Metal electrode potential relative

Knowledge of the Volta potential of a metal/solution interface is relevant to the interpretation of the absolute electrode potential. According to the modem view, the relative electrode potential (i.e., the emf of a galvanic cell) measures the value of the energy of the electrons at the Fermi level of the given metal electrode relative to the metal of the reference electrode. On the other hand, considered separately, the absolute value of the electrode potential measures the work done in transferring an electron from a metal surrounded by a macroscopic layer of solution to a point in a vacuum outside the solotion. ... 

The reducing powers of metals parallel their relative electrode potentials, i.e. potentials developed when the metal is in contact with a normal solution of its salts. The potential of hydrogen being equal to 0, the potentials or electrochemical series of some elements are as shown in Table 4. 

Table 4 Physical constants and relative electrode potentials of some metals...

Of course, if this constant is taken as zero for all the metals with which the potentials of the other systems are compared, there will be no effect of this constant, but one may never forget that the relative electrode potential, to which reference is so often made, is in fact not a metal-solution potential difference. 

Define the following terms used in Section 6.3 (a) electrochemical cell, (b) ideally nonpolarizable and polarizable interfaces, (c) relative electrode potential, (d) outer potential, (e) inner potential, (1) surface potential, (g) image forces, (h) Coulombic forces, (i) electrochemical potential, (j) chemical potential, (k) electron work function, (1) just outside the metal, and (m) absolute potential. (Gamboa-Aldeco)... 

This equation is virtually identical to the Jdnetically deduced version of Eq. (7.40). However, it is not yet formally identical with that of Nernst, which was deduced long before the concept of a Galvani potential difference (MdS< >) across the metal/solution interface was introduced (Lange and Misenko, 1930). Nernst s original treatment was in terms of the electrode potential and symbolized by V. It is possible to show (see Section 3.5.15) that for a given electrode, M S< > - V + const. (i.e., the factors that connect the measured electrode potential to the potential across the actual interface) do not depend on the activity of ions in the solution. Hence, using now the relative electrode potentials, Vt in place of the absolute potentials ,... 

Sacrificial anode — is a piece of metal used as an anode in electrochemical processes where it is intended to be dissolved during the process. In -+ corrosion protection it is a piece of a non-noble metal or metal alloy (e.g., magnesium, aluminum, zinc) attached to the metal to be protected. Because of their relative -+ electrode potentials the latter is established as the -+ cathode und thus immune to corrosion. In -+ electroplating the metal used as anode may serve as a source for replenishing the electrolyte which is consumed by cathodic deposition. The sodium-lead alloy anode used in the electrochemical production of tetraethyl lead may also be considered as a sacrificial anode. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

As shown above, the electrode potential of two different metals in an electrode can be compared. Each metal in contact with an electrolyte of its ion forms a half cell. The most practical method of obtaining reliable and consistent value of relative electrode potential is to compare the value of each half cell with a common reference electrode. 

It is assumed that the contribution of potential differences due to liquid junctions and the contribution of the I drop are properly eliminated in the experimental determination of the relative electrode potential L/ref Frequently the word relative is omitted. The designation relative electric tension has been suggested [1] for L/ref- The polarization of an electrode is equal to the value of with current flow less the value of L/ref the absence of a current. When, at a given current density, a metallic electrode is the site of a definite and unique reaction, its overvoltage rj (overtension [2]) at a given instant is ... 

Thus the tendency for an electrochemical reaction at a metal/solution interface to proceed in a given direction may be defined in terms of the relative values of the actual electrode potential E (experimentally determined and expressed with reference to the S.H.E.) and the reversible or equilibrium potential E, (calculated from E and the activities of the species involved in the equilibrium). 

To obtain comparative values of the strengths of oxidising agents, it is necessary, as in the case of the electrode potentials of the metals, to measure under standard experimental conditions the potential difference between the platinum and the solution relative to a standard of reference. The primary standard is the standard or normal hydrogen electrode (Section 2.28) and its potential is taken as zero. The standard experimental conditions for the redox... 

In general, the baser the metal, the lower (more negative) the electrical potential at the anode and the higher the potential rate of corrosion. Carbon steel and low-alloy steels (which are widely used in boiler plants) have a relatively low potential with respect to the standard hydrogen electrode and can therefore be expected to corrode readily unless active prevention measures are taken. Copper and brasses have a relatively higher potential. 

In principle, a measurement of upon water adsorption gives the value of the electrode potential in the UHV scale. In practice, the interfacial structure in the UHV configuration may differ from that at an electrode interface. Thus, instead of deriving the components of the electrode potential from UHV experiments to discuss the electrochemical situation, it is possible to proceed the other way round, i.e., to examine the actual UHV situation starting from electrochemical data. The problem is that only relative quantities are measured in electrochemistry, so that a comparison with UHV data requires that independent data for at least one metal be available. Hg is usually chosen as the reference (model) metal for the reasons described earlier. 

We start by noting that an electrode potential, Uwr, as measured with respect to a reference electrode R, is a relative measure (in volts) of the energy of electrons at the Fermi level of the metal constituting the electrode ... 

