Eh electrode

Fig. 1.3. Experimental setup for electrochemical thermal desorption mass spectroscopy (ECTDMS). C = electrochemical cell, W = working electrode, El = electrolyte inlet, EO = electrolyte outlet, EH = electrode holder, V = valve, TP = turbo pump, VC = vacuum chamber, L = light source, W = window, P = protective jacket, A = aperture to analysis chamber, GI = grid ion source, S = SEM detector.

In principle, the calculation of the oxidation states of plutonium requires knowledge of the redox potential. Eh, of the aqueous phase. However, the Eh measured with a certain type of electrode may not be the potential for the particular redox couple with which the plutonium reacts. One of the reasons for this is that the Eh-electrode usually catalyses the reaction rate for its specific redox couple. For example, in surface sea water, the measured Eh is about 0.8 V and is due to the O2/H2O couple. In the log Eh versus pH diagram,... 

Figure A. Column end-assembly configured for mlcrovolcammecric electrochemical detector AE, auxiliary electrode CA, cartridge holder CH, column holder CM, Column EH, electrode holder FR, frit MM, micromanipulator PP, piezoelectric positioner RE, reference electrode SC screw cap WE, working electrode. (Reproduced from ref. 11. Copyright 1988 American Chemical Society.)...

Reducing and oxidizing conditions m a sediment determine the chemical stability of the solid compounds and the direction of spontaneous reactions. The redox state can be recognized as a voltage potential measured with a platinum electrode. This voltage potential is usually referred to as E or Eh defined by the Nemst equation, which was introduced in Chapter 5, Section 5.3.1 ... 

If a system is not at equilibrium, which is common for natural systems, each reaction has its own Eh value and the observed electrode potential is a mixed potential depending on the kinetics of several reactions. A redox pair with relatively high ion activity and whose electron exchange process is fast tends to dominate the registered Eh. Thus, measurements in a natural environment may not reveal information about all redox reactions but only from those reactions that are active enough to create a measurable potential difference on the electrode surface. 

In agreement with the theory of electrolysis, treated in Sections 3.1 and 3.2, the parts of the residual current and the limiting current are clearly shown by the nature of the polarographic waves because for the cathodic reduction of Cd2+ and Zn2+ at the dme we have to deal with rapid electron transfer and limited diffusion of the cations from the solution towards the electrode surface and of the metal amalgam formed thereon towards the inside of the Hg drop, we may conclude that the half-wave potential, Eh, is constant [cf., Fig. 3.13 (a ] and agrees with the redox potential of the amalgam, i.e., -0.3521V for Cd2+ + 2e - Cd(Hg) and -0.7628 V for Zn2+ + 2e -> Zn(Hg) (ref. 10). The Nernst equation is... 

In general we work in an analyte solution without stirring, as in polarography, unless mentioned otherwise. The simplest situation is that where a reversible redox process such as ox + ne red takes place at an inert electrode such as Pt, Pd, Ir or Eh (and sometimes Au or Ag) and where both ox and red... 

This means, in agreement with Fig. 4.17, that a negative Eh+/h2 value yields an additional increase in the Eredo range. Finally, solvation in itself contributes to extending the E OI range, as a more solvated ion is less accessible to electrolysis however, it is doubtful whether separate consideration of this effect makes any sense provided that E nodic and E +/h2 have been determined absolutely, i.e., versus the ferrocene-ferrocinium+ electrode. 

Complicating matters further is the fact that the platinum electrode, the standard tool for measuring Eh directly, does not respond to some of the most important redox couples in geochemical systems. The electrode, for example, responds incorrectly or not at all to the couples SO -HS-, O2-H2O, CO2-CH4, NOJ-N2, and N2-NH4 (Stumm and Morgan, 1996 Hostettler, 1984). In a laboratory experiment, Runnells and Lindberg (1990) prepared solutions with differing ratios of selenium in the Se4+ and Se6+ oxidation states. They found that even under controlled conditions the platinum electrode was completely insensitive to the selenium composition. The meaning of an Eh measurement from a natural water, therefore, may be difficult or impossible to determine (e.g., Westall, 2002). 

