Polarizable and nonpolarizable electrodes

In electrochemistry, the electrode at which no transfer of electrons and ions occurs is called the polarizable electrode, and the electrode at which the transfer of electrons and/or ions takes place is called the nonpolarizable electrode as shown in Fig. 4-4. The term of polarization in electrochemistry, different from dipole polarization in physics, indicates the deviation in the electrode potential from a specific potential this specific potential is usually the potential at which no electric current flows across the electrode interface. To polarize means to shift the electrode potential from a specific potential in the anodic (anodic polarization) or in the cathodic (cathodic polarization) direction. 

With nonpolarizable electrodes the polarization (the shift of the electrode potential) does not occur, because the charge transfer reaction involves a large electric current without producing an appreciable change in the electrode potential. Nonpolarizable electrodes cannot be polarized to a significant extent as a result. 

The nonpolarizable electrode may also be defined as the electrode at which an electron or ion transfer reaction is essentiaUy in equilibrium i. e. the electron or ion level in the electrode is pinned at the electron level of hydrated redox particles or at the hydrated ion level in aqueous electrolyte. In order for the electrode reaction to be in equilibrium at the interface of nonpolarizable electrode, an appreciable concentration of redox particles or potential determining ions must exist in the electrolyte. 

In electrochemistry, the electrode current is conventionaUy classified into the faradaic current and the nonfaradaic current. The former is the electric current associated with charge transfer reactions at nonpolarizable electrodes and the latter is the current that is required to establish the electrostatic equilibrium at the interfacial double layer on both polarizable and nonpolarizable electrodes. The nonfaradaic ciurent, sometimes called a transient current, flows also in the course of establishing the adsorption of ions on electrodes. 

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The polarizable and nonpolarizable electrodes may be connected to a potential source such as a power supply or a potentiostat, and the resulting current from the titration may be recorded with the aid of a... 

Fig. 8 Current-voltage relationships of highly polarizable and nonpolarizable electrodes...

For an ideally polarizable electrode, q has a unique value for a given set of conditions.1 For a nonpolarizable electrode, q does not have a unique value. It depends on the choice of the set of chemical potentials as independent variables1 and does not coincide with the physical charge residing at the interface. This can be easily understood if one considers that q measures the electric charge that must be supplied to the electrode as its surface area is increased by a unit at a constant potential." Clearly, with a nonpolarizable interface, only part of the charge exchanged between the phases remains localized at the interface to form the electrical double layer. 

The electrode potential defined in Sec. 4.3 applies to both nonpolarizable electrodes at which charge transfer reactions may take place and polarizable electrodes at which no charge transfer takes place. For nonpolarizable electrodes at which the charge transfer is in equilibrium, the interfacial potential is determined by the equilibrium of the charge transfer reaction. 

At the other end of the scale is the idealization of a nonpolarizable electrode—and that means an electrode that is completely leaky, i.e., when one flows electrons in from the outside circuit to give excess electrons to the electrode, they do not stay there, but go straight across and neutralize particles on the other side. In contrast to the behavior of a polarizable electrode, the potential of the electrode does not change because, of the electrons that flow in, none stay, i.e., no extra charge builds up on the electrode surface, but instead flows away to the solution. In the same way, when a nonpolarizable electrode is stimulated to flow electrons from the solution to the... 

The real case, a partly polarizable (and hence partly nonpolarizable) electrode, can be described in terms of the exchange current density i0. From the linearized Butler-Volmer equation [Eq. (7.25)], then ... 

Impedance spectroscopy a single interface. Draw the equivalent circuits for the following electrode/electrolyte interfaces, then derive their impedance expression and explain what their Cole-Cole plot will look like (a) An ideally polarizable interface between electrode and electrolyte, (b) An ideally nonpolarizable interface between electrode and electrolyte, (c) A real-life electrode/... 

