Potential halfwave

The measurement of the current for a redox process as a fiinction of an applied potential yields a voltaimnogram characteristic of the analyte of interest. The particular features, such as peak potentials, halfwave potentials, relative peak/wave height of a voltaimnogram give qualitative infonnation about the analyte electrochemistry within the sample being studied, whilst quantitative data can also be detennined. There is a wealth of voltaimnetric teclmiques, which are linked to the fonn of potential program and mode of current measurement adopted. Potential-step and potential-sweep... 

Determination of limiting current and halfwave potential in linear scan hydrodynamic voltammetry. 

Fig. 3. Evans-diagram for the cementation of Cu2+ and Pb2 with zinc amalgam of different zinc content. If the zinc concentration in the mercury employed for this special extraction technique is low, the anodic zinc-dissolution current density may be diffusion controlled and below the limiting cathodic current density for the copper reduction. The resulting mixed potential will lie near the halfwave potential for the reaction Cu2+ + 2e j Cu°(Hg) and only Cu2 ions are cemented into the mercury.

So, corresponding to id for a reversible electrode reaction, Jp is a linear function of concentration the greater sensitivity of the latter permits determinations down to 10 1M(instead of 10 6 Mfor id). Apart from this advantage, the second derivative curve, by means of the difference between its maximum and minimum as a function of concentration, offers an even better check on reaction reversibility32 than the straight-line plot of E (according to eqn. 3.49) against log(tcd - i)/i (see also p. 120), especially because Ip, as a property at the halfwave potential, is more sensitive to the occurrence of irreversibility (cf., pp. 124-127). 

A different view of the OMT process is that the molecule, M, is fully reduced, M , or oxidized, M+, during the tunneling process [25, 26, 92-95]. In this picture a fully relaxed ion is formed in the junction. The absorption of a phonon (the creation of a vibrational excitation) then induces the ion to decay back to the neutral molecule with emission (or absorption) of an electron - which then completes tunneling through the barrier. For simplicity, the reduction case will be discussed in detail however, the oxidation arguments are similar. A transition of the type M + e —> M is conventionally described as formation of an electron affinity level. The most commonly used measure of condensed-phase electron affinity is the halfwave reduction potential measured in non-aqueous solvents, Ey2. Often these values are tabulated relative to the saturated calomel electrode (SCE). In order to correlate OMTS data with electrochemical potentials, we need them referenced to an electron in the vacuum state. That is, we need the potential for the half reaction ... 

In dimethylformamide, halcinonide is reduced in two steps.63 The 21a-chloroketo group exhibits a halfwave reduction potential of -1.17 volts vs Hg. This is easily distinguished from the half-wave potential of -1.62 volts vs Hg of the A -S-keto group. Thus, halcinonide can be... 

The electrode potential obtained with linear-sweep polarography, for example, at a dropping-mercury electrode (DME), is different again and is called the halfwave potential, 1/2, which is also discussed in Chapter 6. 

The potential of the mercury-drop working electrode at the point where the current has reached exactly half its maximum value (that is, 1 /2 x /j) is called the halfwave potential ( 1/2). The latter occurs at a potential value that is characteristic of the analyte. A single polarogram is sufficient to both identify the analyte (from the value of 1/2 - comparing it with values in standard tables) and quantify its concentration (from la and a known standard). 

When a tetraalkylammonium cation is used as a counterion in solvents of high polarity, such as AN or DME, the alkyl groups of the cation hinder the mutual approach of species with different charges. Ion pairs with the potassium cation are stable. This follows from a comparison of the polarographic behavior of the three isomeric dinitrobenzenes in the same solvent (DMF) using tetraethylammonium or potassium perchlorate as the carrier electrolyte (Todres 1970). The halfwave potentials corresponding to the conversion of p- and m-dinitrobenzenes into anion-radicals are independent of whether tetraethylammonium or potassium counterions are employed. The anion-radical is formed from o-dinitrobenzene at a potential that is less negative by almost 100 mV when... 

