activity of sulfate-reducing bacteria (e.g., desulfovibrio) in the anaerobic zone would be a more effective way of removing metals and sulfates from AD (Kleinmann, 1989). These sulfate-reducing bacteria consume acidity and most of the hydrogen sulfide they produce reacts with heavy metals to create insoluble precipitates. The reactions are shown below ... 

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