Electrochemistry standard hydrogen electrode

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 Tf carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubiUty of Pu(IV) in aqueous carbonate solutions has been measured, and the stabiUty constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

Figure 7.12 shows the relationship between the standard oxygen electrode (soe) scale of solid state electrochemistry, the corresponding standard hydrogen electrode (she) scale of solid state electrochemistry, the standard hydrogen electrode (she) scale of aqueous electrochemistry, and the physical absolute electrode scale. The first two scales refer to a standard temperature of 673.15 K, the third to 298.15 K. In constructing Figure 7.12 we have used the she aqueous electrochemical scale as presented by Trasatti.14... 

The combination of hydrogen gas, H3 O ions, and a platinum electrode is referred to as a hydrogen electrode. This electrode appears in the right-hand portion of Figure 19-8. When a hydrogen electrode operates under standard conditions, PH2 — 1-00 bar and H3 O ] — 1.00 M, it is a standard hydrogen electrode (SHE). The standard hydrogen electrode is particularly important in electrochemistry, as we describe in Section 19-1. 

Cyclic voltammetry is perhaps the most important and widely used technique within the field of analytical electrochemistry. With a theoretical standard hydrogen electrode at hand, one of the first interesting and challenging applications may be to try to use it to make theoretical cyclic voltammograms (CVs). In following, we set out to do this by attempting to calculate the CV for hydrogen adsorption on two different facets of platinum the (111) and the (100) facets. 

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 ... 

We have seen already that an absolute potential at an electrode cannot be known, so, in accord with all other electrochemistry, it is the potential difference between two electrodes which we measure. However, if the potential of the electrode of interest is cited with respect to that of a second electrode having a known, fixed potential, then we can know its voltage via the concept of the standard hydrogen electrode (SHE) scale (see Section 3.1). We see that a reliable value of overpotential requires a circuit containing a reference electrode. 

First of all, the important role of platinum as the metal part of the standard hydrogen electrode (SHE), which is the primary standard in electrochemistry should be mentioned. The standard potential of an electrode reaction (standard electrode potential) is defined as the value of the standard potential of a cell reaction when that involves the oxidation of molecular hydrogen to solvated (hydrated) protons (hydrogen ions) ... 

Hydrogen electrode — A gas electrode where purified hydrogen gas is dissolved, usually in an aqueous solution, in which an inert electrode, preferably a - platinized platinum (- platinum black, -> electrode materials) electrode is inserted. The hydrogen electrode is of exceptional importance in electrochemistry because the -> standard hydrogen electrode (SHE) provides by convention a reference potential for all half reactions and thus the thermodynamic reference point for all energy calculations. An alternative form is the -> dynamic hydrogen electrode (DHE). 

Both the - standard hydrogen electrode (SHE), which is the primary standard in electrochemistry [iv,v] and the relative hydrogen electrode (RHE) are widely used in aqueous acidic solutions. In RHE the nature and concentration of acid is the same in the reference and the main compartments. In general, it is advantageous to use the same solution in both compartments to decrease the - junction potential. By the help of RHE the -> activity effect can also be eliminated when the -> pH dependence of a — redox reaction is to be determined, since the H+ ion activity influences both the redox reactions under study and the redox reaction occurring in the reference system (1/2H2 -> H+ + e-) in the same way. 

Standard hydrogen electrode — The standard -> hydrogen electrode (SHE) is the primary standard in electrochemistry [i-v]. It is also called normal hydrogen electrode (NHE), however, this usage is no longer recommended. 

In electrochemistry, it is usual to measure potentials with respect to a stable and reproducible system, known as - reference electrode. For the vast majority of practical electrochemical problems there is no need to determine - absolute potentials. However, this is necessary in cases where one wants to connect the relative electrode potential with the absolute physical quantities of the system, like electronic energies, as is the case of the work function. It is possible to convert all relative values of electrode potential to absolute-scale values and to electronic energies. For aqueous systems the - standard hydrogen electrode potential corresponds to -4.44 V in the physical scale taking electrons at rest in vacuum as reference and the absolute potential is given by the relation T(abs) = T(SHE) + 4.44 [vii]. 

