Oxidation state buffering

Analogous examples of nonuniqueness can be constructed using any mineral or gas of intermediate oxidation state. Buffering the fugacity of N2(g) or S02(g), for example, would be a poor choice for constraining oxidation state, since the gases can either oxidize to NO3 and SO4 , respectively, or reduce to NH4 and H2S(aq) species. 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

As a second example, we constrain a fluid s oxidation state by assuming equilibrium with pyrite (FeS2). As before, direct information on this variable can be difficult to obtain, so it is not uncommon for modelers to use mineral equilibrium to fix a fluid s redox state. The choice of pyrite to buffer oxidation state, however, is perilous because pyrite sulfur, which is in the S1- oxidation state, may dissolve by oxidation to sulfate (S6+),... 

As a second example, we consider how the presence of pyrite (FeS2) can serve to buffer a fluid s oxidation state. We set an initial system at 100 °C containing a 1... 

With a final example, we consider how the presence of a gas phase can serve as a chemical buffer. A fluid, for example, might maintain equilibrium with the atmosphere, soil gas in the root zone, or natural gas reservoirs in deep strata. Gases such as 02 and H2 can fix oxidation state, H2S can set the activity of dissolved sulfide, and C02 (as we demonstrate in this section) can buffer pH. 

Fig. 12.17 Determination of the oxidation state of submicro traces of Lr (open circles) and No (solid circles). Solvent 0.2 mol dm TTA-MIBK aqueous phases buffers at various pHs. (From Ref. 67.)...

To summarize, the special bonding characteristics of 7r-acceptor ligands in organotransition metal compounds enable these complexes to coordinate small molecules such as ethylene, CO, and H2 and also provide an electronic buffer system to facilitate changes of metal oxidation state and coordination... 

The arsenic oxidation state data and the calculated pH at 300°C (see Table H) allow an upper limit on the Eh of the solution in the basalt-water experiment to be estimated from Equation (2). Assuming aH,0 = 1 and As(V) = 15 pg/L, this upper limit Eh value is -400 100 mV. The basalt-fluid redox buffer mechanism of Jacobs and Apted (2) gives an Eh of about -600 mV at 300°C and pH 7.8 (19). This mechanism involves ferrous ironbearing basalt glass + water reacting to magnetite + silica. 

Sulfur functions in low oxidation states have been oxidized to sulfoxides, sul-fones, and sulfonic acids, often in very good yields in spite of the fact that cpe was not employed. This is probably due to the resistance towards oxidation of the products, making control of the anode potential a less critical factor, and to the use of a potential-buffering SSE (Sect. 5.3). Illustrative examples include the preparation of 2,2 -bishydroxyethyl sulfone 1 24 > dibenzyl sulfoxide 125 ethanesulfonic acid 126 dibenzyl disulfoxide 125) and dimethyl sulfone 127 ... 

If material can be lost by volatilization, as with phases containing alkalis, fluoride, or sulphate, it may be necessary to use a sealed platinum container and to test its effectiveness by chemically analysing the product. Control of the furnace atmosphere, e.g. by employing mixtures of CO and CO2 to buffer the oxygen pressure, may be needed if variable oxidation states are possible, as with iron compounds. Special methods may be needed to make single crystals of sufficient size for X-ray structure deterniinations or other purposes. These are described in papers on X-ray structure determination. Single crystals of C3S can be obtained from CaClj melts (NIO). 

Fig. 9. A reductive titration of the crystalline bovine heart cytochrome c oxidase with dithionite. Absolute spectra for each oxidation state are shown for the Soret (A) and visible (B) regions. The difference spectra against the spectrum in the fully reduced state are given for the near-infrared region (C). The insets show titration curves against the electron equivalent per enzyme. The reaction mixture contained 7.5 jlM bovine heart cytochrome c oxidase in 0.1 M sodium phosphate buffer, pH 7.4. The enzyme preparation was stabilized with a synthetic non-ionic detergent, CH3(CH2)ii(0CH2CH2)80H. The light path was 1 cm.

The control that the ene-dithiolate ligand has upon the first ionization energy of these complexes illustrates the importance of this ligation to the reactivity found for molybdoenzymes. The ene-dithiolate ligand acts as an electronic buffer , effectively dampening the harsh electronic changes that would otherwise be expected to take place with changes in the metal formal oxidation states and atom transfer reactions at the active site in these enzymes. 

Studies in buffered solutions showed that an Fe-As complex was formed, the solubility of which was also dependent on the oxidation state of the As and the solution pH. 

Redox properties of all the complexes except 7 were studied in acetonitrile. The complex 7 was studied in aqueous phosphate buffer solution (pH, 6.88). While the complexes 1 - 3 are isolated in trivalent (III,III) states, and 4 in tetravalent (IV,IV) and one-electron reduced mixed-valence (in,rV) states, their cyclic voltammograms all display two one-electron reversible redox processes between the oxidation states (III,III) and (IV,IV) in acetonitrile. The mixed-valence diosmium(in,IV) complex has been isolated and its stmeture has been determined by the X-ray stmctural analysis. Three dinudear Ru(ni) complexes are reduced with a quasi-reversible one-electron and irreversible one-electron processes. The oxo-centered trinuclear mthenium(in) complex displays four one-electron reversible processes om (II,II,III) to (in,IV,IV). Complexes with three different oxidation states (II,III,III),... 

Laser flash photolysis time-resolved spectrophotometry, utilizing deazariboflavin-EDTA as a photochemical reductant, has been used with this system in order to characterize the initial step in the ET mechanism. Figure 3 shows examples of the type of data obtained in these studies. In the top panel, a transient is shown [54] that was obtained at 507 nm in 100 mM phosphate buffer, pH 7.0, containing 35 pM Fd, and in the middle panel, 10.3 pM FNR has been added to the solution prior to photolysis. This wavelength corresponds to an isosbestic point for the FAD cofactor of the reductase, and thus the absorbance change monitors the oxidation state of the [2Fe-2S] cluster of Fd (and also the formation and decay of the dRfH species). As is evident, immediately after the laser flash there is a rapid rise in absorbance due to dRfH formation. This is followed by a sharp absorbance decrease corresponding to Fd reduction and dRfH oxidation. The subsequent slow increase in absorption shown in the middle panel is a consequence of Fd reoxidation that is due to electron transfer to FNR. The latter is confirmed by measurement at 610 nm (bottom panel), a wavelength which monitors FAD neutral semiquinone formation the rate constant obtained from the 610 nm absorbance rise is the same as that obtained from the slow absorbance increase at 507 nm, consistent with this interpretation. 

The drawbacks of a simplified relative fo approach become apparent in the case of the relative volumetric properties of EeO and Ec203 in sihcate melts even at relatively low pressures (Kress and Carmichael, 1991). There are two problems first, the pressure dependence of EeO-Ec203 equilibria is different from the pressure dependence of the nickel-nickel oxide (NNO) buffer such that use of NNO as a normalization for relative/oj can be misleading. The volume change of the FMQ buffer (0.17 log/o units per GPa) is much closer to the redox state of a silicate melt than is that of the NNO buffer (0.51 log/o units per GPa). Second, the compressibilities of FeO and Fe203 are much different, so that even pressures of 1-2 GPa will have a large effect on their partial molar volumes. Any calculation of relative /o should include an assessment of the volumetric properties of both the buffer phases, and the phases involved in the redox reaction. 

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