Water redox stability field

Figure 15.2 Redox stability field of liquid water at 25 °C. Solid lines refer to gas partial pressures of 1 bar (standard state), broken lines to 10-6 bar.

Fig. 12.2. Redox-pH diagram for the Fe-S-H20 system at 100 °C, showing speciation of sulfur (dashed line) and the stability fields of iron minerals (solid lines). Diagram is drawn assuming sulfur and iron species activities, respectively, of 10-3 and 10-4. Broken line at bottom of diagram is the water stability limit at 100 atm total pressure. At pH 4, there are two oxidation states (points A and B) in equilibrium with pyrite under these conditions.

The main usefulness of Eh-pH diagrams consists in the immediacy of qualitative information about the effects of redox and acid-base properties of the system on elemental solubility. Concerning, for instance, cerium, figure 8.20 immediately shows that, within the stability field of water, delimited upward by oxidation boundary curve o and downward by reduction boundary curve r, the element (in the absence of other anionic ligands besides OH groups) is present in solution mainly as trivalent cerium Ce and as soluble tetravalent hydroxide Ce(OH)2. It is also evident that, with increasing pH, cerium precipitates as trivalent hydroxide Ce(OH)3. 

Figure 8.21C shows the Eh-pH diagram for phosphorus at a solute total molality of 10 ". Within the stability field of water, phosphorus occurs as orthophos-phoric acid H3PO4 and its ionization products. The predominance limits are dictated by the acidity of the solution and do not depend on redox conditions. 

Fig. 19 Redox buffer and subdivision of natural ground waters into 4 redox ranges within the stability field of water black dashed lines indicate the boundaries of the four redox ranges (after Drever 1997)...

An electrolytic cell is similar to a voltaic cell except the electrochemical reactions involved do not occur spontaneously but require the input of current from an external source. Wires connected to each end of a battery and submerged in a suitable electrolyte can represent an electrochemical cell. As with voltaic cells, the creation and/or removal of ions at the electrodes facilitates the transfer of current into and out of solution. If the electrolytes in solution are redox-inert within the stability field of water (e.g., Na and Cf) and the voltage is over 1.2 volts, the hydrolysis of water may transfer current at the electrodes ... 

Almost any natural redox-active substance that can exist in the Eh stability field of water could potentially contribute to this redox gradient. Natural terrestrial materials more oxidising than oxygen are virtually non-existent and consequently natural redox... 

The theoretical stability field of water in Eh and pH terms (see Fig. 11.2) bounds all theoretical redox reactions taking place in water. The upper boundary of the water stability field is defined by Eh and pH values for which liquid water is in equilibrium with 02(g) at 1 bar pressure. The lower boundary is defined by Eh and pH values for which liquid water is in equilibrium with H2(g) at 1 bar pressure. 

Figure 7.8. Stability field diagram for soluble and solid forms of Mn under a range of redox potential and pH conditions, assuming a dissolved CO concentration of 10 M. Shaded areas delineate conditions that are unattainable because of water decomposition. (Adapted from W. Stumm and J. J. Morgan. 1981. Aquat/c Chemistry. 2nd ed. New York Wiley.)...

Alternatively, fulvalene complexes have been prepared from alkynylferrocenes by Pauson Khand reactions [106]. Although numerous complexes 103 with metals different from iron and also chiral indenyl-derived dibenzofulvalene complexes are known [107], 106 has remainedthe most prominent representative of its class. As an example, recent research in the field involves the work of Schmittel et al., who report a higher redox stability for 106 in water at pH = 0-12 as compared to... 

Spent fuels vary in microstructure, and phase and elemental distribution depending on the in-core reactor operating conditions and reactor history. The chemical stability of spent U oxide fuel is described by local pH and Eh conditions, redox being the most important parameter. However, the redox system will also evolve with time as various radionuclides decay and the proportion of oxidants and reductants generated at the fuel/water interface changes with the altering a-, (J-, y-radiation field and with the generation of other corrosion products that can act as... 

For gold transported at temperatures of less than 400 °C in reduced sulfide-bearing solutions, gold solubility is maximized near the H2S-HS -S04 equal predominance point at any given temperature, pressure, and activity of water (see Figure 3). As conditions become more oxidized, the sulfide complexes that stabilize gold in aqueous solution break down, and gold metal precipitates. As pH increases (in the HS predominance field), the redox reaction... 

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