Water standard electrode potentials

Reductions. Hydrazine is a very strong reducing agent. In the presence of oxygen and peroxides, it yields primarily nitrogen and water with more or less ammonia and hydrazoic acid [7782-79-8]. Based on standard electrode potentials, hydrazine in alkaline solution is a stronger reductant than sulfite but weaker than hypophosphite in acid solution, it falls between and Ti ( 7). 

In this equation is the standard electrode potential of the water/oxygen reaction, i.e. —AG% o/nF. Simplifying, equation 12.4 at 298K becomes... 

Meta Standard electrode potential, hydrogen scale (V) Corrosion potential in flowing sea-water, hydrogen scale (V)... 

The values of Hn and E are zero for water, by virtue of the constants 1.74 and 2.60. In these definitions, pKa refers to the acid ionization constant of the conjugate acid of the nucleophile, and E° to the standard electrode potential for the two-electron half-reaction ... 

The standard Galvani potential difference can be related to the standard electrode potentials and oxi/rediCo) oii hydrogen scale in water or organic... 

The review of Martynova (18) covers solubilities of a variety of salts and oxides up to 10 kbar and 700 C and also available steam-water distribution coefficients. That of Lietzke (19) reviews measurements of standard electrode potentials and ionic activity coefficients using Harned cells up to 175-200 C. The review of Mesmer, Sweeton, Hitch and Baes (20) covers a range of protolytic dissociation reactions up to 300°C at SVP. Apart from the work on Fe304 solubility by Sweeton and Baes (23), the only references to hydrolysis and complexing reactions by transition metals above 100 C were to aluminium hydrolysis (20) and nickel hydrolysis (24) both to 150 C. Nikolaeva (24) was one of several at the conference who discussed the problems arising when hydrolysis and complexing occur simultaneously. There appear to be no experimental studies of solution phase redox equilibria above 100°C. 

The standard electrode potentials, E°(V) for some chelates of the Fe /Fe redox couple areas follows o-phenanthroline, 1.20 2,2 -bipyridyl 1.096 water, 0.77 cyanide, 0.10 oxalate, -0.01 and 8-hydroquinone, -0.15 (Latimer, 1952). In the case of bipyridyl... 

The most common oxidation states of iron are +2 (ferrous) and -i-3 (ferric). The standard electrode potential for Fe —> Fe2+ + 2e- is -0.440 volts. Thus, the metal can replace hydrogen from water at ordinary temperatures ... 

Passage of 1.0 mol of electrons (one faraday, 96,485 A s) will produce 1.0 mol of oxidation or reduction—in this case, 1.0 mol of Cl- converted to 0.5 mol of Cl2, and 1.0 mol of water reduced to 1.0 mol of OH- plus 0.5 mol of H2. Thermodynamically, the electrical potential required to do this is given by the difference in standard electrode potentials (Chapter 15 and Appendix D) for the anode and cathode processes, but there is also an additional voltage or overpotential that originates in kinetic barriers within these multistep gas-evolving electrode processes. The overpotential can be minimized by catalyzing the electrode reactions in the case of chlorine evolution, this can be done by coating the anode with ruthenium dioxide. 

The chemistry of fluorine is dominated by its electronegativity, which is the highest of all elements. The colorless gas F2 has an estimated standard electrode potential E° (Chapter 15) of +2.85 V for reduction to F (cf. + 1.36 V for Cl2 to Cl-), and thus F2 immediately oxidizes water to oxygen (E° = +1.23 V), and 2% aqueous NaOH to the gas F20. Obviously, F2 cannot be made by electrolysis of aqueous NaF. The usual preparation involves electrolysis of HF-KF melts in a Monel (Cu-Ni alloy) or copper apparatus. 

However, it is not an easy matter to relate these scales of E° to the standard electrode potentials in water. This is because of the unknown liquid junction potential that is inevitably introduced when one attempts to calibrate the potential of a reference electrode in a given non-aqueous solution against a common reference electrode, such as the standard hydrogen electrode or saturated calomel electrode in aqueous solution. 

As for the reoxidation of reduced heteropoly compounds in the solid state, few reliable studies have been reported. It was reported that the reoxidizability increases with an increase in standard electrode potentials of countercations (108). In the case of reoxidation by O2 of le -reduced CsxHj - PMo 12O40, the rates divided by the surface area show a monotonic variation (Fig. 53e) as in Figs. 53c and d, indicating a surface reaction. A similar variation was observed for the Na and K salts. The presence of water vapor sometimes accelerates the migration of oxide ion, probably in the form of OH- or H20, and makes surface-type reactions more like bulk type II reactions (266). 

Bratsch, S.G. 1989. Standard electrode potentials and temperature coefficients in water at 298.15... 

Fig. 1. Reduction potential E (referenced to the standard hydrogen electrode) versus pH for various species of vanadium. Boundary lines correspond to E, pH values where the species in adjacent regions are present in equal concentrations. The short dashed lines indicate uncertainty in the location of the boundary. The upper and lower long dashed lines correspond to the upper and lower limits of stability of water. Standard reduction potentials are given by the intersections of horizontal lines with the abscissa pH = 0. The half reactions are 02 + 4H+ + 4e = 2H20, E° = 1.23V V02+ + 2H+ + e = V02+ + H20, E° = 1.0V V02+ + 2H+ + e = V3+ + H,0, E° = 0.36V 2H+ + 2e = H2, E° = 0.0V and V3+ + e = V2+, E° = -0.25V. V2+ is therefore a strong reductant. Air oxidation of V02+ presumably proceeds by the reaction 4V02t + 02 + 2H20 = 4VOJ + 4H+, E° = 0.23V which is favored at higher pH. Not all known species are represented on this diagram. Reproduced with permission from Ref. 30...

AG ° (i) = standard free energy change for the transfer of i from water into the mixed solvent N = Avogadro s number e = electronic charge Dw = dielectric constant of water Ds = dielectric constant of the mixed solvent Th2o = radius of the water molecule Ew° = standard electrode potential in water EB° = standard electrode potential in the mixed solvent M = cation X = anion... 

The values of E, log y , Mxy> A, and B from Equations 5-10 substituted into Equation 4, make it possible to calculate Em0 at known molalities of hydrobromic acid, solvent compositions, and temperatures. By plotting values of Em° at a given solvent composition and temperature vs. molality, one can find the standard electrode potential E° of the Ag-AgBr electrode at that solvent composition and temperature from the value of Em° extrapolated to infinite dilution. This method has been used successfully in water and in organic solvent-water mixtures of higher dielectric constants, but if the mixed solvents have low dielectric constants, ca. 50 or below, the curvatures of the Em0f vs. m plots are sufficient to prevent accurate determinations of Em0 and hence of E°. 

All cell potentials reached equilibrium in 1 or 2 hr, except when the solvent was anhydrous terf-butanol, in which the electrodes reached equilibrium only in dilute soltuions of HBr and even then only in a sluggish manner. This sluggish behavior has been reported (27) for the silver-silver bromide electrode in anhydrous ethanol when the acid was concentrated. In the dilute hydrobromic acid solutions used here, this phenomena was not observed in anhydrous ethanol. It is estimated that the standard electrode potential of the silver-silver bromide electrode in anhydrous terf-butanol is accurate to only d=l mV. However, these data are reported to the same degree of precision found in the other tert-buta-nol-water solvents in order to facilitate comparisons of the emf s in the various dilutions of tert-butanol used. 

Table 8 Absorption maxima, extinction coefficients, stability constants, and standard electrode potentials for selected Tris-(l,4-diimine)iron(II) complexes in water...

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