It may be appropriate to ask here why the potential at a reversible electrode should change at all with current density. This does not occur because "no system is really ideally polarizable", and one is observing a small polarization. Indeed the relationship shown in Eq. 17D holds strictly only when the interphase is ideally nonpolariz-able. Each value of the potential given by Eq. 17D represents the reversible potential for the concentration of the species at the surface, C(s). These concentrations deviate, however, from the corresponding bulk concentrations C° as a result of mass-transport limitations, according to Eq. 13D. 

When at open circuit, a highly nonpolarizable electrode assumes its iBVersible potential, whereas a highly polarizable electrode may deviate -from it significantly. In either case, the overpotential t] is defined M the difference between the actual potential measured (or applied) and Ihe reversible potential... 

The ITIES formed at the pipet tip is polarizable, and the voltage applied between the micropipet and the reference electrode in organic phase provides the driving force for facilitated IT reaction. The interface between organic (top) and water (bottom) layers is nonpolarizable, and the potential drop, A)[Pg.325]

Figure 1.3.5 Current-potential curves for ideal (a) polarizable and ib) nonpolarizable electrodes. Dashed lines show behavior of actual electrodes that approach the ideal behavior over limited ranges of current or potential.

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. 

Current-potential curves for ideal (a) polarizable and (b) nonpolarizable electrodes. 

A redox reaction is not only dependent on the electrode material, but also on the electrolyte solution. As we have seen, platinum was highly polarizable in the NaCl solution. However, if the surface is saturated with dissolved hydrogen gas, a redox system is created (H/H ), and then the platinum electrode becomes a nonpolarizable reference electrode. Surface oxidation, adsorption processes, and organic redox processes may reduce the polarizability and increase the applicability of a platinum electrode in tissue media. 

A simple two-electrode electrochemical cell consisting of either single or dual polarizable electrode(s) is normally required for amperometric titrations of various organic and inorganic substances. By definition, a polarizable electrode is a suitable electronic conductor whose potential changes even with the passage of relatively small current. In contrast, the potential of a nonpolarizable electrode, such as the saturated calomel and silver - silver chloride electrodes that are commonly employed as reference electrodes, remains reasonably constant even when a large current is passed. 

Ideally Polarizable Electrodes and Ideally Nonpolarizable Electrodes. 101... 

Two limiting cases for the description of an electrode are the ideally polarizable electrode and the ideally nonpolarizable electrode [8, 9, 14], The ideally polarizable electrode corresponds to an electrode for which the Zfaiadaic element has infinite resistance (i.e., this element is absent). Such an electrode is modeled as a pure capacitor, with Cdi = Aq 6V (equation 26), in series with the solution resistance. In an ideally polarizable electrode, no electron transfer occurs across the electrode/electrolyte interface at any potential when current is passed rather all current is through capacitive action. No sustained current flow is required to support a large voltage change across the electrode interface. An ideally polarizable electrode is not used as a reference electrode, since the electrode potential is easily perturbed... 

Consider a metal electrode consisting of a silver wire placed inside the body, with a solution of silver ions between the wire and ECF, supporting the reaction Ag" + e <— Ag. This is an example of an electrode of the first kind, which is defined as a metal electrode directly immersed into an electrolyte of ions of the metal s salt. As the concentration of silver ions [Ag" ] decreases, the resistance of the interface increases. At very low silver ion concentrations, the Faradaic impedance Zfaradaic becomes very large, and the interface model shown in Fig. 3(a) reduces to a solution resistance in series with the capacitance C. Such an electrode is an ideally polarizable electrode. At very high silver concentrations, the Faradaic impedance approaches zero and the interface model of Fig. 3(a) reduces to a solution resistance in series with the Faradaic impedance Zfaradaic. which is approximated by the solution resistance only. Such an electrode is an ideally nonpolarizable electrode. 

In the case of an ideally polarizable electrode, the transfer resistance of the charge carrier through the interface is infinitely high (the classical example from electrochemistry is the interface between Hg and an inert aqueous electrolyte such as KCl or sulfuric acid) in the case of the ideally nonpolarizable electrode, in contrast, it is zero (approximately for Ag/AgCl) (cf. Chapter 7). 

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