Irreversible behavior

For quasi-reversible systems (with 10 1 > k" > 10 5 cm s1) the current is controlled by both the charge transfer and mass transport. The shape of the cyclic voltammogram is a function of k°/ JnaD (where a = nFv/RT). As k"/s/naD increases, the process approaches the reversible case. For small values of k°/+JnaD (i.e., at very fast i>) the system exhibits an irreversible behavior. Overall, the voltaimnograms of a quasi-reversible system are more drawn-out and exhibit a larger separation in peak potentials compared to those of a reversible system (Figure 2-5, curve B). 

The influence of the reversibility of the electrochemical reaction on the SW net charge-potential curves ( (Gsw/Gf) - (Eindex is plotted in Fig. 7.48 for different values of the square wave amplitude ( sw = 25,50,100, and 150mV) and three values of the dimensionless surface rate constant (1° ( k°t) = 10,0.25, and 0.01), which correspond to reversible, quasi-reversible, and fully irreversible behaviors. Thus, it can be seen that for a reversible process (Fig. 7.48a), the (Gsw/Gf) — (Eindex EL°) curves present a well-defined peak centered at the formal potential (dotted line), whose height and half-peak width increase with Esw (in line with Eqs. (7.118) and (7.119)), until, for sw > lOOmV, the peak becomes a broad plateau whose height coincides with Q s. This behavior can also be observed for the quasi-reversible case shown in Fig. 7.48b, although in this case, there is a smaller increase of the net charge curves with sw, and the plateau is not obtained for the values of sw used, with a higher square wave amplitude needed to obtain it. Nevertheless, even for this low value of the dimensionless rate constant, the peak potential of the SWVC curves coincides with the formal potential. This coincidence can be observed for values of sw > 10 mV. 

The fully irreversible behavior is shown in Fig. 7.48c. In this case, a negative net charge appears (see the curve with sw = 25 mV). An increase of the square wave pulse leads to a growing peak in the SWVC curve, with its peak potential being shifted up to values close to that corresponding to formal potential. From these results, the following reversibility criteria as a function of the dimensionless rate constant... 

In unier tu cuklcuuy prevent proteolysis, the inhibitor should fulfill at least two requirements (1) inhibit the target enzyme in an irreversible or pseudo-irreversible manner and (2) suppress the enzyme activity before significant proteolysis has occurred. From a kinetic viewpoint, the classical example of vn irreversible protein inhibitor of NE is ai-PI. Proteolysis can also be prevented by reversible inhibitors if the in vivo inhibitor concentration is much greater than Kj, the equilibrium constant far inhibition (Ujvta >>Kj), resulting in a pseudo-irreversible behavior. 

They remarked that SbCl2+ was the dominant species in the acidic ionic liquids. The reduction of this species on glassy carbon exhibited irreversible behavior. In the basic melts SbCL+ and SbClf, were believed to be the dominant species. In basic media the reduction of Sb(III) to the metal on glassy carbon was also irreversible while its oxidation to Sb(V) showed quasi-reversible behavior. 

Such a plot is shown (Figure 8) and displays both reversible and irreversible behavior. The critical scan rate is vc = 10.5 millivolts sec l. At vc equations for both reversible and irreversible behavior apply. 

In Figure 8 the positive slope for irreversible behavior should be given by ... 

By equating the peak current equations for reversible and irreversible behavior at vc the stoichiometric electron number may be found from peak potential characteristics. Thus, at 298°K ... 

The very important irreversibility of all observable processes can be fitted into the picture in the following way. The period of time in which we live happens to be a period in which the //-function of the part of the world accessible to observation decreases. This coincidence is really not an accident, since the existence and the functioning of our organisms, as they are now, would not be possible in any other period. To try to explain this coincidence by any kind of probability considerations will, in my opinion, necessarily fail. The expectation that the irreversible behavior will not stop suddenly is in harmony with the mechanical foundations of the kinetic theory. 

Figure 25 Typical chronoamperograms and schematic view of the structure of lithiated graphite electrodes in three classes of electrolyte solutions (as indicated). 1. Reversible behavior 2. partially reversible behavior, low capacity (x < 1 in LijC6 3. irreversible behavior, the electrode is deactivated and partially exfoliated before reaching intercalation stages. Note that in reality the graphite particles are usually flakes, the electrode s structure is porous and the surface films are also formed inside the electrode among the particles, and thus have aporous structure [87]. (With copyrights from Elsevier Science Ltd., 1998.)...

The -> polarization curves for irreversible and quasireversible systems are shown in Figure (a). The respective -> Tafel plots are presented in Figure (b). Tafel plots can be constructed only for electrochemically irreversible systems, and kinetic parameters can be determined only when irreversible kinetics prevails. A switching from reversible to irreversible behavior and vice versa may occur. It depends on the relative values of ks and the -> mass transport coefficient, km. If km ks irreversible behavior can be observed. An illustration of the reversibility-irreversibility problem can be found in the entry -> reversibility. 

