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Normal hydrogen electrode, potential

Fig. 4-27. Work fimction 4> of emersed electrodes of gold, silver and platinum in ultra hig vacuum (UHV) as a function of electrode potential E at which the electrodes have been maintained in 0.1 M per chloric add solution before emersion Esm => normal hydrogen electrode potential = 4.5 or 4.44 V Cnhe = emersed normal h3rdiogen electrode potential in UHV = 4.85 V arrow = work f mction of free dean surfaces of gold, platinum and silver in vacuum. [From Kota-Neff-Moller, 1986.]... Fig. 4-27. Work fimction 4> of emersed electrodes of gold, silver and platinum in ultra hig vacuum (UHV) as a function of electrode potential E at which the electrodes have been maintained in 0.1 M per chloric add solution before emersion Esm => normal hydrogen electrode potential = 4.5 or 4.44 V Cnhe = emersed normal h3rdiogen electrode potential in UHV = 4.85 V arrow = work f mction of free dean surfaces of gold, platinum and silver in vacuum. [From Kota-Neff-Moller, 1986.]...
In electrochemistry we have customarily employed, instead of the absolute electrode potential / abs scale, a relative scale of the electrode potential, E yila scale, referred to the standard or normal hydrogen electrode potential E m at which the hydrogen electrode reaction, 2H + 2e dox = H2(gas), is at equilibrium in the standard state unit activity of the hydrated proton, the standard pressure of 101.3 kPa for hydrogen gas, and room temperature of 298 K. Since Eniie is + 4.44 V (or + 4.5 V) in the absolute electrode potential scale, we obtain Eq. 9.9 for the relation between abs scile and [Refs. 4 and 5.] ... [Pg.87]

The electrode potential as defined earlier is called the absolute electrode potential, and it is compared to the electrode potential referred to normal hydrogen electrode. The Fermi level of the normal hydrogen electrode has been estimated near —4.5 eV, and the normal hydrogen electrode potential is 4.5 V on the scale of the absolute electrode potential. [Pg.540]

Practical developers must possess good image discrimination that is, rapid reaction with exposed silver haUde, but slow reaction with unexposed grains. This is possible because the silver of the latent image provides a conducting site where the developer can easily give up its electrons, but requires that the electrochemical potential of the developer be properly poised. For most systems, this means a developer overpotential of between —40 to +50 mV vs the normal hydrogen electrode. [Pg.473]

To obtain comparative values of the strengths of oxidising agents, it is necessary, as in the case of the electrode potentials of the metals, to measure under standard experimental conditions the potential difference between the platinum and the solution relative to a standard of reference. The primary standard is the standard or normal hydrogen electrode (Section 2.28) and its potential is taken as zero. The standard experimental conditions for the redox... [Pg.64]

The potentials of the metals in their 1 mol U salt solution are all related to the standard or normal hydrogen electrode (NHE). For the measurement, the hydrogen half-cell is combined with another half-cell to form a galvanic cell. The measured voltage is called the normal potential or standard electrode potential, E° of the metal. If the metals are ranked according to their normal potentials, the resulting order is called the electrochemi-... [Pg.7]

Sometimes the term normal hydrogen electrode (and respectively normal potential instead of standard potential) has been used referring to a hydrogen electrode with a platinized platinum electrode immersed in 1 M sulfuric acid irrespectively of the actual proton activity in this solution. With the latter electrode poorly defined diffusion (liquid junction) potentials will be caused, thus data obtained with this electrode are not included. The term normal hydrogen electrode should not be used either, because it implies a reference to the concentration unit normal which is not to be used anymore, see also below. [Pg.411]

The Vacuum Reference The first reference in the double-reference method enables the surface potential of the metal slab to be related to the vacuum scale. This relationship is determined by calculating the workfunction of the model metal/water/adsorbate interface, including a few layers of water molecules. The workfunction, — < ermi. is then used to calibrate the system Fermi level to an electrochemical reference electrode. It is convenient to choose the normal hydrogen electrode (NHE), as it has been experimentally and theoretically determined that the NHE potential is —4.8 V with respect to the free electron in a vacuum [Wagner, 1993]. We therefore apply the relationship... [Pg.101]

