Metals nonpolarizable

Thermodynamically, all metal/solution interfaces are nonpolarizable, i.e., they can exchange electrical charges freely across the phase boundary. It is the extreme slowness of these exchanges that turns a nonpolarizable into a polarizable interface. Therefore polarizable interfaces are a limiting case of nonpolarizable interfaces.2... 

Nonpolarizable interfaces correspond to interfaces on which a reversible reaction takes place. An Ag wire in a solution containing Ag+ions is a classic example of a nonpolarizable interface. As the metal is immersed in solution, the following phenomena occur3 (1) solvent molecules at the metal surface are reoriented and polarized (2) the electron cloud of the metal surface is redistributed (retreats or spills over) (3) Ag+ ions cross the phase boundary (the net direction depends on the solution composition). At equilibrium, an electric potential drop occurs so that the following electrochemical equilibrium is established ... 

Murphy and Waynewright, and change of upthrust on emersed metal, as a method of measuring, 34 Nikitas, at the air-solution interface, 30 in non-aqueous solutions, 71 for a nonpolarizable electrode, 4... 

Next, we consider the interface M/S of a nonpolarizable electrode where electron or ion transfer is in equilibrium between a solid metal M and an aqueous solution S. Here, the interfadal potential is determined by the charge transfer equilibrium. As shown in Fig. 4-9, the electron transfer equilibrium equates the Fermi level, Enn) (= P (M)), of electrons in the metal with the Fermi level, erredox) (= P s)), of redox electrons in hydrated redox particles in the solution this gives rise to the inner and the outer potential differences, and respectively, as shown in Eqn. 4-10 ... 

Consider mercury as the liquid metal under study. One of the advantages of this metal is that the mercuiy/solution interface approaches closest to the ideal polarizable interface (see Section 6.3.3) over a range of 2 V. What this means is that this interface responds exactly to all the changes in the potential difference of an external source when it is coupled to a nonpolarizable interface, and there are no complications of charges leaking through the double layer (charge-transfer reactions). 

Now consider a polarizable interface that consists of a metal electrode in contact with a solution of a l l-valent electrolyte (i.e., Z+ = 1 and z = -1). It will be remembered that in order to apply electrocapillaiy thermodynamics to a polarizable interface Mj/S, the interface has to be assembled in a cell along with a nonpolarizable interface. Suppose that the nonpolarizable interface is one at which negative ions interchange charge with the metal surface, i.e., Zj = — 1. Hence, Eq. (6.99) for the polarizable interface becomes... 

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)... 

At this stage, two facts may be recalled. First, the potential difference across an electrochemical cell, or system, is measurable. Thus, if the Cu2+/Cu interface is incorporated into an electrochemical along with a second metal/solution interface, the potential difference across the whole cell is measurable (Fig. 7.14). Second, if the second interface is nonpolarizable (i.e., its potential does not depart significantly from the equilibrium value on the passage across it of a small current), it contributes a constant value to the potential difference across the cell. Thus, by choosing a standard hydrogen electrode as the nonpolarizable interface, the following system can be built (Fig. 7.14) ... 

Central to the stability and chemistry of complexes formed by mixed donor ligands are two key concepts of coordination chemistry. The first is the chelate effect, which applies to all polydentate ligands, and reflects the increase in stability of a type of complex as monodentate donor molecnles are replaced by polydentates with donors linked by chelate rings. The hard-soft acid-base theory is particnlarly relevant to mixed donor ligands where donors of distinctly different character may bind to a central metal ion. The like prefers like concept means hard nonpolarizable donor atoms (N and O, for example) bond preferentially to hard nonpolarizable metal... 

The last two terms on the right-hand side of this equation may be considered to be negligible, since metal-metal interphases behave as ideally nonpolarizable Interphases. The voltage drop across the... 

