Redox-buffer

For example, assuming anhydrite-magnetite-calcite-pyrite-pyrrhotite buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations of 21 °N EPR fluid indicate a temperature of 370-385°C. However, the estimated temperatures are higher than those of the measurement. This difference could be explained by adiabatic ascension and probably conductive heat loss during ascension of hydrothermal solution from deeper parts where chemical compositions of hydrothermal solutions are buffered by these assemblages. 

Fig. 2.33. H2Saq concentration.s as a function of temperature for hot spring fluids at midocean ridges as a function of redox. Assuming AMPC (anhydrite-magnetite-pyrite-calcite) and PPM (pyrite-pyrrhotite) buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations indicate temperatures of approximately 370-385°C. Solid star Okinawa. (Modified after Seyfried and Ding, 1995.)...

Although this treatment of buffers was based on acid-base chemistry, the idea of a buffer is general and can be extended to equilibria involving complexation or redox reactions. For example, the Nernst equation for a solution containing Fe + and Fe + is similar in form to the Henderson-Hasselbalch equation. 

The purity of a sample of Na2S203 was determined by a coulometric redox titration using as a mediator, and as the titrant. A sample weighing 0.1342 g is transferred to a 100-mL volumetric flask and diluted to volume with distilled water. A 10.00-mL portion is transferred to an electrochemical cell along with 25 mL of 1 M KI, 75 mL of a pH 7.0 phosphate buffer, and several drops of a starch indicator solution. Electrolysis at a constant current of 36.45 mA required 221.8 s to reach the starch indicator end point. Determine the purity of the sample. 

Hydrogen chloride, 203,562-563 Hydrogen fluoride, 183,562-563 Hydrogen ion acceptors, donors of, 353 and balancing redox equations, 88-89 in buffer systems, 387-390 and hydroxide ion, 354—355 and indicator color, 391-393 and pH, 355... 

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

These measurements indicate that it is not possible to identify a single value of pe surrounding the O2/H2S interface in the environment. Redox couples do not respond to the pe of the environment with the same lability as hydrogen ion donors and acceptors. There is no clear electron buffer capacity other than the most general states of "oxygen containing" or "H2S containing." The reason for the vast differences in pec in the oxic waters is the slow oxidation kinetics of the reduced forms of the redox couples. The reduced species for which the kinetics of oxidation by O2 has been most widely studied is Mn. This oxidation reaction... 

TABLE 7 Redox Peak Potential of Cytochrome P450scc at Different pH of Phosphate Buffer... 

Figure 26 shows the redox potential of 40 monolayers of cytochrome P450scc on ITO glass plate in 0.1 KCl containing 10 mM phosphate buffer. It can be seen that when the cholesterol dissolved in X-triton 100 was added 50 pi at a time, the redox peaks were well distinguishable, and the cathodic peak at -90 mV was developed in addition to the anodic peak at 16 mV. When the potential was scanned from 400 to 400 mV, there could have been reaction of cholesterol. It is possible that the electrochemical process donated electrons to the cytochrome P450scc that reacted with the cholesterol. The kinetics of adsorption and the reduction process could have been the ion-diffusion-controlled process. 

The redox condition of the deep fluids may be buffered by the chlorite-pyrite equilibrium. 

TABLE 2 Concentrations of Redox Products After the Electrolysis for 4 h by Applying a Definite E appi at the Stationary Interface Between W Containing 10 M H2O2 and DCE containing 10 M Tetrachlorohydroquinone, CQH2, Under the Deaerated Condition. As Supporting Electrolytes, 0.5 M Li2SO and 0.05 M TPenA+TEPB Were Added in W and DCE, Respectively. The pH of W Was Adjusted with the Aid of 0.1 M Phosphate Buffer to be 7.0... 

