Electrode potential scale

The implications of Equations (7.11) and (7.12) are quite significant. They allow for the establishment and straightforward measurement of a unique absolute electrode potential scale in solid state electrochemistry. 

To decide whether a unified electrode potential scale is of some advantage, consider cells (3.1.40) in both aqueous medium and in protic medium s ... 

Several descriptions of electrode reaction rates discussed on the preceding pages and the difficulty to standardize electrode potential scales with respect to different temperatures imply several definitions of activation energies of electrode reactions. The easiest way to determine this quantity, for example, for an irreversible cathodic process, employs Eqs (5.2.9), (5.2.10) and (5.2.12) at a constant electrode potential,... 

Figure 2.19 Linear sweep voltammograms of a platinum electrode immersed in N -saturated 0.5 M H2SO t showing the anodic stripping of adsorbed CO. The CO was adsorbed from the CO-saturated electrolyte for 10 minutes at the designated potential. The scan rate was 1 mV s The adsorption potential was (a) 0.05 V and (b) 0.4 V vs. NHE. Note the different electrode potential scales for the two plots. From Kunimatsu et at. (1986).

Then, knowing F H2, it was relatively easy to determine values of electrode potentials for any other couple. With this methodology, they devised the standard electrode potentials ° scale (often called the E nought scale , or the hydrogen scale ). 

Fig. 12. Position of the band edges of various semiconductors in absolute and conventional electrode potential scale at pH = 7 GaP (40). CdSe (41), CdS (42), ZnO (43), T1O2 (44, 45), SnC>2 (46) and the electron transfer terms of the excited rhodaminc in solution...

Table E.l is a list of reduction potentials of several reactions given in the relative-electrode potential scale. Write the potentials of the same reactions in... 

Measuring the flat band potential reference electrode), one may determine the position of the Fermi level F of a semiconductor on the electrode potential scale. Next, formulas (9)—(11) can be used to find (on the same scale) the position of Ec and v relative to F in the electrode bulk. For a chosen electrode potential one may determine, using (pn, the quantity and, hence, the band bending after that, the position of the band edges at the surface can easily be found. Finally, since the equilibrium potential for a given redox couple is known, the Fredox level can also be found with the help of Eq. (8). The diagrams thus constructed will often be used below. 

In electrochemistry we have customarily employed, instead of the absolute electrode potential / abs scale, a relative scale of the electrode potential, E yila scale, referred to the standard or normal hydrogen electrode potential E m at which the hydrogen electrode reaction, 2H + 2e dox = H2(gas), is at equilibrium in the standard state unit activity of the hydrated proton, the standard pressure of 101.3 kPa for hydrogen gas, and room temperature of 298 K. Since Eniie is + 4.44 V (or + 4.5 V) in the absolute electrode potential scale, we obtain Eq. 9.9 for the relation between abs scile and [Refs. 4 and 5.] ... 

Electrochemically. The hydrogen electrode reaction, which is the basis of the standard electrode potential scale, is given by the overall Reaction 12. After more than fifty years of intensive and sophisticated... 

Calomel Electrode. The normal hydrogen electrode [a platinum wire in 1.288 N HCl solution ( H+ = 1) with H2pressure bubbling through it] was used to define the standard electrode potential scale (see Section 7.3.2). This electrode is not convenient to use on an everyday basis, so a series of secondary reference electrodes has been developed for this purpose. One of the most commonly used laboratory reference electrodes is the saturated calomel electrode. The electrode (Fig. 7-36) consists of a platinum wire set in a paste that is a mixture of mercury (Hg(fj), mercurous chloride (calomel, HgaClajsj), and potassium chloride (KCl). The paste is in contact with a solution that is saturated with KCl and Hg2Cl2(s), The electrode can be represented as... 

Fig. 11.5 Potential energy surface profile for the oxygen reduction reaction at the standard hydrogen electrode potential scale the proton was modeled by two shells of water molecules, H 0H2(H20)3(H20),5, and the data in parentheses are Gibbs free energies [50]...

