Formal potential determination

The electrode reaction of tetraheme cytochrome C3 adsorbed at the silver electrode surface was monitored by SERRS spectroscopy [104, 105] and compared with the results obtained by voltammetric techniques. The formal potential determined on the basis of the SERS measurement is more positive compared to the potential determined by the voltammetry, but it is in good agreement with the macroscopic formal potential of the heme-1. This is because cytochrome C3 is adsorbed on the silver electrode particularly via lysine residues surrounding heme-1, which is in fact responsible for the SERRS spectrum of this protein [105]. 

The dependences of the conventional formal potentials determined here enable us to calculate the conventional equilibrium constants for the reaction ... 

Although strictly speaking this type of reference electrode has not been used in IL electrochemistry, sufficient data does exist for this type of reference electrode to be constructed. Compton et al. have shown that H2 can be oxidised in a number of ILs at a platinum electrode (Table 7.5) [33-35]. Additionally for selected ILs the reduction of can also occur [35]. The formal potential determined for the H" H2 redox couple in selected ILs is presented in Table 7.5. 

Determining the Standard-State Potential To extract the standard-state potential, or formal potential, for reaction 11.34 from a voltammogram, it is necessary to rewrite the Nernst equation... 

The reduction potentials for the actinide elements ate shown in Figure 5 (12—14,17,20). These ate formal potentials, defined as the measured potentials corrected to unit concentration of the substances entering into the reactions they ate based on the hydrogen-ion-hydrogen couple taken as zero volts no corrections ate made for activity coefficients. The measured potentials were estabhshed by cell, equihbrium, and heat of reaction determinations. The potentials for acid solution were generally measured in 1 Af perchloric acid and for alkaline solution in 1 Af sodium hydroxide. Estimated values ate given in parentheses. 

Standard potentials are determined with full consideration of activity effects, and are really limiting values. They are rarely, if ever, observed directly in a potentiometric measurement. In practice, measured potentials determined under defined concentration conditions (formal potentials) are very useful for predicting the possibilities of redox processes. Further details are given in Section 10.90. 

Electrodes of the second type can formally be regarded as a special case of electrodes of the first type where the standard state (when E = °) corresponds not to flAg+ = 1 but to a value of == 10 mol/L, which is established in a KCl solution of unit activity. In this case, the concentration of the potential-determining cation can be varied by varying the concentration of an anion, which might be called the controlling ion. The oxides and hydroxides of most metals (other than the alkali metals) are poorly soluble in alkaline solutions hence, almost all metal electrodes in alkaline solutions are electrodes of the second type. 

Standard redox potentials can be determined approximately from the titration curves for suitably selected pairs of redox systems. However, these curves always yield only the difference between the standard potentials and a term containing the activity coefficients, i.e. the formal potential. The large values of the terms containing the activity coefficients lead to a considerable difference between the formal potential and the standard potential (of the order of tens of millivolts). 

This simplified model of electronic polarization may be used within a KS like formalism to determine the electron density p(r). For instance, if we place the model within the Hartree-Fock-Slater X — a approximation [33], the exchange-correlation potential reduces to ... 

The formal potential is the quantity determined from the analysis of a volta-mmogram, but the true thermodynamic quantity (the standard potential) can be derived by obtaining 0/(R/R ) for different bulk concentrations (c) and extrapolating to c = 0 (unit activity coefficients). The procedure is, however, seldom adopted in practice, (R/R-) is identified with the standard potential. The lower the concentration of the electroactive species, the better the assumption. 

C. P. Andrieux, P. Hapiot, J. Pinson, J.-M. Saveant. Determination of Formal Potentials of Chemically Unstable Redox Couples by Second-Harmonic Alternating Current Voltammetry and Cyclic Voltammetry. Application to the Oxidation of Thiophenoxide Ions. J.Am. Chem. Soc. 1993,115, 7783-7788. 

Formal potential of redox couple, determined at ionic strength noted in parentheses. Data from ref. 9 unless otherwise indicated. 

In addition, the split peaks can be used for estimation of electron-transfer coefficient as well as for precise determination of the formal potential of the surface electrode reaction. The potential separation between split peaks is insensitive to the electron-transfer coefficient. However, the relative ratio of the heights of the split peaks depends on the electron-transfer coefficient according to the following function ... 