Polarography of nitrogen containing aromatic compounds, values of the halfwave potential for the first reduction wave in dimethylformamide, tetraethyl-... 

In acidic aqueous buffers, secondary nitramines [116] show a six-electron po-larographic wave which is converted into two two-electron waves above pH 5 (Fig. 11.4). Nitrosamines [117,118] show a four-electron wave in acid solution and this becomes a single two-electron wave above pH 5 (Fig, 11,5). Above pH 5 the halfwave potentials are independent of pH. Primary nitramines are relatively acidic. They show a six-electron wave in acidic aqueous buffers, the height of which falls to zero around the pKg value for the nitramine because the anion is not reducible [119],... 

Radiopolarography measurements for the cathodic reduction of Bk(III) to Bk(0) at a dropping mercury electrode in 0.1 M LiCl at pH 2 give an amalgamation halfwave potential value of 1.63 V versus SHE and an estimated of 2.18 V [169]. Analysis of the electrochemical data leads the authors to conclude that the Bk(III)/Bk(0) electrode process is irreversible. 

Fig. 4.2 Relation between the halfwave potentials (V vs BCr+/BCr) of Rb+ and those of l<+ in various organic solvents [3], Solvents are ...

Fig. 8.4 Relation between the halfwave potentials of metal ions (V vs BCG/BCr) and the donor number of solvents O Na+ O TT Ba2 ...

They are used as voltammetric solvent alone or by mixing with C02. Usually they have wide potential windows. Olsen and Tallman [26] measured in supercritical chlorodifluoromethane the oxidation wave of ferrocene (Fc° -> Fc+) and the reduction wave of cobaltocenium ion (Cc+->Cc°). The difference between their halfwave potentials was 1.28 V, in good agreement with 1.31 V obtained in various non-aqueous solutions (Section 8.2.2). Recently, Abbott and Eardley [27] studied the reduction of C02 in a mixed SCF (C02/HFC-134a, %HFc-i34a = 0.3) using Pt and Pb electrodes. At 60 °C and 260 bar, the faradaic efficiencies (%) of (COOH)2,... 

Polarography. In principle, it should be possible to relate the halfwave potentials Exjl for oxidation of organometallic molecules, or of the critical oxidation potential Ec, to the energy of the highest occupied orbital, since oxidation (or reduction) in many of these systems involves simple removal or addition of an electron to this orbital. 

The halfwave reduction potential (in acetonitrile with tetraethylammo-nium perchlorate as supporting electrolyte) is more negative for 387 than for 385 (difference 70mV) and becomes more negative when going from Y = Se over S to O (in 388 X = N Ph, Y = O,S,Se in 389 X = O,S,Se). ... 

Figure 14-11 Reduction potential of ascorbic acid, showing its dependence on pH. (a) Graph of the function labeled formal potential in Equation 14-34. (b) Experimental polarographic halfwave reduction potential of ascorbic acid in a medium of ionic strength = 0.2 M. The half-wave potential, discussed in Chapter 17. is nearly the same as the formal potential. At high pH (>12), the half-wave potential does not level off to a slope of 0, as Equation 14-34 predicts. Instead, a hydrolysis reaction of ascorbic acid occurs and the chemistry is more complex than Reaction 14-32. [J. J. Ruiz, A. Aldaz. and M. Dominguez. Can. J. Chem. 1977,55.2799 ibid. 1978, 56. 1533.]...

Fig. 41. Potential dependence of log aa = log (Ao o + (JR) in the case of a preceding chemical reaction (CE). Numbers indicate values of KA. Vertical bars indicate the halfwave potential at the corresponding d.c. polarograms. Numerical data as in Fig. 40(a) except that kAt — 3 X 10s (see text). From ref. 175.

The Born equation, modified by the inclusion of empirical correction terms for the radii of the solvated ions, was used to calculate shifts in the polarographic halfwave potentials of the alkali metal ions in different solvents.35... 

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