Absolute single electrode potential

Absolute single electrode potential is a characteristic property of a metal. All metals have characteristic electrode potentials. Absolute single electrode potentials cannot be measured directly. They can be measured with respect to a standard electrode such as standard hydrogen electrode (SHE). The standard hydrogen electrode is a widely used standard electrode. It is arbitrarily assumed to have zero potential at all temperatures by definition. Thus, it provides a zero reference point for electrode potentials. The construction of the standard hydrogen electrode is shown in Fig. 2.10. As shown in the figure. 

In the cell used for measuring the electrode potential, in which the two electrodes are immersed in a single phase of electrolyte solution, the outer potential, tps, ofthe test electrode-solution is equal to the outer potential, ips, of the reference electrode-solution as shown in Fig. 4—24. The difference in the Fermi level of electrons, CFtu)- between the test electrode M and the reference electrode M , then, is represented by the difference in the real potential of electrons, M/aw) - .(M0/ V). tuid hence by the difference in the electrode potential (absolute electrode potential), AE = E-E°, between the two electrodes. This difference also equals the difference in the work function, 4>no/3/v - 4>ji/s/v> between the two electrodes. Thus, the potential E of the test electrode relative to the reference electrode is the difference in the electrode potential (absolute electrode potential) between the two electrodes as indicated in Eqn. 4-35 . 

Trasatti14 16 has done a very thorough and lucid work in clarifying the concept of absolute electrode potentials in aqueous electrochemistry. He has pointed out that at least four different absolute, or single , electron potentials can be defined, depending on the choice of the reference state of electrons. 

Owing to the existence of relation (3), it is possible to define three other single (i.e., absolute) electrode potentials. However, only the potential defined by Eq. (16) has the reference state at a point close to the surface where the Volta potential is measured. This case corresponds to a truly universal absolute state ( electrons at rest in a vacuum ) adopted by physicists to measure electronic work fimctions. 

The emersed electrode, in principle, may be treated as the experimental realization of a single electrode. However, it is doubtful whether its liquid layer has the same bulk properties. This is probably the main reason for the different results of E°H(abs) found for emersed electrodes, e.g., -4.85 V.83 Samec et al. have found that emersion of electrodes in a nitrogen atmosphere decreases the Volta potential and therefore the absolute electrode potential by ca. 0.32 V relative to the value in solution. They have attributed this mainly to the reorientation of the water molecules at the free surface. 

Thus the EMF has been separated into two terms, each containing a quantity related to a single electrode. If the surface potential of the electrolyte x(S) is added to each of the two expressions in brackets in Eq. (3.1.73), then the expression for the EMF contains the difference in the absolute electrode potentials for the absolute electrode potential of metal M we have... 

The experiments were performed with single crystal (111) p-Si electrodes with a resistivity of about 5.5 ohm cm non-aqueous electrolytes were used consisting of absolute methanol containing tetramethylammonium chloride (TMAC) or acetonitrile containing tetraethyl ammonium perchlorate (TEAP). The flat-band potentials or p-Si in the two electrolytes were determined from Mott-Schottky plots (in the dark) in the depletion range of the p-Si electrode, from open-circuit photopotential measurements, and from the values of electrode potential at which anodic photocurrent is first observed in n-type Si electrodes. These three methods all yielded consistent flat-band potential values for p-Si of + 0.05V (vs SCE)... 

Figure 5.19 summarizes the positive and negative voltage limits for some commonly used electrode materials in several solvents. Wherever possible, the data for a particular solvent has been referred to a single reference electrode. Absolute values of the electrode potential for different solvent systems cannot be directly compared, however, because they are often referred to different reference electrodes and because of the uncertainty in our knowledge of junction potentials between different solvent systems. 

To express the absolute values of single potentials is made difficult by the fact that the absolute zero electrode is not known, in respect of which other elements could be measured. It is, therefore, necessary to be satisfied with comparative values. These will be obtained by referring each potential to an exactly defined arbitrary standard electrode the potential of which is conventionally taken as zero. Such comparative potential valuos, of course, do not prevent the calculation of the EMF s of cells composed of two elements because in such instance the zero electrode potential proper appears in the corresponding equation twice once with a positive, and once with the negative sign, so being annuled in the result. 

Absolute potential (also called single electrode potential) — is a hypothetic p. of an isolated - electrode without referring it to any reference electrode. Although it has long been known that only relative - electrode p. can be measured experimentally, numerous attempts were undertaken to determine such a value (see in [i-x]). The problem was also formulated as a search for the hypothetical reference state determined as reckoned from the ground state of - electron in vacuum (a physical scale of energy with the opposite sign). In... 

Absolute Single Electrode Potentials.— The electrode potentials discussed hitherto are actually the e.m.f. s of cells resulting from the combination of the electrode with a standard hydrogen electrode. A single electrode potential, as already seen, involves individual ion activities and hence has no thermod3mamic significance the absolute potential difference at an electrode is nevertheless a quantity of theoretical interest. Many attempts have been made to set up so-called null electrodes ... 

Since kxjk is a constant at definite temperature, this equation is obviously of the same form as the electrode potential equations derived by thermodynamic methods, e.g., equation (85) for an electrode reversible with respect to positive ions. The first term on the right-hand side of equation (30) is clearly the absolute single standard potential of the electrode it is equal to the standard free energy of the conversion of solid metal to solvated ions in solution divided by and its physical significance has been already discussed. 

The two bracketed expressions in eq. 1 A.6 do not contain quantities relating to other interfaces and are termed single electrode potentials. We can turn them into absolute potentials expressed with respect to the local solution vacuum level by adding to each the surface potential / of the solution, which is the electric potential difference between a point just inside the solution phase and just outside it (points P4 and Pi in Fig. lA.l)... 

