Normal hydrogen electrode, definition

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. 

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). 

We have seen that there is no ambiguity in the thermodynamic definition of the NHE scale. A detailed analysis, in terms of the various A(t) values at different interphases in the cell, is less straightforward. At first we might be tempted to assign a zero value to the metal-solution potential difference at the normal hydrogen electrode... 

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. 

Other abbreviations may also be used in tables, such as SHE for the standard hydrogen electrode or SCE for the saturated calomel electrode. The abbreviation NHE has been widely used for the normal hydrogen electrode , which is by definition identical to the SHE. It should nevertheless be noted that NHE customarily refers to a standard state pressure of 1 atm, whereas SHE always refers to a standard state pressure of 0.1 MPa (1 bar) in this review. 

These electrodes are electrochemical half cells providing a constant potential they are basically nonpolarizable, which means that their potential is independent from a current flow through the cell, or at least they follow Ohm s law. They contain a redox couple where both partners are present in well-defined concentrations (in fact, activities). Basically it is possible to realize a normal hydrogen electrode (NHE) as a potential reference with 0.000 V (per definition) but the practical efforts are enormous (proton activity 1.000 mol/L, hydrogen gas pressure 1.000 atm using a specially activated platinum electrode) additionally the potential is not very robust because small current flows or temperature changes may cause significant deviations already. 

Reference electrode An electrochemical half-cell with a stable and known potential. The most common is the saturated calomel electrode (SCE,-1-0.242 V vs NHE). The primary standard reference electrode is the normal hydrogen electrode (NHE, 0.0000 V by definition). 

In the above equation, if a = 1, then E = eP. The standard potential of an electrode eP is the potential of an electrode in contact with a solution of its ions of unit activity. The standard potentials are always expressed against the standard hydrogen electrode (SHE), the potential of which is zero by definition. The standard potentials are a function of temperature they are usually tabulated for 25° C. Standard electrode potential is also called normal electrode potential. 

Normal/Standard Hydrogen Electrode (NHE/SHE) it is the standard reference electrode, all standard potentials are referred to NHE, its potential is by definition 0.000 V. Oxidation refers to the process in which a chemical species loses one or more electrons ... 

It is in principle impossible to measure the potential drop at only one electrode/ electrolyte interface. Two electrodes always have to be combined in an electrochemical cell. To normalize all potential measurements, the standard hydrogen electrode is used as a reference with [H" ] = 1.0 M (pH = 0) and p(H2) = 1.013 bar at an inert metal electrode such as platinum. Its potential is °=0 V by definition. This definition is directly linked to the definition in thermodynamics of AGf (H" ) = 0 for the formation of hydrogen ions under standard conditions. The cell voltage between an electrode and the standard hydrogen electrode is called the electrode potential E. Work function measurements have related this hydro-gen-based potential scale to the vacuum scale used in physics. The standard hydro-... 

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