The energy of interaction of an ion with a solvent may be represented by three parts its electrostatic interaction, a solvophobic component, and a specific interaction due to the donor-acceptor interactions. In recent considerations of the electrostatic interaction energy, the basic ideas of the Born model [21] are accepted, though its shortcomings and limitations are evident and the original equation has been modified. The ion, M", in this model is represented by a nonpolarizable metallic sphere with a radius r. 

Up to this point, we have considered potentials associated with a single metal/solution interface (i.e., (])m>s> and( ) ). It is, of course, not possible to measure directly either the absolute potentials or differences between them. Potential is only experimentally measurable or controllable relative to that of another electrode of defined, invariant potential (i.e., a nonpolarizable reference electrode). Apart from defining the applied potential and enabling it to be measured, a reference electrode is required in order to complete the circuit and maintain electrical neutrality with zero current flow throughout the potentialmeasuring circuit of the cell. 

In general, various metal ions fall into two categories hard acids such as Fe3+ and Al3+, which are nonpolarizable, and soft acids such as Cu+ and Ag+, which are highly polarizable. There are several acids such as Fe2+ and Cu2+ on the borderline, and they act either as hard acids or as soft acids depending on the partners to associate with. It is convenient to divide bases also into two categories hard bases such as Cl-, OH-, and SO4, which are nonpolarizable, and soft bases such as I- and S2-, which are polarizable. It is a useful generalization that hard acids prefer associating with hard bases, and soft acids prefer soft bases. 

Pearson " designated the class (a) metal ions of Ahrland, Chatt, and Davies as hard acids and the class (b) ions as soft acids. Bases are also classified as hard or soft on the basis of polarizability the halide ions range from F , a very hard base, through less hard Cl and Br to I , a soft base. Reactions are more favorable for hard-hard and soft-soft interactions than for a mix of hard and soft reactants. Hard adds and bases are relatively small, compact, and nonpolarizable soft acids and bases are larger and more polarizable. The hard acids include cations with a large positive charge (3-t or larger) or those whose... 

The silver-silver chloride electrode has characteristics similar to a perfectly nonpolarizable electrode and is practical for use in many biomedical applications. The electrode (Figure 4.1a) consists of a silver base structure that is coated with a layer of the ionic compound silver chloride. Some of the silver chloride when exposed to light is reduced to metallic silver hence, a typical silver-silver chloride electrode has finely divided metallic silver within a matrix of silver chloride on its surface. Because silver chloride is relatively insoluble in aqueous solutions, this surface remains stable. Moreover, because there is minimal polarization associated with this electrode, motion artifact is reduced compared to polarizable electrodes such as the platinum electrode. Furthermore, owing to the reduction in polarization, there is also a smaller effect of frequency on electrode impedance, especially at low frequencies. 

Figure 7.4 shows the result of a clinical measurement of biopotentials with three different metals in the PU electrodes. The signal source is endogenic electro-oculography registrations are related to eye position and are measured with DC response biopotential amplifiers. It is clear that AgCl is the only material making the electrodes sufficiently nonpolarizable. 

This is a much used electrode metal (silver covered by silver chloride) in biology and medicine for DC applications both because it is simple and because it has a well-defined DC potential not very dependent on DC current flow. It is therefore a nonpolarizable DC reference electrode. It usually consists of silver metal covered by an AgCl layer, often electrolytically deposited. Ag and AgCl are toxic and cannot be used in long-term living tissue contact. A salt bridge is often used to remove the electrode metal from direct tissue contact. 

Cation polarizability is also a factor in selectivity. Kodama et al studied the complexation of aza-18-crown-6 with alkali, alkaline earth, and some d-block metals. This ligand showed higher selectivity toward the more polarizable, soft d-block metal ions than the hard, nonpolarizable alkali and alkaline earth metal cations. 

Nonpolarizable ions Na+, NH, K+ Mg2+, Ca + Transition metals Amines < Cg Alkah metals Alkahne-earth metals... 

Pearson s classification of ligands and metals as soft acids and bases can readily be applied to metal 71-complexes. Here the term soft means polarizable and hard indicates nonpolarizable. According to the general guideline,... 

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