Electrode inert

In addition to the case of a metal in contact with its ions in solution there are other cases in which a Galvani potential difference between two phases may be found. One case is the innnersion of an inert electrode, such as platinum metal, into an electrolyte solution containing a substance S that can exist m either an oxidized or reduced fomi tlirough the loss or gain of electrons from the electrode. In the sunplest case, we have... 

If two redox electrodes both use an inert electrode material such as platinum, tlie cell EMF can be written down iimnediately. Thus, for the hydrogen/chlorine fiiel cell, which we represent by the cell Fl2(g) Pt FICl(m) Pt Cl2(g) and for which it is clear that the cathodic reaction is the reduction of CI2 as considered in section... 

For a simple electron transfer reaction containing low concentrations of a redox couple in an excess of electrolyte, the potential established at an inert electrode under equilibrium conditions will be governed by the Nemst equation and the electrode will take up the equilibrium potential for the couple 0/R. In temis of... 

Electron transfer can be established experimentally in reactions involving only ions in solution. Inert electrodes, made from platinum, are used to transfer electrons to and from the ions. The apparatus used is shown in Figure 4.3. the redox reaction being considered... 

Note that the Pt cathode is an inert electrode that carries electrons to the reduction half-reaction. The electrode itself does not undergo oxidation or reduction. 

An inert electrode that serves as a source or sink for electrons for a redox halfreaction. 

Dilution with water reverses the reaction, and heating the solution Hberates sulfur dioxide. Upon being added to a solution of teUurides, teUurium forms colored polyteUurides. Unlike selenium, teUurium is not soluble in aqueous sodium sulfite. This difference offers a method of separating the two elements. Like selenium, teUurium is soluble in hot alkaline solutions except for ammonium hydroxide solutions. Cooling reverses the reaction. Because teUurium forms solutions of anions, Te , and cations, Te" ", teUurium films can be deposited on inert electrodes of either sign. 

The electrolyte thus formed can conduct electric current by the movement of ions under the influence of an electric field. A cell using an electrolyte as a conductor and a positive and a negative electrode is called an electrolysis cell. If a direct-current voltage is appHed to a cell having inert electrode material such as platinum, the hydrogen ions (cations) migrate to the cathode where they first accept an electron and then form molecular hydrogen. The ions... 

Tafel Extrapolation Corrosion is an elec trochemical reac tion of a metal and its environment. When corrosion occurs, the current that flows between individual small anodes and cathodes on the metal surface causes the electrode potential for the system to change. While this current cannot be measured, it can be evaluated indirectly on a metal specimen with an inert electrode and an external electrical circuit. Pmarization is described as the extent of the change in potential of an electrode from its equilibrium potential caused by a net current flow to or from the electrode, galvanic or impressed (Fig. 28-7). 

The concept of the fuel cell, that is, a cell in which inert electrodes immersed in an electrolyte could he intimately contacted with a reacting fuel (e.g., hydrogen) and oxidant (e.g., air) and so generate an electric current, was demonstrated in 1839 by Grove and intensively studied by him during the next decade. 

The properties of platinum as an inert electrode in a variety of electrolytic processes are well known, and in cathodic protection it is utilised as a thin coating on a suitable substrate. In this way a small mass of Pt can provide a very large surface area and thus anodes of this type can be operated at high current densities in certain electrolyte solutions, such as seawater, and can be economical to use. 

A voltaic cell using this reaction is similar to the Zn-Cu2+ cell the Zn Zn2+ half-cell and the salt bridge are the same. Because no metal is involved in the cathode half-reaction, an inert electrode that conducts an electric current is used. Frequently, the cathode is made of platinum (Figure 18.3, p. 484). In the cathode, Co3+ ions are provided by a solution of Co(N03)3. The half-reactions occurring in the cell are... 

At this stage reference may be made to potential mediators, i.e. substances which undergo reversible oxidation-reduction and reach equilibrium rapidly. If we have a mixture of two ions, say M2+ and M +, which reaches equilibrium slowly with an inert electrode, and a very small quantity of cerium(IV) salt is added, then the reaction ... 

The chemical composition of the SEI formed on carbonaceous anodes is, in general, similar to that formed on metallic lithium or inert electrodes. However some differences are expected as a result of the variety of chemical compositions and morphologies of carbon surfaces, each of which can affect the i() value for the various reduction reactions differently. Another factor, when dealing with graphite, is solvent co-intercalation. Assuming Li2C03 to be a major SEI building material, the thickness of the SEI was estimated to be about 45 A [711. 

In practice, for a ternary system, the decomposition voltage of the solid electrolyte may be readily measured with the help of a galvanic cell which makes use of the solid electrolyte under investigation and the adjacent equilibrium phase in the phase diagram as an electrode. A convenient technique is the formation of these phases electrochemically by decomposition of the electrolyte. The sample is polarized between a reversible electrode and an inert electrode such as Pt or Mo in the case of a lithium ion conductor, in the same direction as in polarization experiments. The... 

