Normal hydrogen electrode reduction potentials

According to its definition, the standard (reduction) potential of the A/A couple is the standard electromotive force of a cell in which an A/A electrode (where the activities of A and A are made unity) is opposed to an NHE (normal hydrogen electrode) whose potential is assigned to zero by convention. 

Figure 18.6 Energetics of the ORR at the heme/Cu site of CcO the enzyme couples oxidation of ferroc3ftochrome c (standard potential about —250 mV all potentials are listed with respect to a normal hydrogen electrode) to reduction of O2 (standard potential at pH 7 800 mV). Of the 550 mV difference, only 100 mV is dissipated to drive the reaction 220 mV is expanded to translocate four protons from the basic matrix compartment to the acidic IMS (inter-membrane space). In addition 200 mV is converted into transmembrane electrostatic potential as ferroc3ftochrome is oxidized in the IMS, but the charge-compensating protons are taken from the matrix. The potentials are approximate.

Electrocatalytic reduction of both O2 and H2O2 starts at potentials close to that of the Fe couple in the absence of a substrate (which for most porphyrins is about 0.2-0 V with respect to a normal hydrogen electrode (NHE) at pH < 6 the exception being Fe(TMPyP), E k 0.5 V). Catalytic reduction of H2O2 by simple/erne porphyrins is too slow to be detectable in typical electrocatalytic experiments whereas ferrous porphyrins catalyze rapid reduction of H2O2,... 

EPR spectrometers use radiation in the giga-hertz range (GHz is 109 Hz), and the most common type of spectrometer operates with radiation in the X-band of micro-waves (i.e., a frequency of circa 9-10 GHz). For a resonance frequency of 9.500 GHz (9500 MHz), and a g-value of 2.00232, the resonance field is 0.338987 tesla. The value ge = 2.00232 is a theoretical one calculated for a free unpaired electron in vacuo. Although this esoteric entity may perhaps not strike us as being of high (bio) chemical relevance, it is in fact the reference system of EPR spectroscopy, and thus of comparable importance as the chemical-shift position of the II line of tetra-methylsilane in NMR spectroscopy, or the reduction potential of the normal hydrogen electrode in electrochemistry. 

For the reaction of hydrogen and oxygen to generate a current in a fuel cell, the anode needs to be polarized more positive than 0 V vs. NHE (Normal Hydrogen Electrode, the reference potential for all electrochemical reactions) for the oxidation of hydrogen, while the cathode needs to be polarized more negative than 1.229 V vs. NHE for the reduction of oxygen. 

The metal-centered redox potential is the most important criterion for the complex to be the SOD mimetic, since the catalytic disproportionation of O2 requires redox reactions between complex and superoxide (Scheme 9) (18). The complex redox potential should fall between the redox potentials for the reduction and oxidation of O2, viz. —0.16 and +0.89 V vs. NHE (normal hydrogen electrode), respectively (Scheme 1) (2). 

Fig. 7.1 Position of band edges and photodecomposition Fermi energies levels of various non-oxide semiconductors. E(e,d) represents decomposition energy level by electrons, while E(h,d) represents the decomposition energy level for holes vs normal hydrogen electrode (NHE). E(VB) denotes the valence band edge, E(CB) denotes the conduction band edge. E(H2/H20) denotes the reduction potential of water, and (H2O/O2) the oxidation potential of water, both with reference to NHE.

The potentials are referred to the normal hydrogen electrode (NHE). The energy levels for the oxidation and reduction of water at pH 7 are shown by horizontal lines. Energy scale in volts. 

The diphenylphenanthroline complex 23 is emissive in fluid solutions, with quantum yields of ca. 10-4 and lifetimes of 0.4-0.7 ps. The estimated excited state reduction potential of 2.2 V (vs. normal hydrogen electrode (NHE)) suggested that the complex is a strong photooxidant, which was demonstrated with the formation of the 1,4-dimethoxybenzene radical cation (DMB+) upon UV-visible irradiation of an MeCN solution of the complex with DMB.22... 

There are two low-valent oxidation states available to the lanthanides under normal conditions the +2 oxidation state and the formally zero oxidation state found in the elemental metals. The zero oxidation state is available to all the lanthanides, but only three members of the series have +2 oxidation states accessible under common organometallic reaction conditions Eu (4/ ), Yb (4/ ), and Sm (4/ ). The Ln VLn" reduction potentials [vs. normal hydrogen electrode (NHE)] (12), - 0.34 V for Eu, - 1.04 V for Yb and - 1.50 V for Sm, indicate that Eu is the most stable and Sm the most reactive of these divalent ions. Sm is also the most reactive based on radial size considerations, since it is the largest and most difficult to stabilize by steric saturation. 

The physical properties of dihydrogen are well understood. Free hydrogen gas has a bond length of 0.74 A, with a bond strength of 103 kcal mol The potential of proton reduction is set at 0 V for the normal hydrogen electrode (NHE) under... 

Figure 6. Size-dependence ofthe reduction potential E° of silver dusters in water ( ) and ofthe ionization potential IP of silver clusters in the gas phase (A). The reduction potentials refer to the normal hydrogen electrode one which is at4.5eVabove vacuum [11],...

Also in 1975, Fajer, Brune, Davis, Forman and Spaulding investigated the possible involvement of bacteriopheophytin in bacterial photosynthesis by examining the spectral properties of electrochemi-cally reduced bacteriopheophytin (BO) in solution. BO molecules in CH2CI2 undergo a reversible, one-electron reduction with a halfwave potential (Ey,) of-0.54 V V5. the normal hydrogen electrode. From the experimentally measured reduced-minus-oxidized difference spectrum of BO, combined with the known... 

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