To learn about the degree of ordering of the water layer at the water interface in which the jump of the dielectric constant takes place, e.g. fi"om water to air, there must be suitable experimental conditions. One of the best and simplest methods is the measurement of the surface potential jump at the interface. As we will show, absolute electrical surface potentials, like single electrode potentials, are not measurable. In every case we need a reference system. 

For most purposes in electrochemistry, it is sufficient to reference the potentials of electrodes (and half-cell emfs) arbitrarily to the NHE, but it is sometimes of interest to have an estimate of the absolute or single electrode potential (i.e., the potential of a free electron in vacuum). This interest arises, for example, if one would like to estimate relative potentials of metals or semiconductors based on their work functions. The absolute potential of the NHE can be estimated as 4.5 0.1 V, based on certain extrather-modynamic assumptions, such as about the energy involved in moving a proton from the gas phase into an aqueous solution (10, 29). Thus, the amount of energy needed to remove an electron from Pt/H2/H ( = 1) to vacuum is about 4.5 eV or 434 kJ. With this value, the standard potentials of other couples and reference electrodes can be expressed on the absolute scale (Figure 2.1.1). 

We note that it is possible to measure absolute single-electrode potentials by nonthermodynamic techniques. In principle, the singleelectrode potential can be determined from a measurement of quad-rupole radiation from the oscillating electrode. 

Consider a Cu electrode in CUSO4 at equilibrium. There is no potential gradient or concentration gradient in the solution. However, there is charge separation at the interface resulting in a potential drop. At equilibrium, this potential drop is representative of the reversible potential. Note that it is impossible to measure the absolute value of a single electrode potential only the potential difference between two electrodes can be measured. To characterize electrode potentials in practice one uses a reference electrode, which is an electrode that has a fixed... 

Of the two half-cell electrode potentials, only the cell potential E can be measured experimentally. Therefore, it is not possible to measure the absolute values of any single half-cell electrode potential. To solve this problem, Nemst suggested that the potential of... 

The measurement of an electrode potential requires the use of a reference electrode [2,28] to complete the electrical circuit (Fig. 4.1.6). The SHE is chosen as the primary reference electrode and its equilibrium potential is assigned the value of zero when the activities of H" " and H2(g) are unity. Thus, the equilibrium single electrode potential values in Table 4.1.2 are relative, as they are measured with respect to the SHE, and are not absolute. 

Although this agreement regularizes the values of the electrode potentials that we use, the problem of measuring or calculating the absolute values of these potentials, or of the activities of single ionic species still remains. 

Ostwald had what appeared to be a very elegant concept. It involved the measurement of a single electrode potential. The method of measurement was in good accordance with his philosophical views and with the chemistry of the times, and it would, in his opinion, yield an absolute potential. An absolute potential was a sharp contrast to the relative potential obtained by referring a measured half-cell to another single electrode reaction arbitrarily set at zero. Ostwald s measurements of half-cell potentials could be directly related to heats of ionization (1 ). In his opinion, an absolute half-cell redox potential would allow the establishment of an electromotive series which would be analogous to the absolute temperature scale. 

The absolute potentials of single electrodes have been a subject of interest since Ostwald, Nernst, and their contemporaries formulated the beginnings of modern electrochemistry in the nineteenth century. The twentieth century brought new methods of applying electrochemical measurements to analytical problems. For these, a knowledge of electrode potentials, or, alternatively, the activities of individual species of ions, could provide simplicity and accuracy not hitherto attainable. Nevertheless, ordinary thermodynamic procedures are incapable of measuring these quantities. 

Reference electrodes of mercury have been used by several investigators in an attempt to measure single electrode potentials. Stastny and Strafelda (5 ) concluded that the zero charge potential of such an electrode in contact with an infinitely dilute aqueous solution is -0.1901V referred to the standard hydrogen electrode. Hall ( ) states that the potential drop across the double layer under these conditions is independent of solution composition when specific adsorption is absent. Daghetti and Trasatti (7, ) have used mercury reference electrodes to study the absolute potential of the fluoride ion-selective electrode and have compared their estimates of ion activities in NaF solutions with those provided by other methods. Their method is based on the assumption that the potential drop across the mercury I solution interface is independent of the electrolyte concentration once the diffuse layer effects are accounted for by the Gouy-Chapman theory. 

The absolute potentials on the right-hand side are identified with single electrode potentials according to the definition of TrasattiT These potentials can be expressed in terms of electronic and ionic work functions. The following two sections are a summary of the heuristic derivation of these expressions, as presented by Fawcett in [9],... 

The potentials of Equations 13.20 and 13.21 depend on the properties of one electrode only and can be regarded as single-electrode potentials. A similar claim could already be made for the quantities between brackets in Equation 13.19. The potentials of Equations 13.20 and 13.21, however, are work functions and can be interpreted as absolute electrode potentials, as anticipated by the notation. This is why the surface potential was added in. With this additional term, we can replace p + by the real potential a° + combining Equation 13.21 with Equation 13.12, i.e.,... 

Thermodynamic calculations are always based on an electrochemical cell reaction, and the derived voltage means the voltage difference between two electrodes. The voltage difference between the electrode and the electrolyte, the absolute potential , cannot exactly be measured, since potential differences can only be measured between two electronic conductors (2). Single electrode potential always means the cell voltage between this electrode and a reference electrode. To get a basis for the electrode-potential scale, the zero point was arbitrarily equated with the potential of the standard hydrogen electrode (SHE), a hydrogen electrode under specihed conditions at 25 °C (cf. Ref. 3). 

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