NHE normal hydrogen electrode

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-... 

The term normal hydrogen electrode (NHE) is frequently employed as a synonym. Since normal may be taken as a concentration unit (although use of this unit is discouraged, but nevertheless numerous results are given with this unit) its use in the designation of this reference electrode should be avoided. 

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... 

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,... 

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 ... 

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... 

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)... 

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. 

The SHE is sometimes erroneously called a normal hydrogen electrode (NHE). 

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. 

Even though the normal hydrogen electrode (NHE) is the best known and internationally accepted reference electrode, it is difficult to construct and handle, rendering it of little practical use. 

Therefore, if a normal hydrogen electrode (NHE) is used as reference electrode, one can describe the reaction in the electrochemical cell as ... 

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). 

In the foregoing reaction steps, , nhe) is the real potential of an equilibrium redox electron of the reaction of normal hydrogen electrode (NHE), which is the energy required for transferring a standard gaseous electron Ccstd) at the outer... 

As described in Sec. 2.11, the electron level in the normal hydrogen electrode (gaseous hydrogen molecules at unit fiigacity and hydrated protons at unit activity) is -4.5 eV (or - 4.44 eV in the lUPAC report [Trasatti, 1986]). We, then, obtain the equilibrium potential of the normal hydrogen electrode nhe (= in Eqn. 4-32) as shown in Eqn. 4-34 ... 

Fig. 4-25. Comparison between the real potential a.M

Metal/Insoluble Salt/Ion Electrodes, Electrode potentials are usually reported relative to the normal hydrogen electrode [NHE a(H ) = 1, 1] but they are... 

Fig. 3.5 Band position of anatase Ti02, bandgap = 3.2 eV, in the presence of a pH = 1 aqueous electrolyte. The energy scale is indicated in electron volts (eV) using either normal hydrogen electrode (NHE) or vacuum level as reference showing the condition for water splitting.

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 value of the constant V, and hence the values of standard potentials, depend on the choice of the reference electrode and on the character of electrode reaction, which takes place on it With the reference electrode potential conventionally taken as zero, we can choose, for example, the normal hydrogen electrode (NHE), i.e., an electrode, for which the equilibrium at the interface is attained due to the reversible redox reaction H+ + e = H2, provided the activity of H+ ions in the solution is 1 mol/liter and the pressure of gaseous hydrogen above the solution is 1 atm. Many of the measured potentials are given below relative to the saturated calomel electrode (SCE) its potential relative to the NHE is 0.242 V. 

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. 

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