Sulfur redox diagram

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.

Table 2.1 states the redox relations at standard conditions. Extended information on the distribution of the redox pairs — still under equilibrium conditions but under varying redox potential and pH — is given in a Pourbaix diagram. Figure 2.4 is an example of such a diagram for the binary sulfur and oxygen system in water at 1 atm and 25°C with the sum of the concentrations of... 

In a similar feshion, fi. n-pH diagrams can be constructed for other redox half reactions. Some examples are given in Figure 7.8. These diagrams suggest that in oxic seawater = +0.4 V and pH = 8), the stable form of iron is Fe(OH)3, nitrogen is stable as nitrate, sulfur as sulfete, and carbon as bicarbonate, if each of these species reaches redox equilibrivun. 

Figure 1. Tridimensional Pourbaix diagram. XYZ plot of redox potentials, MDC of sulfur and pH. Aqueous solutions at 25°C. X = redox potential, volt vs. NHE Y = log (MDCU defined by Equation 1, i — moieties identified on curves (Sc,...

The Figure is a schematic polarization curve for a metal exhibiting typical thin-film active-passive behavior (e.g., Ni or Cr in sulfuric acid). Note that this diagram is for a single redox system, namely M/M+ (i.e.,... 

Figure 8.13. Equilibrium concentrations of biochemically important redox components as a function of pe at a pH of 7.0 (a) nitrogen (b) nitrogen, with elemental nitrogen N2 ignored (c) iron and manganese (d) sulfur (e) carbon. These equilibrium diagrams have been constructed from equilibrium constants listed in Tables 8.6a and 8.6b for the following concentrations Cr (total carbonate carbon) = 10 M [HjSCaq)) + [HS ] -I- [SOri = 10 M [NOj-] + [NOj"] + [NH ] = 10 M = 0.78 atm and thus [NjCaq)] = 0.5 x 10 M. For the construction of (b) the species NH4 , NOj, and NO are treated as metastable with regard to Nj.

The Eh-pH diagram for thermodynamically most stable sulfur species is shown in Fig. 12.16. The acid-base boundaries have been considered above. We will derive two of the redox boundaries. Probably the S0 /H2S and SO /HS boundaries are the most important. The SOl /HzS redox reaction is... 

Less common among the metastable sulfur species are the polythionates (S Oi" with n = 4 to 9). TTiey are formed, for example, in volcanic lakes by the bubbling of H2S gas through sulfurous acid (Takano 1987). The polythionates break down eventually into sulfate and elemental sulfur. The polysulfides and polythionates are themselves metastable relative to sulfite, thiosulfate, S , and the sulfide species. In order to examine the redox-stability environments of these species, it is useful to construct an Eh-pH diagram ignoring sulfate, which is relatively inert. Such a plot is given in... 

The driving force of the reaction at Eq. (1) comes from the thermodynamic instabihty of elemental sulfur in water. The Pourbaix diagram of the S8/H2O system at 20 °C shows elemental sulfur to exist as a separate phase only at very low pH values and redox potentials in the range 0.1-0.3 V [11]. 

Fig. 4. Simplified diagram, illustrating the proposed oxidation states and coordination of the [NiFe]-dinuclear center in D. gigas hydrogenase. States with the highest oxidation level are at the top, each lower level representing reduction by one equivalent. The redox states of the Fe-S clusters are not shown, e represents electrons exchanged with the iron-sulfur clusters H+ protons from exchange with the proton channels.

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