Protons, standard reduction potentials

Proton Transfer Standard Reduction Potential Group Transfer... 

Dilute aqueous solutions of strong acids (e.g. HCl or H2SO4) contain sufficient concentrations of hydrated protons to oxidize many metals, to produce their most stable states in solution. The only thermodynamic condition for metal oxidation is that the reduction potential of the metal ion produced should be negative. In general, for the metal ion M + undergoing reduction to the metal, if the standard reduction potential for the half-reaction ... 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

The hydroxyl radical is a powerful oxidant with a standard reduction potential of 2.73 V in acidic solution. In neutral solution where the free energy of neutralization of OH by the proton is not available, the reduction potential is calculated to be 1.9 V (Table 2). 

Pumps require energy to function. The "pumps" of the ETS chain derive their energy to transport protons from oxidation/reduction (called redox) reactions that occur as electrons move from one complex to another. To quantitate the amount of energy in these transfers, the standard reduction potential can be used. 

Many complex biochemical reactions are electron-transfer processes, and as such have a standard reduction potential. For example, nicotinamide adenine dinucleotide (NAD" ") accepts a proton and two electrons to become NADH ... 

The first element, hydrogen, has an Allred Rochow electronegativity coefficient of 2.1, and an electronic configuration Is1. The atom may lose the single electron to become a proton, which exists in aqueous solutions as the hydroxonium ion, H30+(aq), in which the proton is covalently bonded to the oxygen atom of a water molecule. The ion is hydrated, as is discussed extensively in Chapter 2. The reduction of the hydrated proton by an electron forms the reference standard half-reaction for the scale of reduction potentials ... 

Table 6 Standard aqueous reduction potentials for protons, water, and oxygen species and the free energy of bond formation for several hydrogen - oxygen species...

Table 3-7 Standard Aqueous Reduction Potentials for Protons, Water, and Oxygen Species and the Free Energy of Bond Formation (-AGbf) for Several Hydrogen-Oxygen Species...

The one-electron reduction potential (E ) values were calculated at several pH values using several redox standards. From these measurements, e) = - 109 mV vs NHE at 25°C was established. A E versus pH plot shows a region independent of pH, i.e. no proton involved in the reduction process, confirming the above structural assignments. 

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