With respect to the formal potential of the surface electrode reaction, Figs. 2.45c and 2.46 show that the split peaks are symmetrically located around the formal potential, which enables precise determination of this important thermodynamic parameter. 

The application of three-phase electrodes for determining the energy of ion transfer is based on precise measurements of the formal potential of reaction (4.1). The latter is defined as [4,5] ... 

Kimura K, Nakajima S, Niki K, Inokuchi H (1985) Determination of formal potentials of multihemoprotein, cytochrome C3 by H nuclear magnetic resonance. Bull Chem Soc Jpn 58 1010-1012... 

Cyclic voltammetry has perhaps become the most popular electroanalytical, electrochemical technique [23, 27], and many reports have appeared in which E° values were determined in this way. However, reliable formal potentials can be determined only for electrochemically reversible systems [28]. For any reversible redox system - provided that the electrode applied is perfectly inert, that is, there are... 

Dropping indium and thallium amalgam electrodes [41] were used to determine kinetic parameters of Zn(II) reduction as a function of the amalgam composition. The formal potentials were shifted to more negative values with increasing thallium and indium amalgam concentrations. 

The electrodeposition of zinc on polycrystalline Au, Pt, and tungsten electrodes was preceded by underpotential deposition (UPD) [172]. The formal potential of Zn(II)/Zn(0) couple and diffusion coefficient of Zn(II) were also determined [172]. 

The electrode process of the Cd(II)/ Cd(Hg) system was investigated in water-DM SO [61] and hexamethylphosph-ortriamide (HMPA) solutions [62]. The formal potentials, charge-transfer rate constant, and diffusion coefficients were determined. In the presence of adsorbed HMPA molecules, the rate constant was found to he dependent only on the surface phase composition. 

Baranski and Lu [209] have carried out, applying microelectrodes, voltammetric studies on ammonium amalgam in propylene carbonate solutions at room temperatures. The sweep rates up to 80 V s were appropriate for the analysis of the formation kinetics of this compound. Experimental and numerical simulation results have shown that ammonium amalgam was formed via fast charge-transfer process and its first-order decomposition was characterized by the rate constant of about 0.6 s . Diffusion coefficient of NH4 radical in mercury was estimated to be about 1.8 X 10 cm s k The formal potential of NH4+ (aq)/NH4(Hg) couple was determined as—1.723 V (SHE). 

The general electrochemical behavior of uranium in aqueous solutions is dominated by the reduction of the hexava-lent uranyl moiety, 1102. As shown in Table 1, the potential for the UO2 /UO2 couple is 0.089 0.002 V versus SHE, as determined from formal potential data in C104 solutions (0.5-3.0 M) [49, 50aj. The electrochemical reduction of uranyl compounds has been a thoroughly studied... 

Neptunium has been characterized from the +3 to +7 oxidation states in aqueous solution. The standard potentials for various Np ions have been determined from measured formal potentials of the various redox couples. These data have been thoroughly reviewed by Martinet [94] and Fahey [95]. Recently the standard potentials for the redox couples Np02 /Np02, Np +/Np +, and Np02 /Np" in acidic aqueous solution have been reevaluated with more detailed consideration of activity coefficients [49,50]. The standard potential accepted here for the Np02 /Np02 couple is 1.161 0.011 V as determined from... 

The vast majority of electrochemical data on americium ions has heen obtained in aqueous solutions. Americium can exist in aqueous solutions in the oxidation states III, IV, V, and VI. The divalent state is difficult to attain in aqueous solutions because of the proximity of the standard potential of the Am(III)/Am(II) couple to the solvent/supporting electrolyte breakdown potential. Previous reviews have presented the formal and standard potentials for the various americium couples and these reviews should be consulted by the interested reader for more detailed discussion [133, 134]. Table 3 contains a summary of selected formal potentials Ef from these reviews in 1 M HCIO4 for convenience. AU values are calculated from various measurement techniques except for the Am(VI)/Am(V) couple (Am02 /Am02" "), which was determined directly. 

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