Standard potential series

The degree to which the reaction can proceed is the greater the further apart both substances are in the standard potential series. Thus e. g. from the potential values of the elements Zn Zn++ and Pt I Co++, Co+++ it follows that the equilibrium constant of the reaction % Zn -f- Co+++ = Co++ -f- % Zn++ at 25 °C is approximately K = 1044, which means that the salt of the trivalent cobalt is reduced by zinc almost completely. The equilibrium constant K — 6.4 will belong on the other hand to the reaction Fe + Cd++ = Cd + Fe++ this comparatively small value shows a considerable amount of cadmium ions remaining in a state of equilibrium and not reduced. 

It is natural that the possibility of ionic reaction can be reliably predicted from the standard potential series only when the activities of all components taking part in the reaction equal unity. At other activities the mutual relations of substances in the potential series can be changed. There is a second limitation, namely, no retardation of the reactions by various foreign phenomena (e. g. by overvoltage or mechanical pasivity of the surface, due to the existence of oxide films). 

The standard potential series obtained is given in Table 1.5. 

The standard potential series can be used as only a rough guide with respect to the ability of a metal to resist corrosion. In most of the corrosion reactions, the potential values shown in the table are not applicable because of the presence of a film on the metal surface, and the change in potential because the activity of metal ions is less than unity. 

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. 

A standard potential series has been established primarily by Flengas and co-workers in this solvent over the temperature range 700-900 0. Oombes et reinvestigated the redox couples Ag+/Ag, Au+/Au,... 

Standard curves in spectrophotometry, 674 Standard deviation I 34 Standard potentials 62, 63, 66 Standard series method 652, 654 Standard solutions 107, 257, 259 for pH, 569, 831 prepn. of, 107, 260, 285, 802 storage of, 108 Standard substances for acidimetry and alkalimetry ... 

The question arises as to which metal is dissolved, and which one is deposited, when combined in an electrochemical cell. The electrochemical series indicates how easily a metal is oxidized or its ions are reduced, i.e., converted into positively charged ions or metal atoms respectively. The standard potential serves for the comparison of different metals. 

Figure 5. Electrochemical series of metals and their standard potentials in volt (measured against NHE).

For transmetallations with a metal (metallo-de-metallations, Scheme 10-95) arylmercury compounds are particularly suitable due to the position of mercury as a noble metal in the electrochemical series of standard potentials (for examples see Makarova, 1970). 

We can use the electrochemical series to predict the thermodynamic tendency for a reaction to take place under standard conditions. A cell reaction that is spontaneous under standard conditions (that is, has K > 1) has AG° < 0 and therefore the corresponding cell has E° > 0. The standard emf is positive when ER° > Et that is, when the standard potential for the reduction half-reaction is more positive than that for the oxidation half-reaction. 

The positive value of the standard voltage obtained in the example indicates that the cell reaction shown is spontaneous. Thus, the standard potentials in Table 6.11 can be used to predict whether a particular reaction will occur, or not. The advantage of Table 6.11 is that it provides quantitative as well as qualitative information. It not only conveys that nickel is a stronger oxidizing agent than silver (because nickel is positioned below silver in the electrochemical series), but it also conveys how much stronger, in terms of the cell emf of+1.05 V. 

The first attempt to describe the dynamics of dissociative electron transfer started with the derivation from existing thermochemical data of the standard potential for the dissociative electron transfer reaction, rx r.+x-,12 14 with application of the Butler-Volmer law for electrochemical reactions12 and of the Marcus quadratic equation for a series of homogeneous reactions.1314 Application of the Marcus-Hush model to dissociative electron transfers had little basis in electron transfer theory (the same is true for applications to proton transfer or SN2 reactions). Thus, there was no real justification for the application of the Marcus equation and the contribution of bond breaking to the intrinsic barrier was not established. 

From the measured electron-transfer equilibrium constant and the known standard potential for the reference D /D- couple it has been possible to determine E for the PhO /PhO- couple. The method, however, is non-trivial and does not lend itself to the rapid determination of standard potentials for a large series of related compounds. 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

Table 7.7 The electrode potential series (against the SHE). The electrode potential series is an arrangement of reduction systems in ascending order of their standard electrode potential...

The method consists of plotting the forward electron transfer rate constant against the standard potential of a series of redox catalysts as illustrated by Figure 2.29. Three regions appear on the resulting Bronsted plot, which correspond to the following reaction scheme (Scheme 2.14). The... 

Using, for example, cyclic voltammetry, the cathodic peak current (normalized to its value in the absence of RX) is a function of the competition parameter, pc = ke2/(ke2 + kin), as detailed in Section 2.2.6 under the heading Deactivation of the Mediator. The competition parameter can be varied using a series of more and more reducing redox catalysts so as eventually to reach the bimolecular diffusion limit. km is about constant in a series of aromatic anion radicals and lower than the bimolecular diffusion limit. Plotting the ratio pc = keij k,n + km) as a function of the standard potential of the catalysts yields a polarogram of the radical whose half-wave potential provides the potential where ke2 = kin, and therefore the value of... 

Since such correlations belong to a series of treatments which are commonly identified as Linear Free Energy Relationships (LFER), and as only the standard potential is an electrochemical quantity directly linked with free energy (AG° = -n F AE°), one can make use of these mathematical treatments only in cases of electrochemically reversible redox processes (or in the limit of quasireversibility). Only in these cases does the measured redox potential have thermodynamic significance. 

Fig. 11 Forward electron transfer (90) rate constant, k, versus the standard potential, F /q, of a series of aromatic anion radicals for rapidly cleaved aryl halide anion radicals (DMF, 20°C). kjy is the bimolecular diffusion limit. (Adapted from Andrieux et al., 1979.)...

Let us consider a set of experimental determinations of the standard potential at a series of temperatures, such as is fisted in Table A.2. A graph of these data (Figure A.2) shows that the slope varies slowly but uniformly along the entire temperature range. For thermodynamic purposes, as in the calculation of the enthalpy of reaction in the transformation... 

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