Half-wave potential, significance

Using these assumptions and conventions, Imoto and co-workers have correlated a number of series of reactions of thiophenes and furans. The reactions studied are the acid-base equilibria pK values) and the acid catalyzed methylations (thiophenes only) of thiophene-and furan-carboxylic acids and the alkaline hydrolyses of their ethyl esters the side-chain bromination of the a-acetylthiophenes, and the a-mercuration of thiophenes and the polarographic half-wave potentials of the methyl esters of thiophene- and furan-carboxylic acids and of nitrothiophenes. The pK values were determined and the ester hydrolyses studied for all three substitution orientations in the thiophene series. For the 4-R-2-Y and 5-R-2-Y series, the p-values do not appear significantly different and the data could probably be combined into a single series unfortunately, however, no limits of accuracy are reported for the p-values, and some of the raw data are not readily available so recalculation is not easily possible. For the 5-R-3-Y series the p-values deviate considerably from the other values however, whereas they are higher for the pK values, they are lower for the ester hydrolyses, and it is possible that the differences are neither systematic nor significant. 

Recently, Muha (83) has found that the concentration of cation radicals is a rather complex function of the half-wave potential the concentration goes through a maximum at a half-wave potential of about 0.7 V. The results were obtained for an amorphous silica-alumina catalyst where the steric problem would not be significant. To explain the observed dependence, the presence of dipositive ions and carbonium ions along with a distribution in the oxidizing strengths of the surface electrophilic sites must be taken into account. The interaction between the different species present is explained by assuming that a chemical equilibrium exists on the surface. 

For each cathodic stripping mechanism, the dimensionless net peak current is proportional to the amount of the deposited salt, which is formed in the course of the deposition step. The amount of the salt is affected by the accumulation time, concentration of the reacting ligand, and accumulation potential. The amount of the deposited salt depends sigmoidally on the deposition potential, with a half-wave potential being sensitive to the accumulation time. If the accumulation potential is significantly more positive than the peak potential, the surface concentration of the insoluble salt is independent on the deposition potential. The formation of the salt is controlled by the diffusion of the ligand, thus the net peak current is proportional to the square root of the accumulation time. If reaction (2.204) is electrochemically reversible, the real net peak current depends linearly on the frequency, which is a common feature of all electrode mechanism of an immobilized reactant (Sect. 2.6.1). The net peak potential for a reversible reaction (2.204) is a hnear function of the log(/) with a slope equal to typical theoretical response... 

In this section, a brief discussion is given of the chronocoulometric method [46, 47, 138, 139] to determine the surface excess Tq of an electroactive species O at some potential Ei chosen in the region far anodic to its half-wave potential. The initial concentration of O is Cq, and the initial concentration of R is zero (i.e. R is initially absent), so that, in the initial situation, no significant electrolysis takes place. 

The value of 0 in a particular radical can be estimated by comparison of the experimental half-wave potential and HFSC with the results obtained from a series of HMO calculations using different assumed values of 0. A large number of phenyl-substituted aromatic compounds [62] and ethylenes [63] have been treated in this fashion. Similar evidence for the twisting of the nitro group in nitroaromatic anion radicals is summarized in Reference 1. Restricted rotation of alkyl substituents is also discussed in Reference 1, but this torsion does not significantly affect the electrochemical behavior. 

Apart from a change in the course of the reduction, which will be discussed below in the section on stereospecific electrosynthesis, the epimers of a,a -dibromosuccinic acid show differences in half-wave potentials (134,135). The free acid of the erythro dibromosuccinic acid is reduced at more positive potentials than the threo-ioxm. For the anions of these acids, however, the reduction at higher pH values occurs at more positive potentials for the threo-epimer than for the erythro-anion. For the esters of dibromosuccinic acids, the difference in half-wave potentials of the two epimers is too small to be significant. 

Hydrodynamic voltammetric techniques have the major advantage of being steady-state techniques (see Section 1). Consequently, it is easy to measure limiting currents and half-wave potentials (see below for their definition) as a function of the convective parameter (i.e. flow rate, electrode angular velocity) in the absence of significant problems arising from capacitative charging currents. 

Both analysis give logical results. As expected, the coefficient 02( 2) in both analyses decreases significantly when going from Li to Cs. However, even in the case of cesium, they still have a significant value, which indicates that such a system would not serve properly as a reference electrode in various nonaqueous solvents. The U] values in Table 3 represent the half-wave potentials at DN = 0, i.e. in... 

There are several reports on the exploitation of ferrocenyl moiety in the building up of polymetallaynes. A novel heterobimetallic platinum acetylide polymer of ferrocenylfluorene, P72, was reported and spectroscopically characterized.95 The presence of significant donor-acceptor interaction leads to a narrow band-gap value of 2.1 eV for this Pt-Fe mixed material, which is much lower than for the parent polymer, P25. The polymer is electroactive, with the half-wave potential of the ferrocene moiety slightly more anodic in P72 than in the diethynyl ligand precursor, which is in line with the transfer of electron density from the ferrocenyl donor unit to the electron-accepting Pt(II) center through the acetylide linkage. 

Finally, since polarography involves mercury, which often solvates metals as amalgams, there can be a significant difference between the potential E° of standard tables and the appropriate value of E° for an amalgam-forming metal. Polarography therefore uses so-called half-wave potentials EU2 instead of standard potentials E°. All the above effects are incorporated in spreadsheet exercise 6.10. 

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. 

Fig. 1.5g shows polarograms for reversible and irreversible processes. It is important to note that the diffusion current limit (/ ) is still attained but not at the same potential. This is an important point in analytical applications. Note however that the significance of the half-wave potential ( i) has changed considerably. Theories... 

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