Reversible electrode standard reduction

Figure 2.87 Schematic of the cyclic voltammogram expected from a reversible electrochemical redox system 0 + e + R having a standard reduction potential °. E is the potential of the working electrode, and I the current.

Figure 2.91 Schematic representation of the rotating disc electrode response Tor the reduction and oxidation of a reversible couple. / = F/RT, /, is the limiting current, / is the current, E is the potential of the electrode and ° is the standard reduction potential of the couple.

Samhoun and David [169] have studied the reduction of Cf(III) by radiopolarography in 0.1 M LiCl at pH 2 and found a reversible electrode process attributed to the Cf(III)/Cf(0) couple at 1/2 = —1.508 V versus SHE with an estimated standard potential of —2.030 V. These results were called into question in a subsequent paper by Musikas etal. [173] in which they determined... 

We start with a simple reversible redox reaction for which we can directly measure the free energy of reaction, Ar<7, with a galvanic cell. This example helps us introduce the concept of using (standard) reduction potentials for evaluating the energetics (i.e., the free energies) of redox processes. Let us consider the reversible interconversion of 1,4-benzoquinone (BQ) and hydroquinone (HQ) (reaction 14-5 in Table 14.1). We perform this reaction at the surface of an inert electrode (e.g.,... 

The two cathodic partner reactions in corrosion are hydrogen evolution and oxygen reduction. Consider the Electrochemical Series (for a full list, see The Handbook of Chemistry and Physics) and work out a rule that gives the standard reversible electrode potential, less negative than which (pH 7 and aMzt = lO 6 M) a metal will no longer have a tendency to corrode (a) in 1 M acid and (b) in 1 M alkali. (Bockris)... 

The third largest class of enzymes is the oxidoreductases, which transfer electrons. Oxidoreductase reactions are different from other reactions in that they can be divided into two or more half reactions. Usually there are only two half reactions, but the methane monooxygenase reaction can be divided into three "half reactions." Each chemical half reaction makes an independent contribution to the equilibrium constant E for a chemical redox reaction. For chemical reactions the standard reduction potentials ° can be determined for half reactions by using electrochemical cells, and these measurements have provided most of the information on standard chemical thermodynamic properties of ions. This research has been restricted to rather simple reactions for which electrode reactions are reversible on platinized platinum or other metal electrodes. 

Cp2 UCl2 undergoes reversible 1-electron reduction during cyclic voltametry in acetonitrile or THF. The Ey2 are -1.30 and -1.22 V, respectively, measured against a standard calomel electrode (424). 

The standard electrode potential is sometimes called the standard reduction potential because it is listed by the reduction half-reactions. However, a voltmeter allows no current in the cell during the measurement. Therefore, the conditions are neither galvanic nor electrolytic—the cell is at equilibrium. As a result, the half-reactions listed in the table are shown as reversible. If the reaction occurs in the opposite direction, as an oxidation half-reaction, E° will have the opposite sign. 

Since in a redox reaction electrons are transferred, and since electrons have charge, there is an electric potential E associated with any redox reaction. The potentials for the oxidation component and reduction component of a reaction can be approximated separately based upon a standard hydrogen electrode (SHE) discussed later in this lecture. Each component is called a hall reaction. Of course, no half reaction will occur by itself any reduction half reaction must be accompanied by an oxidation half reaction. There is only one possible potential for any given half reaction. Since tire reverse of a reduction half reaction is an oxidation half reaction, it would be redundant to list potentials for both the oxidation and reduction half reactions. Therefore, half reaction potentials are usually listed as reduction potentials To find the oxidation potential for the reverse half reaction, the sign of the reduction potential is reversed. Below is a list of some common reduction potentials. 

Finally, it must be noted that standard electrode potential and half-wave potential are nearly identical only for reversible reduction processes 32). This is true for the alkali metal ions, T1+ and in most cases for Zn2+ and G12+. Similar trends are observed for both reversible and irreversible reductions (Figs. 3—6) and this may justify the conclusion that partial irreversibihty resiilts in most cases in a shift of curves along the E1/2 axis without any significant change in the general pattern. 

Selected values of standard reduction potentials are listed in Table 7.1 (see also Appendix 11). Most of these values have been obtained directly from potential difference measurements, but a few values have been calculated from data obtained by calorimetric methods. This latter technique is for systems that cannot be investigated in aqueous media because of solvent decomposition (e.g. F2/2F ) or for which equilibrium is established only very slowly, such that the electrode is non-reversible (e.g. O2, 4H /2H20). Table... 

Such emf values, under standard conditions, are known as standard electrode potentials or standard reduction potentials, and are given the symbol Alternatively, the standard hydrogen electrode may be placed on the right-hand side the potential so obtained is known as the standard oxidation potential The latter potentials are the standard electrode potentials with the signs reversed, the only difference being that the cells have been turned around. 

Perhaps the most common example of overvoltage encountered in electrochemistry is that needed to reduce H" " ions at a mercury electrode. On a catalytic Pt surface (platinized Pt, which is a large surface-area, black Pt deposit on Pt metal) the H2/H ion couple is said to behave reversibly. This means that one can oxidize hydrogen gas, or reduce H ions, at the standard reduction potential, 0.00 V, under standard conditions. At a mercury... 

For biological systems this tendency is expressed by the standard reduction potential, Eq, defined as the electromotive force (emf) in volts given by a half-cell in which the reductant and oxidant species are both present at 1.0 M concentration unit activity, at 25°C and pH 7.0 in equilibrium with an electrode which can reversibly accept electrons from the reductant species, according to the equation ... 

In addition, galvanic corrosion can be predicted by using the electromotive force (emf) or standard potential series for metal reduction listed in Table 2.1. These reactions are reversible. The standard metal potential is measured against the standard hydrogen electrode (SHE), which is a reference electrode having an arbitrary standard potential equals to zero. Details on types of reference electrodes are included in chapter 2. 

Standard potentials are also called standard electrode potentials and standard reduction potentials. If in an older source of data you come across a standard oxidation potential, reverse its sign and use it as a standard reduction potential. 

The standard reduction potentials, corresponding to the oxidation potentials in Table 1.2 but involving the reverse electrode processes, would be obtained by reversing the sign in each case thus, for example, for the zinc electrode,... 

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