Cathodic irreversible wave

Definition of symbols AEp = peak potential difference, Epa = peak potential at cathodic peak current, Epc = peak potential at anodic peak current, tpa = anodic peak current, ipc = cathodic peak current, s = scan rate, t = time after peak (the Cottrell region), n = number of electrons involved in redox reaction. Rate parameters (acn ) and heterogeneous rate constant can be found from irreversible wave. 

Besides extensive kinetic investigations, less detailed polarographic studies can provide useful information on the course of certain reactions. For example, in the reaction of 2,3-dimercaptopropanol with 1,4-ben-zoquinone (34), the anodic wave of 2,3-dimercaptopropanol vanishes with increasing concentration of quinone, when the ratio of thiol to quinone reaches 1 2 (Fig. 13). An anodic wave corresponding to oxidation of hydroquinone is also obtained. In the reaction, which is irreversible, neither cathodic disulphide waves nor anodic waves corresponding to monothiols appear. Thus an addition-reduction mechanism (14) can be postulated. 

The irreversible wave at high cathodic potential (1, 5, 8, 9, 11) can be attributed to reduction processes of the dicyanovinyl group [7]. For the Fp-substituted compounds 1-4 all the oxidation steps display complete electrochemical irreversibility and can be related to the Fe-Si skeleton. Introduction of the Fp donor results in distinct lowering of the oxidation potentials (2-4 compared... 

However, only a few organic compounds behave in a polarographically reversible manner although many may involve a reversible electron transfer step, this is often followed by irreversible chemical reactions. Irreversible processes are those for which the current is limited mainly by the kinetics of the process at the electrode surface and not by diffusion. The nature of such current-potential curves can be described by reference to Figure 6. If electrochemical equilibrium obtains at the electrode surface, then a reversible wave is obtained (curve a). The irreversible wave (curve b) is more drawn out, i.e., for a given current, say, /i or I2, a higher cathodic potential is required. 

Fig. 8. Voltammetric I — E waves at microelectrodes represented in dimensionless I/I units. Cathodic reduction and D = Db are considered. 1, reversible wave according to Eq. (19a), (see Table 2) 2, quasi-reversible wave according to Eq. (19c), Aq= 1 3, analogous wave characterized by Aq = 0.25 4, irreversible wave according to Eq. (19e) at Aq = 0.05. Inlaid disc...

The irreversible cathodic/anodic polarographic wave is shifted to more negative/positive potentials with respect to the wave produced by a reversible process of the same standard potential E . If the electrode process is quasi-reversible the shift of the wave is no longer as pronounced as it is in the case of the totally irreversible reaction. The slope of the logarithmic presentation A ln[(ii — T)/T]/zlE is always lower than the slope of the reversible process involving the same number of electrons. At the foot of the irreversible wave virtually no current is observed. At the plateau of the wave the current becomes diffusion controlled. The T — E curves are like the curves recorded under steady state conditions at microelectrodes (cf. Fig. 9, curves 2 and 3). 

There is a cathodic shift of the Ep for the totally irreversible wave of about 30/afiia (mV) for each ten-fold increase in the potential scan rate. 

The cyclic voltammogram (cv) of [Ru((bpy)2(CO)(Cl)] in an argon saturated DMF solution (Figure 6) showed two successive one-electron reductions a reversible reduction at -1.39 V and an irreversible wave at -1.64 V vs SCE. The peak separations between the cathodic and anodic... 

Figure 6.7 shows a typical special feature of the polarization curves. In the case of reversible reactions (curve 1), the anodic and cathodic branches of the curve form a single step or wave. In the case of irreversible reactions, independent, anodic and cathodic, waves develop, each having its own inflection or half-wave point. The differences between the half-wave potentials of the anodic and cathodic waves will be larger the lower the ratio fH. ... 

The above considerations also apply to the ion of an amalgamating metal with the reversible equilibrium M"+ + ne M(Hg) at a stationary mercury electrode such as an HMDE (hanging mercury drop) or an MTFE (mercury thin-film) with the restriction, however, that the solution can contain only ox, so that merely the cathodic wave (cf., eqn. 3.15) represents a direct dependence of the analyte concentration, whilst the reverse anodic wave concerns only the clean-back of amalgam formed by the previous cathodic amplitude. When one or both of the electrodic reactions is or becomes (in the case of a rapid potential sweep) irreversible, the cathodic wave shifts to a more negative potential and the anodic wave to a more positive potential (cf., Fig. 3.10) this may even result in a complete separation of the cathodic and anodic waves (cf., Fig. 3.11). 

Another situation occurs in the case of a totally irreversible electrode reaction, notwithstanding the solubility of ox and red for an irreversible cathodic wave it means that the current i is determined only by the first term on the right-hand side in eqn. 3.8, so that... 

Obviously, in the case of a cathodic reaction, the process at more positive potentials (at the foot of the wave) is reversible while at more negative potentials irreversible. Increasing rate of stirring makes the irreversibility more pronounced. 

The situation is different on GC where only one irreversible cathodic wave is observed at —1.6 V, associated with disintegration of the compound, and one anodic wave on the reverse scan, caused by stripping of the lead released by the reduction ... 

The reduction potentials were determined by polarography, and by CV on a hanging Hg cathode, and were irreversible, though the presence of a reverse wave was noted. Evidently, here the group 14 elements are directly affected (Table 14). The cathodic shift is understandable when L in Ph3MCo(CO)3L is changed from CO (an acceptor ligand) to the donors phosphine or phosphite. Anodic shift of the first reduction potential in the... 

The discussion is simplified if electron transfer is assumed to obey the Volmer-Butler law, at least in the potential range of a wave recorded at a given scan rate. Under these conditions, the dimensionless expression of the cathodic trace of the irreversible voltammogram is given by (see Section 6.2.1)... 

Since the wave is irreversible, interest is concentrated on the cathodic trace ... 

In order to explain the formation of the product 9 during oxidation at two different potentials we have performed the experiments with cyclic voltammetry [45]. The cyclic voltammogram of catechol (QH2) exhibits the anodic wave at 0.25 V vs SCE (Fig. la) corresponding to the formation of o-quinone (Q) which is reduced in the cathodic sweep at 0.05 V vs SCE. The cathodic counterpart of the anodic peak disappears, when a sufficient amount of 4-hydroxycoumarin was added, and a second irreversible peak at 0.95 V vs SCE appeared (Fig. lb). 

Kinetics and mechanism of electroreduction of Zn(II) ammonia and hydroxy-ammonia complexes on the DME were investigated by Kravtsov and coworkers [93]. The reduction of Zn(II) on DME in solution of pH 9.2-12 and [NH3] = 0.05-2 M occurred in one irreversible diffusion-limited cathodic wave. 

U(III) species and a second three-electron reduction to give U(0) metal. The first reduction, U(IV)/U(III) couple, is elec-trochemically and chemically irreversible except in hexamethylphosphoramide at 298 K where the authors report full chemical reversibility on the voltammetric timescale. The second reduction process is electrochemically irreversible in all solvents and only in dimethylsulfone at 400 K was an anodic return wave associated with uranium metal stripping noted. Electrodeposition of uranium metal as small dendrites from CS2UCI6 starting material was achieved from molten dimethylsulfone at 400 K with 0.1 M LiCl as supporting electrolyte at a platinum cathode. The deposits of uranium and the absence of U CI3, UCI4, UO2, and UO3 were determined by X-ray diffraction. Faradaic yield was low at 17.8%, but the yield can be increased (55.7%) through use of a mercury pool cathode. 

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