Convention for electrode potentials

Bearing in mind that the modern sign convention for electrode potentials relates to the potential of the solution with respect to the metal, whereas Nernst uses the old convention of the f)otential of the metal with respect to the solution, eqs. (7a) and (7b) are formally equivalent if P and E for each electrode are related by P = exp [-3E /RT]. 

ASTM G 3, Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing—Conventions used for electrochemical corrosion testing are presented in this standard. Sign conventions for electrode potential, electrode potential temperature coefficients, and current tmd current density are discussed. 

By convention, a reference electrode is always the left -hand electro(ie. js shown in Figure 23 1. This practice is consistent with the International Union of Pure and Applied Chemistry (lUPAC) convention for electrode potentials, discussed in Section 22C-4 m which the reference is the standard hydrogen electrode and is the electrode on the left in a cell diagram Note that the term "saturated" in the name refers lo the concentration oi K( l (about 4,6 M) and not to the concentration of Hg CI all calomel electrodes are saturated with Mg Ch,... 

By convention, the electrode potentials for redox are referred to only in terms of reduction (E n ). For electron-transfer oxidation, however, the values of are employed for RH donors here with the understanding that = -E i of the radical cation. 

By convention, the electrode potential of any half-reaction is expressed relative to that of a standard hydrogen electrode (half-reaction 2H+ -p 2e -H2) and is called the standard electrode potential, E . Table 34.1 shows the values of E" for selected half-reactions. With any pair of half-reactions from this series, electrons will flow from that having the lowest electrode potential to that of the highest. " is determined at pH = 0. It is often more appropriate to express standard electrode potentials at pH 7 for biological systems, and the symbol is used in all circumstances, it is important that the pH is clearly stated. 

The thermodynamic convention of cell potential defined in Eq. 5 is not practical for most electrochemical work. For a battery switching from discharge to charge, cathode and anode interchange, but the electrode polarities remain the same. It is not practical to switch the leads of the voltmeter when the battery switches to the charge mode. Rather, the voltmeter stays connected during the switch with the positive terminal attached to the positive terminal of the battery and likewise for the negative terminal. The practical convention for ceU potential is... 

By convention, standard electrode potentials are written for reduction half-reactions. We write the standard electrode potentials for the two half-reactions just discussed as ... 

By convention, the electrode potential refers to the reduction reaction. So the electrons appear on the left-hand side of the half-equation. For example ... 

For simplicity, they treated a solution containing only one supporting electrolyte in addition to dissolved metal ions. Then, using a reference electrode (indicator electrode) reversible to one of the three kinds of ions (a dissolved metal ion, and an anion and cation of the supporting electrolyte), they examined the difference in potential between the working and indicator electrodes. Furthermore, the expressions derived were rewritten to correspond to the actual case when the potential of the working electrode was measured with respect to a conventional reference electrode as follows ... 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

Description of the cell composition is based - as far as possible - on the Stockholm convention (1953), i.e. the left-hand electrode constitutes the negative terminal of the cell. Cells are listed according to the metallic constituent of the electrode mentioned first which is involved in the electrode reaction establishing the respective electrode potential. Contact materials and conductive additives may be mentioned first before the actual element of interest only for the sake of correct materials sequence. The sequence of electrode components is stated as reported in the original publications. When an oxygen electrode is used as reference electrode an oxygen partial pressure of 0.21 atm is assumed. 

A parameter that is convenient for said purpose is the electrode potential E it must not be confused with the concept of a potential difference between the electrode and the electrolyte. By convention the term electrode potential E is used to denote the OCV of a galvanic cell that consists of the given electrode (the one that is studied) and a reference electrode selected arbitrarily. Thus, the potential of this electrode is compared with that of a reference electrode that is identical for all electrodes being studied. In accordance with this dehnition, the electrode potential of the reference electrode itself is (conventionally) regarded as zero. Any electrode system for which the equilibrium Galvani potential is established sufficiently rapidly and reproducibly can be used as a reference electrode. We shall write the electrode system to be used as the reference electrode, generally, as M /E ... 

Thus, the temperature coefficient of Galvanic potential of an individual electrode can be neither measured nor calculated. Measured values of the temperature coefficients of electrode potentials depend on the reference electrode employed. For this reason a special scale is used for the temperature coefficients of electrode potential It is assumed as a convention that the temperature coefficient of potential of the standard hydrogen electrode is zero in other words, it is assumed that the value of Hj) is zero at all temperatures. By measuring the EMF under isothermal conditions we actually compare the temperature coefficient of potential of other electrodes with that of the standard hydrogen electrode. 

The following explanation can be provided. With Cu2+ ions there is a tendency for them to be reduced to Cu metal and precipitated on the electrode, which is reflected by a positive standard reduction potential (+ 0.34 V). For Zn metal there is a tendency for it to be oxidized to Zn2+ ions and dissolved in the electrolyte, which is reflected by a negative standard reduction potential (- 0.76 V). In fact, with Zn one could speak of a positive oxidation potential for the electrolyte versus the electrode, as was often done formerly however, some time ago it was agreed internationally that hence forward the potentials must be given for the electrode versus the electrolyte therefore, today lists of electrode potentials in handbooks etc. always refer to the standard reduction potentials (see Appendix) moreover, these now have a direct relationship with the conventional current flow directions. 

The terms EAgCl/AgjCi- and h+/h2 are designated as the electrode potentials. These are related to the standard electrode potentials and to the activities of the components of the system by the Nernst equations. By a convention for the standard Gibbs energies of formation, those related to the elements at standard conditions are equal to zero. According to a further convention, cf. Eq. (3.1.56),... 

For the operation of an STM in a conventional two-electrode configuration, the presence or absence of significant faradaic current in the tip-sample circuit depends on three factors 1) the redox potential(s) of the solution species, 2) the reversibility of the electron transfer events for the dissolved redox couple(s), and, 3) the extent to which solution species are permitted access to the tunneling gap. We have identified four limiting cases of electrochemical interest, and discuss each separately below. 

Using the unique four-electrode STM described above, Bard and coworkers (Lev, 0. Fan, F-R.F. Bard, A.J. J. Electroanal. Chem.. submitted) have obtained the first images of electrode surfaces under potentiostatic control. The current-bias relationships obtained for reduced and anodically passivated nickel surfaces revealed that the exponential current-distance relationship expected for a tunneling-dominated current was not observed at the oxide-covered surfaces. On this basis, the authors concluded that the nickel oxide layer was electrically insulating, and was greater than ca. 10 A in thickness. Because accurate potential control of the substrate surface is difficult in a conventional, two-electrode STM configuration, the ability to decouple the tip-substrate bias from... 

The standard free energy of the reaction, AG° = E — °, is fixed for each reactant couple as soon as the electrode potential is fixed. For simplicity we have written above that AG° is equal to E — E° rather than the usual relationship AG° = F(E — E°). This convention, which is used throughout the book, implies that when potentials are expressed in volts, energies are expressed in electron volts. The term driving force will be used frequently to designate —AG°, in line with the expectation that an increase in the driving force usually speeds up the reaction. 

A CV voltammogram can be recorded under either a dynamic or a steady state depending on the electrode design and solution convection mode. In a stationary solution with a conventional disk electrode, if the scan rate is sufficiently high to ensure a non-steady state, the current will respond differently to the forward and backward potential scan. Figure 63 shows a typical CV for a reversible reduction.1... 

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