Irreversibility — Figure. Quasireversible and irreversible behavior, a Current-potential curves and b lg j -E plots (Tafel plots) for a redox system at different angular velocities of a rotating disc electrode [Pg.374]

Reversibility — Figure 1. The illustration of the reversibility problem. The standard rate constants (fcs(l) and ks(2)) are characteristic to the charge transfer rate of the given systems. The diffusion rate constants (ImR or f mo) are varied by the rotation rate of the electrode. If ks k , the system is reversible, while in the case of km k, irreversible behavior can be observed. The values of the diffusion coefficients are taken equal for both systems... 

Benzophenone is another example of a molecule showing small-molecule behavior in a specific region of its absorption spectrum 30<31). Here it concerns intersystem crossing between the lowest excited singlet and triplet states, which are separated by about 2800 cm-1. Very fast intersystem crossing is induced by inter molecular interactions 3°). Under isolated-molecule conditions relative to the radiative lifetime as calculated from the integrated oscillator strength, irreversible behavior is not obtained. 

Preliminary emf measurements were made on Cell I, and the standard potential of the Ag-AgBr electrode was determined as 0.07106 V from data taken in 0.01000 mol kg"1 hydrobromic acid. This value of Em° was identical with that given in the literature (20). The emf values were reproducible up to m = 1.0 mol kg"1. There was some evidence of irreversible behavior for m = 1.5 mol kg"1. In order to avoid this kind of drift in the emf values at the highest constant total molality tested, the cell with the hydrogen electrode was allowed to equilibrate for 45 min before the Ag-AgBr electrode (which was kept in a separate standard-joint test tube containing a solution of the same composition) was transferred to the electrode compartment. The equilibrium emf value was recorded every 5 min until no deviation was noticed. 

Similar study was performed with cytochrome e using bare GCE, GCE modified with untreated CNTs and GCE modified with treated CNTs [53], While no response was observed at the GCE and an irreversible behavior was obtained at the unactivated SWCNT-modified GCE a quasi reversible bevahior was observed at the activated SWCNT-modified GCE, with a peak potential separation of 73.7 mV. These results point out the advantages of CNTs in the electron transfer reaction. A linear relationship between peak current and cytochrome c concentration was found between 3.0 x 10 M and 7.0 x 10"" M with a detection limit of 1.0 X 10 M. 

Pang et al. [54] studied the electrochemical behavior of L-dopa at SWCNT-modified GCE. Before starting, the electrode was immersed for 120 s in the L-dopa solution. L-dopa showed an irreversible behavior at bare GCE with peak potential separation of 161 mV. On the contrary, a quasi reversible behavior with peak potential separation of 55 mV was obtained at the SWCNTs-modified electrode. Experiments performed by differential pulse voltammetry showed a... 

The Oxygen Electrode.—The standard potential of the oxygen electrode cannot be determined directly from e.m.f. measurements on account of the irreversible behavior of this electrode (cf. p. 353) it is possible, however, to derive the value in an indirect manner. The problem is to determine the e.m.f. of the cell... 

The borderlines between reversible, quasi-reversible, and irreversible behavior were originally defined by Matsuda and Ayabe [12] on the basis of mathematical reasoning. However, in practical work it is more convenient to define borderlines reflecting where deviations from the two limiting cases, reversible and irreversible, may be observed experi-... 

The anodic behavior of A -substituted alkenes can be described as the oxidation of an electron-rich double bond. Tetraamino-substituted alkenes are extremely easily oxidized. Tetrakis(dimethylamino)ethylene exhibits two reversible one-electron processes at —0.75 and —0.61 V vs. SCE at a dropping mercury electrode in acetonitrile [140]. The anodic behavior of A, A -dimethylaminoalkenes has been studied intensively by cyclic voltammetry and electron spin resonance (ESR) spectroscopy [141]. The anodically E° = 0.48 V vs. SCE) generated cation radical of l,l-bis(iV,iV-dimethylamino)ethylene is shown to undergo C-C coupling, forming l,l,4,4-tetrakis(A, iV-dimethylamino)butadiene, which subsequently is further oxidized to its dication at —0.8 V [141,142]. With vicinal diamino ethylenes, usually two reversible one-electron oxidations are observed [143], while gem-inal diamino ethylenes exhibit an irreversible behavior [141]. Aryl-substituted vicinal diamino ethylenes (endiamines) can undergo a double cyclization to give an indolo-oxazoline when oxidized at 0.4 V vs. SCE in acetonitrile in the presence of 2,6-lutidine [144] ... 

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