A 1.0 m NiS04(aq) solution was electrolyzed by using inert electrodes. Write (a) the cathode reaction (b) the anode reaction, (c) With no overpotential or passivity at the... 

The electrochemical reduction and oxidation of sulfur and of polysulfide dianions at inert electrodes has been studied in aprotic solvents and in liquid ammonia. In the latter case, sulfur-nitrogen compounds are involved and these systems [90] will not be discussed here. 

Baranski AS, Fawcett WR (1984) The mechanism of electrodeposition of cadmium sulfide on inert electrodes from diethylene glycol solutions. J Electrochem Soc 131 2509-2514... 

In the other examples, the electrode materials are not involved in the reactions chemically, but constitute the source [sink] of electrons. Such electrodes are called nonconsumable. The term inert electrodes sometimes used is unfortunate insofar as the electrode itself is by no means inert rather, it has a strong catalytic effect on the electrode reaction. For reactions occurring at such electrodes, the terms oxidation- reduction... 

In the present chapter we want to look at certain electrochemical redox reactions occurring at inert electrodes not involved in the reactions stoichiometrically. The reactions to be considered are the change of charge of ions in an electrolyte solution, the evolution and ionization of hydrogen, oxygen, and chlorine, the oxidation and reduction of organic compounds, and the like. The rates of these reactions, often also their direction, depend on the catalytic properties of the electrode employed (discussed in greater detail in Chapter 28). It is for this reason that these reactions are sometimes called electrocatalytic. For each of the examples, we point out its practical value at present and in the future and provide certain kinetic and mechanistic details. Some catalytic features are also discussed. 

In general, the electrolysis of a molten salt at inert electrodes produces the metal at the cathode, e.g., calcium from calcium chloride (melting point 774 °C). The anion is often a halide ion which, on discharge, yields the halogen, e.g., chlorine from calcium chloride. 

When the solution is electrolyzed between inert electrodes, Ch ions are discharged at the anode (Ch - e — Cl) and H+ ions at the cathode (H+ + e — H). This leaves Na+ ions and OH ions in the solution to form sodium hydroxide. The chlorine liberated escapes almost immediately unless, of course, it is held in some chemical combination. If the objective is to produce chlorine and sodium hydroxide, the anodic and the cathodic products are kept... 

A description of an electrolytic cell has already been given under cell features (Section 1.3.2, Fig. 1.1c). Another example is the cell with static inert electrodes (Pt) shown in Fig. 3.1 where an applied voltage (Eappl) allows a current to pass that causes the evolution of Cl2 gas at the anode and the precipitation of Zn metal on the cathode. As a consequence, a galvanic cell, (Pt)Zn 2 ZnCl2 Cl2 iPt+, occurs whose emf counteracts the voltage applied this counter- or back-emf can be calculated with the Nernst equation to be... 

Let us consider a redox system at a static inert electrode. Whilst thermodynamics only describe the equilibrium of such a system (cf., Section 2.2.1.2.1), kinetics deal with an approach to equilibrium and assume a dynamic maintenance of that state. For that purpose the equilibrium reaction... 

Shape of the polarographic curve. The kinetic theory of electrolysis (Section 3.2) for a redox system at a static inert electrode for partial and full exhaustion at the electrode under merely diffusion-controlled conditions leads, for ox + ne - red, to the relationship... 

In comparison with eqns. 3.14 and 3.15 for a static inert electrode, eqns. 3.34 and 3.35 differ only in the power of D /D as for the usual reversible diffusion-controlled redox couples Drei and Dox are approximately equal, ( red/F)ox)1/2 will even be closer to unity, so that we can simply write... 

In Fig. 3.21 this is illustrated for the same redox couple in the case of reversibility and of irreversibility in the latter situation E ted and Ei(ox) are so different that both the reduction and the oxidation waves can be separately determined. In fact, this is in agreement with the picture in Fig. 3.11 for irreversibility at a static inert electrode. 

In general we work in an analyte solution without stirring, as in polarography, unless mentioned otherwise. The simplest situation is that where a reversible redox process such as ox + ne red takes place at an inert electrode such as Pt, Pd, Ir or Eh (and sometimes Au or Ag) and where both ox and red... 

The application of this technique (even in its various modes such as cyclic voltammetry) to other electrodes has already been mentioned in the description of LSV at the dme [Section 3.3.1.2.1(5)]. Especially with stationary electrodes LSV becomes fairly simple, under the conditions of sufficient solubility of ox and red, because of the constant and undisturbed electrode surface at an inert electrode the residual faraday current can be adequately eliminated by means of "J compensation (cf., Fig. 3.23) or by subtractive [cf., Section 3.3.1.2.1(3)] and derivative59 [cf., Section 3.3.1.2.1(4)] voltammetry at a stationary mercury electrode (e.g., HMDE), in addition to the residual faradaic current,... 

Therefore, as the pH varies from 0 to 14, the red(ox) potential may vary from - 0.059 14 = - 0.83 V to 0 V without risk of water reduction. Further, let us assume for the moment the existence of an inert electrode at the surface of which the following reversible oxidation can take place ... 

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