Electrodes, oxidation-reduction table

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

A second way of expressing the same information is to give electrode potentials (Table 6-8). Electrode potentials are also important in that their direct measurement sometimes provides an experimental approach to the study of oxidation-reduction reactions within cells. To measure an electrode potential it must be possible to reduce the oxidant of the couple by flow of electrons (Eq. 6-62) from an electrode surface, often of specially prepared platinum. 

Because any two oxidation-reduction reactions can be combined to make a cell, the tabulation of standard electrode potentials becomes a very efficient way of calculating cell potentials under standard conditions. As indicated by Eq. (54), if the electrode reactions involve the metals of the cell terminals, the metal-metal potential due to the cell terminals is automatically included in the result. A short table of standard electrode potentials is given in Table 2. 

TABLE 16.3 Selected Standard Electrode Potentials for Oxidation-Reduction Half Reactions... 

Table 2 Standard oxidation-reduction potentials 25° C, volts Di. hydrogen electrode...

The standard electrode potential for an oxidation-reduction process is often called the standard redox potential of the pair of ions involved. A table of redox potentials finds immediate application in inorganic chemistry. 

Standard electrode potential data are available for an enormous number of halfreactions. Many have been determined directly from electrochemical measurements. Others have been computed from equilibrium studies of oxidation/reduction systems and from thermochemical data associated with such reactions. Table 18-1 contains standard electrode potential data for several half-reactions that we will be considering in the pages that follow. A more extensive listing is found in Appendix 5. ... 

In order that a compound be used as a mediator, it must satisfy quite a number of requirements 1) the interaction stage between the mediator and the enzyme active center must be fast (the mediator must be a specific substrate of the enzyme) 2) the normal oxidation-reduction potential of the mediator must be close to that of the reaction concerned 3) the mediator should be subject to electrochemical oxidation (or reduction) on the electrode made from a given material under conditions close to reversible ones. By no means are all of the known mediators able to meet the above requirements. In Table 3 the characteristics of certain mediator compounds which have been used in bioelectrocatalysis are given. 

Strontium cerates or strontium zirconates proton conductor can be used as the electrolyte membrane, with the results of nitrogen oxide reduction summarized in Table 8.8. When Pt/Ba/AljOs or EVSr/AljOs is used as working electrodes, it is possible to reduce the NO even in the presence of excess O2 (Kobayashi et a/., 2000,2002).The reduction of NO proceeds through the electrochemical reduction of NO absorbed into Sr/AljOa but not through the chemical reduction of NO by H2 gas. 

D Eha Camacho et al. (2011) proposed a novel concept using an assisted electrochemical reaction to produce atomic hydrogen from water electrolysis for different heterorganic compounds conversion. The electrochemical reactor is divided into two compartments by a palladium membrane in which atomic hydrogen is absorbed and permeated. Organic sulfur in the oil can be desulfurized and transformed to H2S in the electrochemical compartment. In addition, Lam et al. (2012) recently presented a review of electrochemical desulfurization technologies for fossil fuels. Various electrodes and electrolytes that have been used for desulfurization accomphshed by oxidation, reduction, or both were summarized by Lam et al. in their paper. Some electrochemical desulfurization processes for transportation fuels were chosen for listing in Table 14.2. 

The Status of the Hydrogen Electrode. Probably no area of electrochemistry is more greatly neglected in current texts than the history of the choice of the hydrogen electrode as the reference standard for electromotive force measurements. Since all tables of potentials of oxidation-reduction half-reactions are based on the half-cell reaction 35H2=H +e , it would seem that the selection of this reaction as the standard should warrant more attention. If the selection is treated at all, it is usually dismissed as an arbitrary choice, which it is, with no reference made to the people and events involved in establishing this fundamental reference point for the EMF scale. One possible exception may be noted ( ). The referenced edition of this work is perhaps the best previously existing source on this topic. However, the subsequent edition omits the subject entirely. 

The standard reference point in the EMF series is the hydrogen electrode, which consists of gaseous hydrogen at 1 atm bubbling over a platinum electrode in an acidic solution with activity 1 for the hydrogen ion (Figure 11-6). Similar electrodes can be made for some other non-metallic elements, and a few of these elements are included in the table, as well as some other oxidation-reduction pairs. 

An indicator of soil corrosivity is the value of soil oxidation-reduction (ORP) or redox potential. It is calculated from the potential difference measured with a probe that contains an inert platinirm (Pt) electrode and a saturated calomel electrode (Hg/HgjClj/KCl, +0.241 V versus SHE) as a reference electrode. The value of this soil redox piotential depends on the dissolved oxygen content in the piore water and provides some information on the conditions under which sulfate-reducing bacteria could grow. The use of redox potentials to predict soil corrosivity is presented in Table 4. 

Table 23-2 lists ihe various types of ion-selective membrane electrodes that have been developed. These differ in the physical or chemical composition of the membrane. The general mechanism by which an ion-sclective potential develops in these devices depend.s on the nature of the membrane and is entirely different from the source of potential in metallic indicator electrodes. We have seen that the potential of a metallic electrode arises from the tendency of an oxidation-reduction reaction to occur at an electrode surface. In membrane electrodes, in contrast. Ihe observed potential is a kind of junction potential that develops across a membrane thal separates the anidyte solution from a reference solution. 

The electromotive force (emf), or cell potential, is the maximum voltage of a voltaic cell. It can be directly related to the maximum work that can be done by the cell. A standard electrode potential, or reduction potential, refers to the potential of an electrode in which molar concentrations and gas pressures (in atmospheres) have unit values. A table of standard electrode potentials is useful for establishing the direction of spontaneity of an oxidation-reduction reaction and for calculating the standard emf of a cell. 

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