Formal potential table

Another problem is that the Nernst equation is a function of activities, not concentrations. As a result, cell potentials may show significant matrix effects. This problem is compounded when the analyte participates in additional equilibria. For example, the standard-state potential for the Fe "/Fe " redox couple is +0.767 V in 1 M 1TC104, H-0.70 V in 1 M ITCl, and -H0.53 in 10 M ITCl. The shift toward more negative potentials with an increasing concentration of ITCl is due to chloride s ability to form stronger complexes with Fe " than with Fe ". This problem can be minimized by replacing the standard-state potential with a matrix-dependent formal potential. Most tables of standard-state potentials also include a list of selected formal potentials (see Appendix 3D). 

Table 1.5. Formal potentials of organic redox systems.

The data of Figures 1-4 have been used to calculate formal potentials for Reactions 7 and 8 for copper, silver, and gold these are summarized in Table I. Analogous redox parameters for aqueous solutions also are included (14). ... 

Electrochemistry. The redox processes for porphyrazines 21, 25, 28, 29, the heteroleptic Zr (pz/porphyrin) 30 and 31 have been measured by cyclic voltammetry and the formal potentials are given in Table VII. The potentials are compared to the available data for the analogous porphyrin and pc complexes. In general, the electrochemical behavior of the pz sandwiches more closely mirror that observed for the phthalocyanines than the porphyrins. In particular, all of the porphyrazines have at least one ring-based oxidation, attributable to the formation of the bis Jt-radical cation for Lu(III) sandwiches and the formation of the 7T-radical cation for the Zr(IV) and Ce(IV) sandwiches. Additionally, all of the porphyrazines exhibit at least one ring-based reduction. 

Table 6 Summary of formal potentials for selected ruthenium and osmium oxo complexes.

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... 

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. 

The standard potential for a redox reaction is defined for a galvanic cell in which all activities are unity. The formal potential is the reduction potential that applies under a specified set of conditions (including pH, ionic strength, and concentration of complexing agents). Biochemists call the formal potential at pH 7 E° (read "E zero prime"). Table 14-2 lists E° values for various biological redox couples. 

The standard potentials for some redox couples in aqueous solution appear in Table 1.2. More information about formal potentials of a great number of electrochemical systems can be found in [14, 15]. 

As can be observed in this figure (see also Table 3.1), for A Ef < —142.4 mV (K = 1/28), the normal pulse voltagram presents two well-resolved waves whose half-wave potentials exactly match with the corresponding formal potentials Ef and Ef (f°r (—142.4 < A Ef < —71.2) mV, two non-well-resolved waves are observed, whose half-wave potentials are not coincident with the corresponding individual formal potentials, Fig. 3.16a. For AE > 71.2... 

Table 4.1 Heterogeneous rate constant (k°), electron transfer coefficient (a), and formal potential ) corresponding to the best fit of theoretical working curves (Eq. 4.120) to the RPV experimental results [48]... 

Table 6.1 Difference between the formal potentials and formal potential of the first electrochemical step obtained from the best fit of staircase cyclic voltammograms at a gold disc macroelectrode (ro = 0.9mm) of 0.5 mM AQ/H20 solutions in different supporting electrolytes (see Fig. 6.3). T=298K[5]...

Table 7.2 Values of the standard rate constant (k°), the transfer coefficient (a), the reorganization energy (2), and the formal potential (Ef, vs. Ag) corresponding to the theoretical curves shown in Fig. 7.21. Taken from [30]...

Fig. 7.21 (continued) reverse scans. EE SS = —2.6V (vs. Ag), sw = 25mV, A s = 10mV. The values of the kinetic parameters and formal potential extracted in each case are given in Table 7.2. Test solution 2 mM 2-methyl-2-nitropropane, 0.1 M tetra-n-butylammonium perchlorate in acetonitrile. Reproduced from [30] with permission... 

An excellent agreement is obtained from the comparison between experimental and theoretical data. The results obtained for the formal potentials are shown in Table 7.4. From these values it can be concluded that an aprotic electrolyte leads to a shift in the formal potential of the different steps which is more noticeable in process III (shift of 240 mV in AE ). This may be due to the increasing formal charge in the immobilized molecules, which cannot be compensated for the addition of protons [87]. 

From a practical standpoint it is often useful to have the observed potential in the medium of measurement for the condition of equal concentrations of the oxidized and reduced species of a half reaction. Such potentials are known as formal potentials, E°, rather than standard potentials, and are not purely thermodynamic quantities. The term formal potential comes from the tradition of having the supporting electrolyte at a one formal concentration. However, other stated solution conditions are also included in many listings. Thus the indicated potential is what one would expect at the half-equivalence point under actual titration conditions. In other words, activity corrections have not been made. Table 2.3 summarizes a number of formal potentials for commonly encountered half-reactions. 

TABLE 9.3 Standard Reduction Potentials for Dioxygen Species in Water [02l 1 atm (1.0 mM>] (Formal Potentials for 02 at Unit Activity)... 

Table 9.3 summarizes the redox potentials for the reduction of various dioxygen species in aqueous media at pH 0, 7, and 14. For those couples that involve dioxygen itself, formal potentials are given in parentheses for 02 at unit activity ( 105 atm [OJ 1 mM at 1 atm partial pressure). 

The redox mediator 2,6-dichlorophenol indophenol, can mediate electron transfer from and to the redox enzyme, cytochrome c. The mediator was switched between the oxidized and reduced forms by the application of a potential using optically transparent electrodes in a thin-layer cell. From the absorbances of the oxidized and reduced form of the enzyme, the ratio of their concentrations at various potentials was obtained. Calculate the formal potential E° of the enzyme from the data given in Table E.l. Confirm that the enzyme redox process involves one electron transfer. (Contractor)... 

Potentials of berkelium redox couples are summarized in Table V. Replicate values for the Bk(IV)-Bk(III) couple are in reasonable agreement with one another. The effect of anions that strongly complex Bk(IV) is clearly reflected in the values of the formal potential for the Bk(IV)-Bk(III) couple and can be seen in the Nemst equation plots for the couple in various media given in Fig. 9 (227). Values of 1.36 (220, 223) and 1.12 V (227) have been reported for the couple in sulfuric and phosphoric acid solutions, respectively. Carbonate ions, apparently... 

Figure 9.1 shows the metals that can be electrodeposited as pure metals or alloys in chloroaluminate ionic liquids. In the discussion below, the potentials are given in relation to the AI/A1(III) electrode. This electrode is composed of A1 immersed in an acidic ionic liquid of 66.7 or 60.0 mol% AICI3. The formal potentials of some redox couples in ionic liquids are illustrated in Figure 9.2 and summarized in Tables 9.1 and 9.2. 

TABLE 9.1 Formal potentials of certain redox couples in acidic chloroaluminate ionic liquids... 

Table I Plutonium Formal Potentials (in Volts) at 25° (3 ) In 1 M HCIO ...

Fig. 2. The change in the formal potential of the phenazine/phenazine radical anion system (against the Foe/Foe electrode) with the acceptor number of the solvent [91], Abbreviations are defined in Table 1.

---