Half-wave potentials, voltammetric

Table 12.1 Solution Cyclic Voltammetric Half-Wave Potentials 1/2 (V versus SCE), Gas-Phase Ionization Potentials/D(eV), and Electron Affinities AA(eV) for Selected Donors D and Acceptors A, along with Metal Workfunctions [4]...

Table 1. Solution cyclic voltammetric half-wave potentials E[,2 (V vs. SCE), and gas-phase ionization potentials (eV) and electron affinities (eV) for selected donors D and acceptors A [78]...

In a practice, is usually identified with the voltammetric half-wave potential for the metal in a solution free of X, so that... 

Note that the top and bottom signs in the sign combinations or T in Eqn. 330 correspond to reduction and oxidation, respectively. Note also that the quantity E 2 denotes the reversible voltammetric half-wave potential emd a represents either the cathodic or anodic transfer coefficient ac or a a, depending on the context. Furthermore and denote the apparent diffusion coefficients for reduction and oxidation. We see that a plot of... 

For an electrochemically irreversible reduction at a microdisc or microsphere electrode of radius fg, the variation of the voltammetric half-wave potential, 1/2, under steady-state conditions can be shown to vary as... 

Figure 4 illustrates the dependence of on Aq for the case when r = 1 at several different values of [Fig. 4(a)] and when = 0.5 and at several different values of r [Fig. 4(b)]. From Fig. 4(a), one can see that takes a maximum around y = 0, i.e., Aq The volume ratio affects strongly the value of as shown in Fig. 4(b), which is ascribed to the dependence of the equilibrium concentration on r through Eq. (25). This simple example illustrates the necessity of taking into account the variation of the phase-boundary potential, and hence the adsorption of i, when one tries to measure the adsorption properties of a certain ionic species in the oil-water two-phase systems by changing the concentration of i in one of the phases. A similar situation exists also in voltammetric measurements of the transfer of surface-active ions across the polarized O/W interface. In this case, the time-varying thickness of the diffusion layers plays the role of the fixed volume in the above partition example. The adsorption of surface-active ions is hence expected to reach a maximum around the half-wave potential of the ion transfer. 

The theory has been verified by voltammetric measurements using different hole diameters and by electrochemical simulations [13,15]. The plot of the half-wave potential versus log[(4d/7rr)-I-1] yielded a straight line with a slope of 60 mV (Fig. 3), but the experimental points deviated from the theory for small radii. Equations (3) to (5) show that the half-wave potential depends on the hole radius, the film thickness, the interface position within the hole, and the diffusion coefficient values. When d is rather large or the diffusion coefficient in the organic phase is very low, steady-state diffusion in the organic phase cannot be achieved because of the linear diffusion field within the microcylinder [Fig. 2(c)]. Although no analytical solution has been reported for non-steady-state IT across the microhole, the simulations reported in Ref. 13 showed that the diffusion field is asymmetrical, and concentration profiles are similar to those in micropipettes (see... 

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

Many handbooks like the CRC Handbook of Chemistry and Physics provide, on behalf of electrochemistry investigation, values of standard reduction potentials, listed either in alphabetical order and/or in potential order. These must be considered as potentials of completely reversible redox systems. In current analytical practice one is interested in half-wave potentials of voltammetric, mostly polarographic analysis in various specific media, also in the case of irreversible systems. Apart from data such as those recently provided by Rach and Seiler (Spurenanalyse mit Polarographischen und Voltammetrischen Methoden, Hiithig, Heidelberg, 1984), these half-wave potentials are given in the following table (Application Note N-l, EG G Princeton Applied Research, Princeton, NJ, 1980). 

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... 

Through voltammetric experiments, the reversible assembly of guest 1 was monitored at the NP periphery. Addition of MMPC 7 to 1 lowers the overall current and shifts the half-wave potential to more positive values. The electrochemical... 

In fact, the potentiometric or voltammetric measurement is carried out using a conventional reference electrode (e.g. Ag+/Ag electrode).3 After measurement in the test solution, Fc or BCr+ (BPhJ salt) is added to the solution and the half-wave potential of the reference system is measured by polarography or voltammetry. Here, the half-wave potential for the reference system is almost equal to its formal potential. Thus, the potential for the test system is converted to the value versus the formal potential of the reference system. The example in Fig. 6.2 is for a situation where both the test and the reference systems are measured by cyclic voltammetry, where E1/2=(Epc+Epi)/2. Curve 1 was obtained before the addition of Fc and curve 2 was obtained after the addition of Fc. It is essential that the half-wave potential of the test system is not affected by the addition of the reference system. 

This is an appropriate point at which to comment on the common practice of evaluating the formal potential from voltammetric measurements. When a reversible response is obtained in voltammetry, what is actually measured is the reversible half-wave potential, E1/2, which (except for hydrodynamic voltammetry) is related to the formal potential by a term involving the diffusion coefficients of the oxidized and reduced forms of the half-reaction, D0 and DR, respectively. 

From Eqs. (3.287) and (3.290), it can be inferred that, when total equilibrium conditions are considered for the precedent and subsequent chemical reactions to the charge transfer reaction, the shape and height of the voltammetric curve are not affected although the half-wave potential shifts toward more cathodic or anodic... 

From Fig. 6.14, it can be deduced that for A Ef = —142.4 mV, the two mechanisms show distinctly different voltammograms for small values of %cw(= (k 1 + ki) ja), whereas the responses become more similar as the chemical kinetics is faster. Thus, for// v >100 the voltammetric signal of the ECE mechanism is equivalent to that of an EE mechanism where the half-wave potentials correspond to those of the EC (first electron transfer) and CE (second electron transfer) mechanisms under fully labile conditions (Eqs. (3.201c) and (3.221b) for <5, > 0, respectively). 

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