As already discussed, the standard hydrogen electrode (SHE) is the chosen reference half-cell upon which tables of standard electrode potentials are based. The potential of this system is zero by definition at all temperatures. Although this reference electrode was often used in early work in electrochemistry, it is almost never seen in chemical laboratories at the present time. It is simply too awkward to use because of the requirement for H2 gas at 1 bar pressure and safety considerations. 

At the heart of electrochemistry is the electrochemical cell. We will consider the creation of an electrochemical cell from the joining of two half-cells. When an electrical conductor such as a metal strip is immersed in a suitable ionic solution, such as a solution of its own ions, a potential difference (voltage) is created between the conductor and the solution. This system constitutes a half-cell or electrode (Fig. 15.1). The metal strip in the solution is called an electrode and the ionic solution is called an electrolyte. We use the term electrode to mean both the solid electrical conductor in a half-cell (e.g., the metal strip) and the complete half-cell in many cases, for example, the standard hydrogen electrode, the calomel electrode. Each half-cell has its own characteristic potential difference or electrode potential. The electrode potential measures the ability of the half-cell to do work, or the driving force for the half-cell reaction. The reaction between the metal strip and the ionic solution can be represented as... 

At each electrode j in contact with an electrolyte, a defined value of electrode potential Ej is set up. It can be measured only relative to the potential of another electrode. By convention, in electrochemistry the potential of any given electrode is referred to as the potential of the so-called standard hydrogen electrode (SHE), which in turn by convention is taken as zero. A practical realization of the SHE is an electrode made of platinized platinum dipped into an acid solution having a mean ionic activity of hydrogen ions of unity and is washed by gaseous hydrogen at a pressure of 1 bar. 

The standard hydrogen electrode is the primary standard in electrochemistry. It is based on the following reversible equilibrium ... 

In order to define a potential reference suitable for electrochemistry, a reference redox couple must firstly be chosen the H /H2 couple in its thermodynamic standard state. This reference system is called Standard Hydrogen Electrode (SHE). It is well-defined even though it is theoretical, and it is the reference used in all contemporary data tables in thermodynamics and electrochemistry... 

Remember that the potential reference chosen in electrochemistry is the virtual system called the standard hydrogen electrode (SHE) ... 

In electrochemistry, we indicate the half-cell potentials relative to that of the standard hydrogen electrode. 

Figure 2.27 compares the standard potentials measured in V relative to the standard hydrogen electrode, electrochemical scale, with the corresponding Fermi energies of the electrons in the metal measured in eV relative to the vacuum level, the physical scale. Dividing (2.167) by F = 96485 C mol we obtain = 4.44 V. The potential of the standard hydrogen electrode therefore has a potential of 4.44 V on the physical scale. The physical scale of potentials is frequently used in the study of the electrochemistry of semiconductors. 

The electromotive series is a list of the elements in accordance with their electrode potentials. The measurement of what is commonly known as the "single electrode potential", the "half-reaction potential" or the "half-cell electromotive force" by means of a potentiometer requires a second electrode, a reference electrode, to complete the circuit. If the potential of the reference electrode is taken as zero, the measured E.M.P. will be equal to the potential of the unknown electrode on this scale. W. Ostwald prepared the first table of electrode potentials in 1887 with the dropping mercury electrode as a reference electrode. W. Nernst selected in 1889 the Normal Hydrogen Electrode as a reference electrode. G.N. Lewis and M. Randall published in 1923 their table of single electrode potentials with the Standard Hydrogen Electrode (SHE) as the reference electrode. The Commission of Electrochemistry of the I.U.P.A.C. meeting at Stockholm in 1953 defined the "electrode potential" of a half-cell with the SHE as the reference electrode. 

Thermo- electrochemistry and the entropy change of the standard hydrogen electrode reaction. Acta Metall. Sinica. Vol. 9,189-192. 

The standard hydrogen electrode (SHE) acts as a primary reference in electrochemistry. The standard potentials of all other reference electrodes are linked to that of the SHE at the same temperature. The SHE contribution to the cell potential is by convention zero at all temperatures (see Chap. 1). 

Note that by definition, in electrochemistry, the reversible potential of the hydrogen oxidation reaction is zero at all temperatures [5]. That is why the standard hydrogen electrode is used as a reference electrode. Therefore, for hydrogen anodes E,a = OV. Activation polarization of the hydrogen oxidation reaction is much smaller than activation polarization of the oxygen reduction reaction. 

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