Potential metal phase

The chemical potential pi, has been generalized to the electrochemical potential Hj since we will be dealing with phases whose charge may be varied. The problem that now arises is that one desires to deal with individual ionic species and that these are not independently variable. In the present treatment, the difficulty is handled by regarding the electrons of the metallic phase as the dependent component whose amount varies with the addition or removal of charged components in such a way that electroneutrality is preserved. One then writes, for the ith charged species. 

Processes in which solids play a rate-determining role have as their principal kinetic factors the existence of chemical potential gradients, and diffusive mass and heat transfer in materials with rigid structures. The atomic structures of the phases involved in any process and their thermodynamic stabilities have important effects on drese properties, since they result from tire distribution of electrons and ions during tire process. In metallic phases it is the diffusive and thermal capacities of the ion cores which are prevalent, the electrons determining the thermal conduction, whereas it is the ionic charge and the valencies of tire species involved in iron-metallic systems which are important in the diffusive and the electronic behaviour of these solids, especially in the case of variable valency ions, while the ions determine the rate of heat conduction. 

The tlrermodynamic activity of nickel in the nickel oxide layer varies from unity in contact with tire metal phase, to 10 in contact with the gaseous atmosphere at 950 K. The sulphur partial pressure as S2(g) is of the order of 10 ° in the gas phase, and about 10 in nickel sulphide in contact with nickel. It therefore appears that the process involves tire uphill pumping of sulphur across this potential gradient. This cannot occur by the counter-migration of oxygen and sulphur since the mobile species in tire oxide is the nickel ion, and the diffusion coefficient aird solubility of sulphur in the oxide are both vety low. 

The orientational structure of water near a metal surface has obvious consequences for the electrostatic potential across an interface, since any orientational anisotropy creates an electric field that interacts with the metal electrons. Hydrogen bonds are formed mainly within the adsorbate layer but also between the adsorbate and the second layer. Fig. 3 already shows quite clearly that the requirements of hydrogen bond maximization and minimization of interfacial dipoles lead to preferentially planar orientations. On the metal surface, this behavior is modified because of the anisotropy of the water/metal interactions which favors adsorption with the oxygen end towards the metal phase. 

Electrodes such as Cu VCu which are reversible with respect to the ions of the metal phase, are referred to as electrodes of the first kind, whereas electrodes such as Ag/AgCl, Cl" that are based on a sparingly soluble salt in equilibrium with its saturated solution are referred to as electrodes of the second kind. All reference electrodes must have reproducible potentials that are defined by the activity of the species involved in the equilibrium and the potential must remain constant during, and subsequent to, the passage of small quantities of charge during the measurement of another potential. 

The capacity of the metal phase (CM) and the potential drop in the thin metal surface layer have been discussed by Amokrane and Badiali,122,348 as well as by Damaskin et a/.349 353 The value of was found to increase in the order Ga < In(Ga) < Tl(Ga) Hg if it was assumed that the capacity of a solvent monolayer C = const. The negative value of the surface charge density <7, at which the Cs,ff curve has a maximum, decreases in the order Ga > In(Ga) > Hg, i.e., as the hydrophilicity of the electrode decreases. 

The temperature dependence of the inner-layer properties has been studied by Vaartnou etal.m m over a wide interval, -0.15°C < T< 50°C. The inner-layer integral capacitance Kh a curves have been simulated using the Parsons308 and Damaskin672,673 models. The experimental Kj, T dependence has a minimum at T = 20°C. The influence of the potential drop in the metal phase has been taken into account. 

If a gas bubble adheres to an electrode surface being in contact with an electrolyte solution, the contact angle can be measured as an indicator of the interfacial tension and its change. The respective relationship is cos 6= (y ni - y,m)/ g,s with g, s, m referring to the gas, solution and metal phase respectively. It was initially observed by Mdller, that 6 changes with E [08M61]. Assuming that s and do not depend on the electrode potential a plot of relationship follows ... 

In the state of equilibrium between both phases, i.e. the solution phase eontaining the M" species and the solid metal phase, the sum of the chemical potentials in both phases are equal. Sinee charged speeies are involved, the usual chemical potential jUi has to be extended by a term representing the work neeessary to bring one mol of charged species with a charge of Zj e into a phase where an eleetrostatie potential E is present... 

The difference in the outer potentials between the metal and the electrolyte solution is measured similarly. In Fig. 3.6, phase jS will now designate an electrolyte solution in contact with the metal phase oc. Also here tue(a) = jue(cc ) and ip(oc ) = (/ ), and U is again expressed by Eq. (3.1.30) however, fie(oc) = as the communicating species between... 

As has already been mentioned, the EMF the electromotive force) of a cell is given by the potential difference between leads of identical metallic material. In view of this, a galvanic cell is represented schematically as having identical metallic phases at either end. 

The interfacial tension always depends on the potential of the ideal polarized electrode. In order to derive this dependence, consider a cell consisting of an ideal polarized electrode of metal M and a reference non-polarizable electrode of the second kind of the same metal covered with a sparingly soluble salt MA. Anion A is a component of the electrolyte in the cell. The quantities related to the first electrode will be denoted as m, the quantities related to the reference electrode as m and to the solution as 1. For equilibrium between the electrons and ions M+ in the metal phase, Eq. (4.2.17) can be written in the form (s = n — 2)... 

It is usually assumed that the components of a metal are ions (with tightly bound charge) and electrons, so that there are no polarizable species in the metal phase. The contribution of the metal to the potential drop across the interface is then... 

Despite the vast quantity of data on electropolymerization, relatively little is known about the processes involved in the deposition of oligomers (polymers) on the electrode, that is, the heterogeneous phase transition. Research - voltammetric, potential, and current step experiments - has concentrated largely on the induction stage of film formation of PPy [6, 51], PTh [21, 52], and PANI [53]. In all these studies, it has been overlooked that electropolymerization is not comparable with the electrocrystallization of inorganic metallic phases and oxide films [54]. Thus, two-or three-dimensional growth mechanisms have been postulated on the basis that the initial deposition steps involve one- or two-electron transfers of a soluted species and the subsequent formation of ad-molecules at the electrode surface, which may form clusters and nuclei through surface diffusion. These phenomena are still unresolved. 

The energy level of surface metal ions is defined as the real potential a. of the ion, which is the energy released to form a surface metal ion from a standard gaseous metal ion located at the outer potential of the metal phase this is the... 

Figure 3—4 shows the eneigy level of surface silver ions on metallic silver, we estimate the unitary level of surface silver ions to be = - Fa = —5.84 eV referred to the standard gaseous silver ion at the outer potential of the metal phase. 

Since the electrochemical potential of electrons in metals is a function of the inner potential of the metal (P ca) = p. - inner potential difference, Mmb, across the interface where electron transfer is in equilibrium is represented by the difference in the chemical potential of electrons between the two metal phases A and B is shown in Eqn. 4-8 ... 

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. 

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