Figure 4. Absolute and conventional electrode potential scale.

A cognate issue is the establishment of a universal standard electrode potential scale in nonaqueous and mixed solvents, based on the aqueous standard hydrogen electrode (SHE), for which (H, aq/Hj) = 0 at all temperatures. The standard potentials E° are obtained on extrapolation of the EMF of a suitable cell to zero of the concentration of the electroactive electrolyte, the one that responds to the electrodes irrespective of the eventual presence of a constant inert background electrolyte, in the solutions of the two half-cells. A proper procedure for such an extrapolation that assures accuracy has been described by Mussini et al. [15]. The standard electrode potentials for a cation/metal pair M /M in a solvent S vs. the SHE is related to its standard potential in water and the standard molar Gibbs energy of transfer of the cation from water to the solvent (or solvent mixture) ... 

Thermodynamic calculations are always based on an electrochemical cell reaction, and the derived voltage means the voltage difference between two electrodes. The voltage difference between the electrode and the electrolyte, the absolute potential , cannot exactly be measured, since potential differences can only be measured between two electronic conductors (2). Single electrode potential always means the cell voltage between this electrode and a reference electrode. To get a basis for the electrode-potential scale, the zero point was arbitrarily equated with the potential of the standard hydrogen electrode (SHE), a hydrogen electrode under specihed conditions at 25 °C (cf. Ref. 3). 

The relative position of two metals on the standard electrode potential scale makes it possible to predict which of the two metals in contact will act as the anode, i.e. which of the two metals will dissolve when the battery so formed starts operating it is always the more electronegative metal which will dissolve. As an example, when copper (E = +340 mV) is coupled with zinc (E = —760 mV) in a copper sulphate solution, an electric generator (the so-called Daniel cell) is formed, with zinc acting as the anode. Its electromotive force corresponds to the sum of the absolute values of the standard electrode potentials, i.e. 1.10 V. 

Similarly to any chemical system, rate of electrochemical reaction can be changed by temperature, pressure and the concentration of reactants. However, additional control parameter for the rate of eleetroehemical reaction is the electrode potential (is). Its absolute value is not accessible to measurements, so the zero of electrode potential scale is set by introduction of hydrogen scale of electrode potentials. Let us consider an electrode reaction ... 

If we could find one electrode whose is very reproducible and where the activities could be made equal to one, we could use this electrode as a reference, measure all other electrodes against it and use this relative electrode potential scale. It was found that the hydrogen electrode is very suitable for this purpose. Thus the standard hydrogen electrode is defined as the electrode where the activity of the acid solution used is 1 (at the used concentration scale, here molar) and the pressure of the hydrogen gas (strictly speaking, the fugacity) is one atmosphere. The potential of this electrode is defined as zero at all temperatures. 

Shortcomings of the choice of the equilibrium state as the electrical reference point in the evaluation of the temperature effect on the rate of electrode reactions, and consequently of the overpotential as an experimental substitute for A(A0) in the WE-RE cell at various temperatures, have been discussed in the previous section. Hence, another reference point should be sought. From a theoretical point of view, the choice is unambiguous—it is the zero point on the relative electrode potential scale, defined by the SHE convention. Basically, this is also an equilibrium state, but of a single reaction selected by convention, namely, the reduction of two hydrogen ions to molecular hydrogen. The value of A0 at the interface when this reaction is held at equilibrium, assuming all species involved are in standard thermodynamic states, is fixed by the SHE convention as zero. The same convention associates additional properties with this reference state temperature, solvent, and solute Independence. Formally, the properties of the SHE satisfy the principal theoretical requirements for the electrical reference point in the evaluation of the effect of temperature on the rate of electrode reactions. 

Swain and Scott nucleophilicity scale. ( >) Electrode potential scale. Polarizability scale. 

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