Half-Wave Potentials effect

The shift of the half-wave potentials of metal ions by complexation is of value in polarographic analysis to eliminate the interfering effect of one metal upon another, and to promote sufficient separation of the waves of metals in mixtures to make possible their simultaneous determination. Thus, in the analysis of copper-base alloys for nickel, lead, etc., the reduction wave of copper(II) ions in most supporting electrolytes precedes that of the other metals and swamps those of the other metals present by using a cyanide supporting electrolyte, the copper is converted into the difficultly reducible cyanocuprate(I) ion and, in such a medium, nickel, lead, etc., can be determined. 

Fig. 7. Effect of pH on the half-wave potential of methyl butyl phonacyl sulphonium perchlorate [2 x m] in water-H 0-2% ethanol. Britton-Robinsonbuffers, ) sulphuric acid, 3 sodium hydroxide. (Taken from Zuman and Tang, 1963.)...

The influence of the N-bonded substituents R on the half-wave potentials can be described by a Taft relation, like is found for Mo, W and Au. The small value of p points to the dominance of metal orbitals in the redox orbital (5(5). The phenyl derivates do not fit this relation, probably because of a mesomeric influence. Here, however, the n-butyl and cyclohexyl also show small deviations, probably because of steric effects. 

Fig. 10. Half wave potentials (at a rotating platinum electrode) vs. d-electron configuration for Et2dtc complexes. The E1/2 values depend upon solvent and reference electrode used (see text), but this is a minor effect as compared with the influence of the d-electron configuration.

FIG. 3 Effect of the hole radius on the half-wave potential for a quasi-inlaid geometry [Fig. 2(c)]. The depth of the microhole is 12/am. (Reprinted with permission from Ref. 13. Copyright 1999 Elsevier Science S.A.)... 

Finally, a remark should be made on the effect of the scan rate an increase in the scan rate, e.g., from 50 through 100 to 200mV s 1, causes a sharper and apprecially higher peak, as expected. If the electrode reaction is reversible, the half-wave potential, Up/2, remains nearly unaltered, otherwise there is a shift to the right (more negative in reductive LSV). It should be borne in mind that in a follow-up reaction such as the system EC (see p. 124) an increase in scan rate may cause a transition from irreversibility to apparent reversibility if the charge-transfer reaction E becomes predominant. 

Table 6.3 The effect of complexation on half-wave potentials (volts)...

For Ni2, Co2 and Tl increasing donicity of the solvent has a more pronounced effect on the half-wave potential (Fig. 26) the solvate bonds become increasingly covalent with strong donor molecules. 

Certain transition metal ions such as Co2, Ti3 are known to form chelates with trimethylphosphate, i.e., dimethoxyphosphato complexes84 8S>. The chelate effect is responsible for the high stabilities of such complexes, which is expressed in the more negative values for the half-wave potentials. All ions producing such complexes are expected to undergo reduction in TMP at more negative potentials than would be expected from interpolation of the curves. 

The half-wave potentials of K+, Tl+ and Ca2+ in water are slightly more negative and thosefor Zn2+, Cd2+, Mn2+, Ni2+ and Co2+ considerably more negative than is expected according to the donicity rule. It has been shown in the previous sections that water is a rather unique solvent. The effect in question may be interpreted by the so-called Katzin-effect according to which water forms a royal core of coordinated water molecules which are hooked together by hydrogen bonds 70,71>122,1231. 

Although no data are available in HMPA, it has been shown that due to steric effects metal ions are weaker coordinated than would be expected from its donicity 83). This observation suggests that the half-wave potentials will be found at more positive potentials than expected from extrapolation of the curves. 

The presence of water may have an appreciable effect onE j, since water is a fairly strong donor. It is known that it is extremely difficult to remove the last traces of water from any solvent and it is therefore of interest to know the influence of water. It is apparent that in solution of a strong donor such as DMF, DMA, DMSO or HMPA the presence of small amounts of water is not reflected in a shift of the half-wave potential. On the other hand, the half-wave potential is shifted to negative potential values by the presence of water in a weak donor solvent. 

Figure 6.9 Polarogram showing a time-averaged current, i.e. redrawn without the sawtoothed effect caused by the cyclic nature of the mercury drops the half-wave potential, 1/2, and the residual current are also indicated. The magnitude of the diffusion current, 7a, is determined with respect to the residual current.

Figure 6.11 Polarogratn of a solution containing three analytes, showing three different waves . The half-wave potential, 1/2, for each is characteristic of the respective analyte couples, while the wave heights reflect the relative concentrations of each ion. The trace has been smoothed to remove the sawtoothed effects seen in Figures 6.7 and 6.8. The solution also contained KCl (0.1 mol dm ) as a swamping ionic electrolyte, and Triton X-lOO (a non-ionic surfactant) as a current maximum suppressor.

Another analytically useful phenomenon in electrolysis at ITIES is ion transfer faciUtated by ionophores present in the non-aqueous phase [8]. If the ionophore is present at a low concentration in the non-aqueous phase and the aqueous phase contains a large concentration of the cation that is bound in a complex with the ionophore (for example as a component of the base electrolyte), then a voltammetric wave controlled by diffusion of the ionophore toward the ITIES or by diffusion of the complex formed away from the ITIES into the bulk of the organic phase appears at a potential lower than the potential of simple cation transfer. The peak height of this wave is proportional to the ionophore concentration in the solution and can be used for the determination (fig. 9.8). This effect has been observed with valinomycin, nonactin, cycUc polyethers and other substances [2,3,23]. The half-wave potential of these waves is... 

This explanation for the two polarographic waves seen in Figure 3.2 suggests that the region of transition between the two waves will be sensitive to buffer concentration and composition. Such effects are seen in the polarography-pH profiles of steroid enones, some of which [88] show behaviour like that of cyclohexenone while others show only a linear variation of half-wave potential over the whole pH range of 2 — 11 [89, 90]. 

The half-wave potentials of Cd(II), Zn(II), and Pb(II) ions electroreduction in 22 nonaqueous solvents were used in the analysis of solvent effect on electrode potential [68]. 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, by ion-dipole or ion-induced dipole interactions and by more direct interactions, such as hydrogen bonding to anions or electron-pair donation to cations. Lewis acidity and basicity of the solvents affect the latter. The redox properties of the ions at an electrode depend on their being solvated, and the solvation effects electrode potentials or polarographic half-wave potentials. 

The half-wave potential for the electrochemical oxidation of NADH to NAD is ca. -bO.6 V vj. SCE at pH 7. The formal potential for the NADH/NAD couple, however, is only —0.56 V. The overpotential therefore is about 1.2 V. As NAD acts as coenzyme in many enzyme-catalyzed oxidations of practical importance, it would be of interest to regenerate NAD electrochemically. For this purpose it is necessary to find a mediator system which is able to lower the overpotential. Mediator systems accepting two electrons or a hydride atom are most effective. Therefore, dopaquinone electro-generated from dopamine 2" and quinone diimines derived from diaminobenzenes applied successfully. 

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