Normal hydrogen electrode, potential absolute

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.] ... 

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. 

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 relative electrode potential nhe referred to the normal (or standard) hydrogen electrode (NHE) is used in general as a conventional scale of the electrode potential in electrochemistry. Since the electrode potential of the normal hydrogen electrode is 4.5 or 4.44 V, we obtain the relationship between the relative electrode potentiEd, Ema, and the absolute electrode potential, E, as shown in Eqn. 4-36 ... 

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

Values differ from those originally reported by Patterson, Cramer, and Truhlar (2001) by 0.08 V per electron consumed. This difference reflects a more accurate measurement of the absolute potential of the normal hydrogen electrode as 4.36 V instead of 4.44 V since the time of that pubhcation. See Lewis et al. (2004) and Section 11.4.1. 

Typically, the reference level for the solution redox potential is chosen to be the normal hydrogen electrode (NHE). Some tabnlations nse the saturated calomel electrode (SCE) as the reference level with the difference between these two scales well-known to be NHE = —0.2412 V versus SCE. The fundamental problem lies in the determination of the absolnte energy of the NHE relative to vacuum. Although a method to determine directly the absolute electrochemical potential of an NHE has not yet been described, a recent indirect measnrement has indicated that it is approximately 4.4 eV below the vacnum level. This value is often used to relate the solution electrochemical potential scale to the solid electrochemical potential scale. It provides the best approximation that is presently available to calculate the... 

As mentioned above, the electrochemical potential of a redox couple is usually given with respect to the Normal Hydrogen Electrode (NHE). Using an absolute energy scale with the vacuum level as a reference, the energy of a redox couple is given by... 

The absolute value of the potential difference across an electrode-electrolyte interphase cannot be measured since each attempt to do that will introduce a new electrode-electrolyte interphase. A reference electrode - by convention the normal hydrogen electrode (NHE) - is used to make relative measurements possible. 

Nearly a century has passed since Ostwald formally introduced the use of absolute potentials in his Lehrbuch der Allgem. Chemie. Although the Nernst forces carried the day and established the normal hydrogen electrode as the basis of the redox scale, Ostwald s elegant concept of an absolute potential as a base for the system has attracted attention of prominent scientists from time to time since then. New concepts and new experimental approaches have been tested, but in no case does there seem to be a system developed likely to supplant hydrogen. The chemistry of the time led both Nernst and Ostwald to believe that they were dealing with systems much less complex than the experience of a century of research has proved. 

Each half-cell reaction has a specific standard potential reported as the potential of the reduction reaction vs. the normal hydrogen electrode (NHE). In an elecdochemical cell, there is a half-cell corresponding to the working electrode (WE), where the reactions under study take place, and a reference half-cell. Experimentally the cell potential is measured as the difference between the potentials of the WE half-cell and the reference electrode/ref-erence half-cell (see Chapter 4). The archetypal reference electrode is the NHE, also known as the standard hydrogen electrode (SHE) and is defined, by convention, as 0.000 V for any temperature. Although the NHE is not typically encountered due to difficulty of operation, all conventional electrodes are in turn referenced to this standard to define their absolute potential (i.e., the Ag/AgCl, 3 M KCl reference has a potential of 203 mV vs. the NHE). In practice, experimental results are either stated as being obtained vs. a specific reference electrode, or converted to potentials vs. NHE. 

To compare or predict experimental redox potentials using theoretical methods, absolute reduction potentials have to be first computed and made relative to the normal hydrogen electrode (NHE). The absolute reduction potential for this process... 

In order to arrive at values of the virtually intrinsic acidity, i.e., an acidity expression independent of the solvent used (Tremillon12 called it the absolute acidity), Schwarzenbach13 used the normal acidity potential as an expression for the potential of a standard Pt hydrogen electrode (1 atm H2), immersed in a solution of the acid and its conjugate base in equal activities analogously to eqn. 2.39 for a redox system and assuming n = 1 for the transfer of one proton, he wrote for the acidity potential... 

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