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. 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,... [Pg.656]

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. [Pg.11]

A typical electrocapillarity system is shown in Figure 2.1(a). The mercury reservoir provides a source of clean mercury to feed a capillary tube the height of mercury in this tube can be varied such that the mass of the Hg column exactly balances the surface tension between the mercury and the capillary walls, see Figure 2.1(b). A voltage V is applied across the mercury in the capillary and a second electrode which is non-polarisable (i.e. the interface will not sustain a change in the potential dropped across it), such as the normal hydrogen electrode, NHE. The potential distribution across the two interfaces is shown in Figure 2.1(c). As can be seen ... [Pg.42]

In order to illustrate the approach suggested above, it is of value to consider a specific case. Visible or near-UV excitation of the complex RuCbpy results in excitation and formation of the well-characterized metal to ligand charge transfer (MLCT) excited state Ru(bpy)32+. The consequences of optical excitation in the Ru-bpy system in terms of energetics are well established, and are summarized in eq. 1 in a Latimer type diagram where the potentials are versus the normal hydrogen electrode (NHE) and are... [Pg.152]

Fig. 11 Correlation between electrochemical potentials and OMTS bands for more than ten compounds including polyacenes, phthalo-cyanines, and porphyrins. OMTS data were acquired both from tunnel junctions and STM measurements. The standard potential relative to the normal hydrogen electrode associated with the half reaction M(solution) + e-(vac) —> M-(solution) is the y axis. The three outliers are assigned to the ring oxidation of porphyrins. (Reprinted with permission from [26])... Fig. 11 Correlation between electrochemical potentials and OMTS bands for more than ten compounds including polyacenes, phthalo-cyanines, and porphyrins. OMTS data were acquired both from tunnel junctions and STM measurements. The standard potential relative to the normal hydrogen electrode associated with the half reaction M(solution) + e-(vac) —> M-(solution) is the y axis. The three outliers are assigned to the ring oxidation of porphyrins. (Reprinted with permission from [26])...
To do this, a second electrode is needed, the reference electrode, which has a defined potential respective to the solution. This used to be a normal hydrogen electrode (nhe), which has a potential of 0.000 V (see Table 2-1), but nowadays usually silver/silver chloride (Ag/AgCl)... [Pg.9]

A water-alone monolayer potential above the pzc is in accordance with an absolute work function measurement for the water monolayer on Pt(lll) of 4.8 eV (29). Comparing this to the hydrogen electrode (4.7 eV below vacuum (30) for the normal hydrogen electrode NHE) corrected by 7x0.059 V for a nominaI pH 7 yields a water-alone mono-layer potential of +0.5 V vs. RHE at pH 7. This lies 0.3 V above our proposed pzc of 0.2 V RHE. This relatively high apparent potential of the water monolayer has been noted previously (Sass, J.K., private communication), and has raised concern about the relevance of the UHV monolayer to real electrochemical conditions, since most electrochemical measurements of the pzc of polycrystalline Pt have been closer to 0.2 V than to 0.5 V (31). By showing that the water monolayer lies above, not at, the pzc, the present H.+H-O data remove part of the apparent discrepancy between the electrochemical and UHV results. If future UHV work function data show a large ( 0.3 V) decrease in the water monolayer work function upon addition of small (<20X saturation) amounts of hydrogen, all of the apparent discrepancy could be quantitatively accounted for. [Pg.80]

ECb. Evb. Ef. ancl Eg are, respectively, the energies of the conduction band, of the valence band, of the Fermi level, and of the band gap. R and O stand for the reduced and oxidized species, respectively, of a redox couple in the electrolyte. Note, that the redox system is characterized by its standard potential referred to the normal hydrogen electrode (NHE) as a reference point, E°(nhe) (V) (right scale in Fig. 10.6a), while for solids the vacuum level is commonly used as a reference point, E(vac) (eV) (left scale in Fig. 10.6a). Note, that the energy and the potential-scale differ by the Faraday constant, F, E(vac) = F x E°(nhe). where F = 96 484.56 C/mol = 1.60219 10"19 C per electron, which is by definition 1e. The values of the two scales differ by about 4.5 eV, i.e., E(vac) = eE°(NHE) -4-5 eV, which corresponds to the energy required to bring an electron from the hydrogen electrode to the vacuum level. [Pg.345]