Their results show that the redox couples in a sample generally failed to achieve equilibrium with each other. For a given sample, the Nernst Eh values calculated for different redox couples varied over a broad range, by as much as 1000 mV. If the couples had been close to redox equilibrium, they would have yielded Nernst Eh values similar to each other. In addition, the authors could find little relationship between the Nernst values and Eh measured by platinum electrode. Criaud et al. (1989) computed similarly discordant Nernst Eh values for low temperature geothermal fluids from the Paris basin. 

As an example of modeling a fluid in redox disequilibrium, we use an analysis, slightly simplified from Nordstrom et al. (1992), of a groundwater sampled near the Morro do Ferro ore district in Brazil (Table 7.2). There are three measures of oxidation state in the analysis the Eh value determined by platinum electrode, the dissolved oxygen content, and the distribution of iron between ferrous and ferric species. 

Runnells, D. D. and R. D. Lindberg, 1990, Selenium in aqueous solutions, the impossibility of obtaining a meaningful Eh using a platinum electrode, with implications for modeling of natural waters. Geology 18, 212-215. 

Supersaturation of up to nearly 4 orders of magnitude is indicated relative to a log K= 4.9 which reflects freshly precipitated HFO. When elimination of all data points which are below the detection limits for Fe(lll) and for electrode measurements, values of Eh measured agree with Eh calculated from Fe(ll/lll) determinations and speciation calculations and the revised ferrihydrite saturation index diagram looks like fig. 3. 

Figure 9.9. Equations relating Eh, Eh°, and pH, where Eh° is the standard electrode potential R is the gas constant T is the absolute temperature n is the number of electrons F is the Faraday constant (Red), (Ox), and (H+) the concentration of the reduced and oxidized species and the hydrogen ion, respectively.

What do the terms Eh and Eh° stand for What types of electrodes are used for the determination of Eh in soil ... 

Because of the complex nature of titration curves obtained using pH, ion-selective electrodes, or millivolt (Eh) measurements on whole soils, these methods, except for organic matter determination, are seldom used. 

Fig. S-S8. Electron levels of dehydrated redox particles, H ld + bh /h = H,d, adsorbed on an interface of metal electrodes D = state density (electron level density) 6 = adsorption coverage shVi - most probable vacant electron level of adsorbed protons (oxidants) eH(d = most probable occupied electron level of adsorbed hydrogen atoms (reductants) RO.d = adsorbed redox particles.

TABLE 6-S. The flat band potential Eh for typical semiconductor electrodes in aqueous solutions SC = semiconductor electrode e, = band gap. [From Morrison, 1980.]... 

The reaction equiUbrivun of the normal hydrogen electrode, H2.g = 2 H , + 2eHVH,> where eH< 2 equilibrium redox electron, can be obtained by the... 

The Mn3+/Mn2+ couple is ostensibly pH independent, but we must bear in mind that manganese(III), in particular, will hydrolyze unless the acidity is very high, and both Mn(OH)3 and Mn(OH)2 will come out of solution in alkaline media. Small degrees of hydrolysis in solution have little impact on E°, but precipitation of hydroxides (and all metal hydroxides except those of the alkali metals and Ca, Sr, Ba, and Ra are poorly soluble in water) affects E° profoundly. Thus, in alkaline media, the electrode potentials (often called Eh by geologists, wherever [H+] is not the standard value) of Mn2+ and Mn3+ are controlled by the solubility products (Ksp) of Mn(OH)2 and Mn(OH)3, respectively. In practice, Mn(OH)3 tends to dehydrate to MnO(OH), so we consider the Mn2+(aq)/Mn(s) couple ... 

This situation cannot persist for long the concentration of copper (II) ions in water will initially be extremely small, unless some other source is also involved, and will quickly be depleted. The important point is that, as soon as electrical contact is made, the zinc becomes an anodic electrode, and the copper a cathode. If another cathodic reaction besides reaction 16.2 is possible, however, then dissolution (i.e., corrosion) of the zinc will continue, while the copper will serve merely as an electrically conducting surface to deliver electrons for the alternative cathodic reaction. In pure water, the obvious alternative reaction is hydrogen evolution (reaction 16.3) for which Eh is —0.414 V at pH 7 ... 

Hence, pE = 0 at a standard hydrogen electrode, and for any system at equilibrium we find the relationship between our new quantities and the reversible redox potential, e (or Eh)... 

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