Ferrocenes potential standard system

A suitable extrathermodynamic approach is based on structural considerations. The oldest assumption of this type was based on the properties of the rubidium(I) ion, which has a large radius but low deformability. V. A. Pleskov assumed that its solvation energy is the same in all solvents, so that the Galvani potential difference for the rubidium electrode (cf. Eq. 3.1.21) is a constant independent of the solvent. A further assumption was the independence of the standard Galvani potential of the ferricinium-ferrocene redox system (H. Strehlow) or the bis-diphenyl chromium(II)-bis-diphenyl chromium(I) redox system (A. Rusina and G. Gritzner) of the medium. 

A potential difference across the NB/water interface (Ao 0) is determined by the concentrations of TBA+ dissolved in both phases, and calculated to be —131 mV on the basis of Eq. 4. A standard ion transfer potential of ferrocene has been reported to be -75 mV [96]. Therefore, FeCp-EtOH+ is likely to exit quickly to the water phase across the droplet/water interface at the present Ao . Diffusion of FeCp-EtOH + in the NB and water phases is thus concluded to be the rate-determining step of MT from GE to CE across the droplet/water interface. If the Ao value is higher than the ion transfer potential of FeCp-EtOH+ in the NB/water system, a slow MT process, such as migration of the compound across the interface, will be detected. A combination of laser trapping with the microelectrode array methods is highly useful for studying directly MT processes between a droplet and the surrounding solution phase. 

Chronoamperometric curves have been used as a standard tool to obtain values of the rate constants of surface-bound molecules and they prove as very useful for validating the Marcus-Hush s formalist, and, indeed, the experimental application of the MH theory to electrode processes has been mainly carried out with surface-bound redox systems. Thus, Chidsey studied the oxidation of ferrocene groups connected to a gold electrode by means of a long alkylthiol chain by using Single Potential Pulse Chronoamperometry (see examples of the experimental responses in Fig. 6.21) [43]. 

The ferrocene/ferricinium ion electrode. For nonaqueous electrochemistry, IUPAC recommends47 the use of the ferrocene/ferricinium ion [Fen(Cp)2/ Fen(Cp)2 , HCp = cyclopentadiene] couple as an internal standard. The couple has been chosen because its potential is largely independent of the solvent (E° = +0.40 V vs. NHE in water 3 and +0.69, +0.72, +0.76, and +0.68 V vs. NHE in MeCN, DMF, py, and Me2SO, respectively).48 The ferricinium ion is unstable in some organic solvents because of decomposition.49,50 Recently the use of bis(pentamethylcyclopentadienyl) iron(II) has been proposed to avoid the problem.51 The Fen(Cp)2/Fem(Cp)2 couple cannot be used as an internal standard for some systems due to overlapping waves.52 In these cases other compounds such as tris-(l,10-phenanthroline)iron(II),4 cobalto-... 

In non-aqueous electrolytes, the different properties of the solvated metal ions lead to different equilibrium and standard potentials. For comparing standard potentials, electrode reactions should be defined as reference systems with similar values in different solvents. Koepp, Wendt, and Strehlow suggested ferrocene/ferrocinium and cobaltocene/ cobaltocenium redox systems. The redox systems are bis-pentadienyl complexes of Fe +/Fe + and Co /Co , respectively. Gritzner and Kuta recommended ferrocene/ferrocinium and bis(biphenyl)Cr(l)/bis(biphenyl)Cr(0). Salt bridges with conventional cells should be avoided. Similar to aqueous electrolytes a reference to the physical potential scale is possible. Similar considerations hold for ionic melts and molten and solid electrolytes. 

Cyclic voltammograms were obtained on a Bioanalytical Systems CV-27 instrument samples were dissolved in dry THE containing 0.1 M [Et4N][PF0] as supporting electrolyte. The voltammograms were obtained at a scan rate of 100 mV/sec, and Ei/2 values were determined relative to ferrocene/ferrocenium as an internal standard. The electrode array consisted of a saturated calomel reference electrode and platinum disk (working) and wire (auxilliary) electrodes. Potentials were uncorrected for junction effects. 

A well-defined redox couple can be used to calibrate an RE or as an internal standard in electrochemical experiments. The reference redox couple must be stable for the duration of the measurement, and must exhibit a repeatable potential in the system used. A good reference redox couple (63) for nonaqueous, and some carefully controlled aqueous systems, is the ferrocene/ferrocenium (FcIFc ) couple at 0.5-10.0 mM concentration. Standard reduction potentials, E°, for various solvents (64) are listed in Table 4.10. Other couples can be found in References (64-66). 

The most common reference electrode systems used in aqueous solutions are Ag/AgCl and the calomel electrode. If aqueous-based references are used in nonaqueous solution, however, large liquid junction is produced and often more serious, aqueous contamination of the nonaqueous cell occurs. Thus this combination is not recommended. The use of an Ag/Ag non-aqueous-based reference is suggested for nonaqueous electrochemistry. To avoid large junction potentials, the RE solvent should be as close in nature as possible to the cell solvent system. Often potentials are calibrated with a standard, such as ferrocene or cobaltocene. Suggested standards are listed in Table 2-2, along with reduction potentials and other properties. Construction of an Ag/Ag reference for nonaqueous use is shown in Figure 2-6. Reference electrodes can drift with time and must be carefully maintained. 

FIGURE 1.22 Determination of the standard redox potential of decamethylferrocene in 1,2-DCE versus ferrocene, and the redox scale for a biphasic system. 

The determination of electrochemical redox potentials requires standard reference compounds whose potential, ideally, does not vary. The redox couple [FeCp " (q -C6Me6)2] has a redox potential E° = -1.85 V vs. SCE that is independent on the nature of the solvent and electrolytes, because solvent molecules and electrolyte ions cannot reach the iron center due to the steric bulk of the ring methyl groups. This provides an excellent reference system, much better than ferrocene, the lUPAC reference, because these interactions can occur between the two ferrocene rings provoking variations of the potential from one solvent to the... 

Figure 16.24 presents the ISM calculations for the system of Chidsey together with the results for the Butler-Vohner model the monolayer is described as before with a refractive index = 1.6 and the absolute potential of ferrocene is Og = 5.17 eV. The calculations incorporated the correction of the images forces for tunnelling of the electrons. All the models interpret the rates well at the standard potential. 

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