Polarity of an electrode

The same situation is found in the oxidation of certain dissolved reducing agents in many cases these reactions occur only by reaction with oxidizing agents, not on anodic polarization of an electrode. Such behavior is observed primarily in systems with organic reactants, more rarely in systems with inorganic reactants. 

Current flow at electrode surfaces often involves several simultaneous electrochemical reactions, which differ in character. For instance, upon cathodic polarization of an electrode in a mixed solution of lead and tin salt, lead and tin ions are discharged simultaneously, and from an acidic solution of zinc salt, zinc is deposited, and at the same time hydrogen is evolved. Upon anodic polarization of a nonconsumable electrode in chloride solution, oxygen and chlorine are evolved in parallel reactions. 

Reductive dissolution may be more complex than the two previous mechanisms in that it involves electron transfer processes. Formation of Fe" via reductive dissolution can be effected by adsorption of an electron donor, cathodic polarization of an electrode supporting the iron oxide and by transfer of an electron from within a ternary surface complex to a surface Fe ". Addition of Fe" to a system containing a ligand such as EDTA or oxalate promotes electron transfer via a surface complex and markedly accelerates dissolution. 

B. Chronopotentiometry (Formerly called Voltammetry at Constant Current). These terms were applied by Delahay et al (Refs 4 5) to measurements in which the course of polarization of an electrode (immersed in an unstirred soln) under forced constant current was followed potentiometrically as a function of time. The potential-time curve recorded in the presence of a depolarizer is characterized by a transition time, during which the rate of change of potential is relatively small. This... 

Depolarization — The partial or complete elimination or counteraction of - polarization of an - electrode during electrochemical processes by adding a - depolarizer. The term refers to a situation where the electrode... 

Depolarizer — A compound that is capable of reducing or eliminating the -> polarization of an electrode when a - current is applied, i.e., a compound that causes - depolarization. The term is used for any compound that is oxidized or reduced on an electrode, independent of the experimental setup, i.e., for experiments with controlled currents and also controlled potentials. 

Polarization (of an electrode) — For a -> cell or an - electrode that has a defined equilibrium potential (see - equilibrium electrode potential), a departure of the electrode (or cell) potential (/ ) from its equilibrium value (Eeq) is called polarization. The measure of the magnitude of polarization is called the -> overpotential and is marked by symbol q q = E- Eeq. 

Polarization is an electrode phenomenon that may affect either or both of the electrodes in a cell. The degree of polarization of an electrode varies widely. In some instances, it approaches zero, but in others it can be so large that the cutrent in the cell becomes independent of potential. Under this circumstance, polarization is said to be complete. Polarization phenomena can be divided into two categories concentration polarization and kinetic polarization. 

Polarization of an electrode Buildup of a product of oxidation or reduction at an electrode, preventing further reaction. 

The degree of polarization of an electrode in an electrolytic cell is measured by the overvoltage, or overpo-te.ntial, 17, which is the difference between the actual electrode potential E and the ihermodynamic. or equilibrium, potential Thai is,... 

The polarity of an electrode where the high electric field is located distinguishes between negative corona (around the cathode) and positive corona (around the anode). Ignition of the negative corona is based on secondary emission from the cathode similarly to the Townsend breakdown (see Section 4.1.1) ... 

In Section 4.2.1 the method of impedance spectroscopy was introduced. The polarization of an electrode with an alternating potential of small amplitude is also influenced by restricted mass transport. The Warburg impedance for semi-infinite diffusion describes the diffusion process. The Warburg impedance is a complex quantity with real and imaginary parts of equal magnitude. The impedance is given by the equation... 

Moreover, as soon as a current flows through the system, one of the electrodes becomes the anode and the other the cathode . However there is no automatic link between the polarity of an electrode and its role as an anode or a cathode . Unlike the notions of anode and cathode, the polarity of an electrode remains defined when the system is at open circuit. Finally, depending on the operating conditions of the electrochemical system in question, an electrode can be either the anode or cathode and also change its polarity... 

The polarization of an electrode-electrolyte interface can result either from the slowness of the electrode reaction, as in the case of nonblocking but kinetically nonreversible electrodes, or from any factor which hmits the transport of any of the species participating in the electrode reaction, for example slow diffusion of the reactant or product species away from the interface or the generation or consumption of one of the species by a slow chemical reaction in the electrolyte. 

The polarization of an electrode triggers the movement of ionic species in the solution. The current lines represent the path of the ionic species which are perpendicular to the equipotential lines. When two macroelectrodes of comparable size are used, the equipotential lines are parallel to the electrode and the current lines are perpendicular to the electrodes. The current lines are confined within the volume defined by the area of the electrodes and the distance between them (Fig. 3c). 

The following theory explains the basic mathematics that may be used to extract the exchange current density from the results obtained. Ageneral representation of the polarization of an electrode supporting one specific reaction is given in the Butler-Volmer equation (5.4) ... 

Activation polarization Polarization of an electrode controlled by a slow step in reaction sequence of steps at the metal/electrolyte inter ce. There is a critical activation energy needed to surmount the energy barrier associated with the slowest step. 

Figure D.6 illustrates the eflect of a limiting current on the polarization of an electrode. For this example three arbitrary limiting current densities were added the activation voltage of the Fe /Fe reaction on palladium. Figure D.7 presents the same data set on a logarithmic current scale.

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 ... 

Daniel -Bek and co-workers [173, 174] have developed a method for combining gasometric and polarization measurements during anodic and cathodic polarization of an electrode in solutions of organic cong>ounds. 

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