Sn/V/Nb/Sb/0 catalysts were prepared with the co-precipitahon technique, developed for the synthesis of ratile Sn02-based systems claimed by Rhodia (8). The preparation involved the dissoluhon of SnCl45H20, VO(acac)2, SbCh and NbCls in absolute ethanol, and by dropping the soluhon into a buffered aqueous soluhon maintained at pH 7. The precipitate obtained was separated from the liquid by filtrahon. The solid was then dried at 120°C and calcined in air at 700°C for 3 hours. The V/Sb/0 catalyst was prepared by means of the slurry method that consists in a redox reachon between Sb203 and NH4VO3 in water medium, for 18h at 95°C. The... 

In fact, any type of titration can be carried out potentiometrically provided that an indicator electrode is applied whose potential changes markedly at the equivalence point. As the potential is a selective property of both reactants (titrand and titrant), notwithstanding an appreciable influence by the titration medium [aqueous or non-aqueous, with or without an ISA (ionic strength adjuster) or pH buffer, etc.] on that property, potentiometric titration is far more important than conductometric titration. Moreover, the potentiometric method has greater applicability because it is used not only for acid-base, precipitation, complex-formation and displacement titrations, but also for redox titrations. 

In the controlled (constant) potential method the procedure starts and continues to work with the limiting current iu but as the ion concentration and hence its i, decreases exponentially with time, the course of the electrolysis slows down quickly and its completion lags behind therefore, one often prefers the application of a constant current. Suppose that we want to oxidize Fe(II) we consider Fig. 3.78 and apply across a Pt electrode (WE) and an auxiliary electrode (AE) an anodic current, -1, of nearly the half-wave current this means that the anodic potential (vs. an RE) starts at nearly the half-wave potential, Ei, of Fe(II) - Fe(III) (= 0.770 V), but increases with time, while the anodic wave height diminishes linearly and halfway to completion the electrolysis falls below - / after that moment the potential will suddenly increase until it attains the decomposition potential (nearly 2.4 V) of H20 -> 02. The way to prevent this from happening is to add previously a small amount of a so-called redox buffer, i.e., a reversible oxidant such as Ce(IV) with a standard... 

The result of the entire procedure, being a 100% conversion of Fe(II) to Fe(III), thus represents a so-called coulometric titration with internal generation of course, it seems possible to titrate Fe(II) with Ce(IV) generated externally from Ce(III), but in this way one would unnecessarily remove the solution of the 100% conversion problem hence the above titration with internal generation in the presence of a redox buffer as an intermediary oxidant represents an extremely reliable method, unless occasional circumstances are prohibitive for the remainder internal generation offers the advantage of no dilution of the analyte solution. 

It is very often necessary to characterize the redox properties of a given system with unknown activity coefficients in a state far from standard conditions. For this purpose, formal (solution with unit concentrations of all the species appearing in the Nernst equation its value depends on the overall composition of the solution. If the solution also contains additional species that do not appear in the Nernst equation (indifferent electrolyte, buffer components, etc.), their concentrations must be precisely specified in the formal potential data. The formal potential, denoted as E0, is best characterized by an expression in parentheses, giving both the half-cell reaction and the composition of the medium, for example E0,(Zn2+ + 2e = Zn, 10-3M H2S04). 

The slope of the tangent to the curve at the inflection point where oc = is thus inversely proportional to the number of electrons n. The E-oc curves are similar to the titration curves of weak acids or bases (pH-or). For neutralization curves, the slope dpH/doc characterizes the buffering capacity of the solution for redox potential curves, the differential dE/da characterizes the redox capacity of the system. If oc — for a buffer, then changes in pH produced by changes in a are the smallest possible. If a = in a redox system, then the potential changes produced by changes in oc are also minimal (the system is well poised ). 

At greater depths, the redox characteristics are mainly determined by another mineral system fayalite (Fe2Si04), magnetite and quartz (Si02) (theFMQ system). The buffer properties of the two systems can be expressed in terms of the following equations ... 

Table 7.1 Comparison of the yields of carbon-containing compounds obtained from an atmosphere of CH4, NH3, H2O and H2 using spark discharges with those obtained under hydrothermal conditions from a mixture of HCN, HCHO and NH3 at 423 K and 10 atm in the presence of pyrite-pyrrhotite-magnetite redox buffer (Holm and Andersson, 1995)...

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. 

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