In the introductory chapter we stated that the formation of chemical compounds with the metal ion in a variety of formal oxidation states is a characteristic of transition metals. We also saw in Chapter 8 how we may quantify the thermodynamic stability of a coordination compound in terms of the stability constant K. It is convenient to be able to assess the relative ease by which a metal is transformed from one oxidation state to another, and you will recall that the standard electrode potential, E , is a convenient measure of this. Remember that the standard free energy change for a reaction, AG , is related both to the equilibrium constant (Eq. 9.1)... 

Electrode potentials are relative values because they are defined as the EMF of cells containing a reference electrode. A number of authors have attempted to define and measure absolute electrode potentials with respect to a universal reference system that does not contain a further metal-electrolyte interface. It has been demonstrated by J. E. B. Randles, A. N. Frumkin and B. B. Damaskin, and by S. Trasatti that a suitable reference system is an electron in a vacuum or in an inert gas at a suitable distance from the surface of the electrolyte (i.e. under similar conditions as those for measuring the contact potential of the metal-electrolyte system). In this way a reference system is obtained that is identical with that employed in solid-state physics for measuring the electronic energy of the bulk of a phase. 

It is noteworthy that the relative proportion of amine 44 and bicumyl (43) which reflects the ratio of the rate of electronation to the rate of reaction with M(H) (the competition between electronation and reaction with M(H)), varies with the Raney metal (compare entries 1 and 3 of Table 1, and entries 2 and 4) and with the electrode potential (compare entries 1 and 2). The more negative is the potential, the faster is the rate of electronation and the higher should be the proportion of bicumyl (43) as observed (entries 1 and 2). The less active the Raney metal as hydrogenation catalyst, the slower is the rate of reaction with M(H) (the lower is the amount of M(H) at the surface of the electrode) and the lower is the amount of aminocumene (44). RCu is the least active catalyst and the proportion of aminocumene (44) is indeed the lowest at the RCu cathode (entry 4). 

The mechanism of cathodic luminescence is distinctly different from other ECL systems. Light is emitted from oxide-covered, so-called valve metal, electrodes, namely aluminium and tantalum, during the reduction of peroxodisulfate, hydrogen peroxide, or oxygen, in aqueous solution, at relatively low potentials (<10 V). The mechanism involving persulfate, for example, is as follows. A conduc-... 

There is a fundamental difference between electron-transfer reactions on metals and on semiconductors. On metals the variation of the electrode potential causes a corresponding change in the molar Gibbs energy of the reaction. Due to the comparatively low conductivity of semiconductors, the positions of the band edges at the semiconductor surface do not change with respect to the solution as the potential is varied. However, the relative position of the Fermi level in the semiconductor is changed, and so are the densities of electrons and holes on the metal surface. 

The relative position of the electronic level eo to the Fermi level depends on the electrode potential. We perform estimates for the case where there is no drop in the outer potential between the adsorbate and the metal - usually this situation is not far from the pzc. In this case we obtain for an alkali ion eo — Ep — where is the work function of the metal, and I the ionization energy of the alkali atom. For a halide ion eo — Ep = electron affinity of the atom. 

Figure 5. An oxidation state diagram for Mo, Cr, Fe and Mn. For Mo and Cr, N = III for Fe and Mn, N = II. Potentials are given at standard states in acid solution relative to the hydrogen electrode. On such a diagram, die slope between any two points equals the redox potential. In conh ast to most other metals, multiple Mo oxidation states are accessible over a small range of potentials. Note also that Mo is oxidized to Mo(VI) at relatively low potential (similar to Fe(III). Figure modified after Frausto da Silva and Williams (2001).

In the state of band edge level pinning, the electron level of redox particles with the state density of DredoxCe), relative to the electron level rf semiconductor with the state density of Dsc(e), remains unchanged at the electrode interface irrespective of electrode potential. On the other hand, in the state of Fermi level pinning, the electron level of redox particles relative to the electron level of semiconductor electrode depends on the electrode potential in the same way as occurs with metal electrodes (quasi-metallization of semiconductor electrodes). 

For some metallic electrodes, such as transition metals, metal ions dissolve directly from the metallic phase into acidic solutions tiiis direct dissolution of metal ions proceeds at relatively low (less anodic) electrode potentials. The direct dissolution of metal ions is inhibited by the formation of a thin oxide film on metallic electrodes at higher (more anodic) electrode potentials. At still higher electrode potentials this inhibitive film becomes electrochemically soluble (or apparently broken down) and the dissolution rate of the metal increases substantially. These three states of direct dissolution, inhibition by a film, and indirect dissolution via a film (or a broken film) are illustrated in Fig. 11-9. 

The state in which the anodic dissolution of metals proceeds from the bare metal stirface at relatively low electrode potentials is called the active state the state in which metal dissolution is inhibited substantially by a superficial oxide film at higher electrode potentials is called the passive state-, the state in which the anodic dissolution of metals increases again at stiU higher (more anodic) potentials is called the transpassive state. 

Fig. 11-13. Anodic polarization curve of a metallic nickel electrode in a sulfuric add solution transpassivation arises at a potential relatively dose to the flat band potential because of p-type nature of the passive oxide film. [From Sato, 1982.]...

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