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. [Pg.315]

NHE OCP ONO OPS PCD PDS PL PLE PMMA PP PP PS PSG PSL PTFE PVC PVDF normal hydrogen electrode (= SHE) open circuit potential oxide-nitride-oxide dielectric oxidized porous silicon photoconductive decay photothermal displacement spectroscopy photoluminescence photoluminescence excitation spectroscopy polymethyl methacrylate passivation potential polypropylene porous silicon phosphosilicate glass porous silicon layer polytetrafluoroethylene polyvinyl chloride polyvinylidene fluoride... [Pg.246]

A scale showing the potential values of the most commonly used reference electrodes is given in Figure 2. The potentials are quoted both with respect to the normal hydrogen electrode and the saturated calomel electrode. [Pg.141]

Pretreatment of electrodes by cychc polarization needs special care because the surface structure depends on the number of cycles and the potential range of polarization. It was shown that during the polarization of Au and Pt electrodes, up to ca. 15V (vs. Normal Hydrogen Electrode) in lAf H2SO4, the quantities of dissolved metals corresponded to the... [Pg.13]

Flg.1 Current density-potential curves for the anodic oxidation of two various reactants and finally of the solvent. The electrode potential is measured against a reference electrode (RE), here for example, the normal hydrogen electrode (NHE). [Pg.32]

Reference Electrodes By definition, the normal hydrogen electrode (N H E) is the reference for electrode potentials (see Sect. 2.3.2.1), but practically it is scarcely usable. A reference electrode (RE) has to provide a well-defined potential between the electrolyte and its electric connector, joined with the input of the measuring instrument. Usually, a metal and a slightly soluble salt of this metal is applied (secondary electrode) [76, 77]. The electrolyte in the RE is connected to the electrolyte in the electrochemical cell via a diaphragm, which has to separate both electrolytes, as far as possible without a potential difference (see below). [Pg.61]

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). [Pg.76]

In electrochemistry, the electron level of the normal hydrogen electrode is important, because it is used as the reference zero level of the electrode potential in aqueous solutions. The reaction of normal hydrogen electrode in the standard state (temperature 25°C, hydrogen pressure 1 atm, and unit activity of hydrated protons) is written in Eqn. 2-54 ... [Pg.55]

Fig. 2-43. Energy balance in the reaction of normal hydrogen electrode H2(sid.p>j = hydrogen molecule in the gaseous standard state (at 1 atm) H( gro. i) = hydrated proton of unit activity = real potential of the hydrated proton of unit activity a.ajHE) = real potential of the equilibrium electron of NHE (= Fermi level cpcnhe) of NHE). Fig. 2-43. Energy balance in the reaction of normal hydrogen electrode H2(sid.p>j = hydrogen molecule in the gaseous standard state (at 1 atm) H( gro. i) = hydrated proton of unit activity = real potential of the hydrated proton of unit activity a.ajHE) = real potential of the equilibrium electron of NHE (= Fermi level cpcnhe) of NHE).

See other pages where Normal hydrogen electrode, potential is mentioned: [Pg.168]    [Pg.543]    [Pg.8]    [Pg.857]    [Pg.8]    [Pg.168]    [Pg.543]    [Pg.8]    [Pg.857]    [Pg.8]    [Pg.75]    [Pg.295]    [Pg.229]    [Pg.66]    [Pg.43]    [Pg.194]    [Pg.683]    [Pg.296]    [Pg.70]    [Pg.111]    [Pg.43]    [Pg.219]    [Pg.229]    [Pg.632]    [Pg.231]    [Pg.13]    [Pg.383]    [Pg.118]   


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