Ferrocene-ferrocinium

Ferrocene/ferrocinium (reference electrode) activity coefficient (also in use y)... 

Strehlow standard ferrocene-ferrocinium+ electrode and obtained a value of - 0.228 V, i.e., a positive pH shift of 3.85. Theoretically the pH range of pyridine is unlimited on the basic side, but in practice it is limited somewhere in that area as a consequence of the protogenic effect of contaminating traces of water. 

This means, in agreement with Fig. 4.17, that a negative Eh+/h2 value yields an additional increase in the Eredo range. Finally, solvation in itself contributes to extending the E OI range, as a more solvated ion is less accessible to electrolysis however, it is doubtful whether separate consideration of this effect makes any sense provided that E nodic and E +/h2 have been determined absolutely, i.e., versus the ferrocene-ferrocinium+ electrode. 

V, in acetonitrile with respect to the ferrocene/ferrocinium couple. " " Peroxodisulfate oxidation of [Fe(ttcn)2] does not give the Fe + complex the major product is [Fe(ttcn)(ttcn-1-oxide)]. Oxidation by lead dioxide does however yield [Fe(ttcn)2]. " The Fe—S bond... 

A stable configuration can only be achieved when nonaqueous solvents are involved. For example, a photoelectro-chemical cell can be operated using the ferrocene/ferrocinium couple at an n-type Si electrode in a pure methanol solvent. This system is of special interest because the rate-determining steps can be analyzed in detail. Using Si as an electrode in CH3OH or CH3CN requires that the solvent is completely water free in order... 

Fig. 10.9 SNIFTIR spectra obtained in the 1700-700 regionfora2mM [Cr(DMSO)g] (0104)3 solution in dimethylacetamide at an Au electrode. The reference potential was 0.16 V against an internal ferrocene-ferrocinium couple spectra were recorded at more negative potentials at intervals of 100 mV (From reference 17, with permission.)...

Fig. 6.2-21 (1) Electrochemical window of [BMP][Tf2N] on HOPC with the ferrocene/ferrocinium (Fc/Fc+) couple (2) cyclic voltammogram of SiCU saturated in the same ionic liquid. Scan rate each 10 mV s (from Ref [71]).

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. 

Figure 16 Self-assembled monolayer of S—(CH2) CH3 on gold, w=13, with a small fraction of the sites substituted by S—(CH2) Fc (Fc = ferrocene/ferrocinium).

Indicates that the actual AG° values are more negative than those given, converted from values versus ferrocene/ferrocinium [135]. 

Figure 7.47. SNIFTIRS spectra obtained in 1700-700-cm region for 2mM [Cr(DMCO)e] (0104)3 solution in dimethyiacetamide at Au eiectrode. Reference potentiai was 0.16 V against ferrocene/ferrocinium reference with sampie potentiais at 100-mV intervais in negative direction as indicated. Reprinted, by permission, from W. R. Fawcett, A. A. Kioss, J. J. Caivente, and N. Marinkovic, Electrochim. Acta 44, 881 (1998), p. 882, Fig. 1. Copyright 1998 Eisevier Science Ltd.

The novelty of these measurements is the spectral and kinetic resolution of quinol formation. Although many species can contribute to the observed trace in the UV region, they can be spectrally and kinetically separated from that of the quinol signal (Fig.1). These include the light-induced P Qa /PQa spectrum ferrocene/ferrocinium, the exogeneous donor to P" " used, which has no ox-red difference bands... 

In practice the main requirement of a reference electrode is that it has a stable potential and that it is not substantially polarised during the experiment. Hence it is common to use the highly convenient aqueous calomel electrode in many experiments in all solvents. Even so, a very wide range of reference electrodes have been used in non-aqueous solvents. Where there is any doubt about the potential of the reference electrode, it is recommended to check the potential of a standard couple, e.g. ferrocene/ferrocinium ion, by cyclic voltammetry. This is also the easiest way to compare potential scales in different solvents it is assumed that the potential of this couple, where both halves of the couple are poorly solvated, is independent of solvent [2j. 

The quantitative analysis requires knowledge of the rate(s) of the heterogeneous electrode reaction(s), reagent diffusion coefficients and the transfer coefficient. If the electrode reaction is reversible, most of these parameters can be determined from the CV experiments. The formal reduction potential, differs from the standard potential, °, because the latter is obtained by extrapolation to infinite dilution, while the former refers to the actual experimental conditions of ionic strength and temperature. For a fast, reversible process, E° => j,2 10 mV if the diffusion coefficients of the oxidized and reduced forms are within a factor of two. Potentials are reported relative to some standard electrode, such as ferrocene/ferrocinium ion, saturated calomel, SCE, or Ag/AgCl, and this must be taken into account in comparing results from different sources. 

A solution of the problem is a reference electrode whose constituents are poorly soluble in any of the media. Such a couple is ferrocene-ferrocinium whose potential does not depend on the solvent. Both forms of this couple have a large volume consequently the free energies of solvation are very small. The application of such reference systems makes comparisons possible in which the potentials are referred to a classical SCE in water. 

Indeed, such correlations are observed and values of k between 10 and 10 cm s arc common. The fastest reactions are those where the structures of O and R are similar (e.g. anthracene/anion radical, ferrocene/ferrocinium ion, Fc(CN)S /Fe(CN) ") and these reactions can be considered as akin to outer sphere electron transfer processes in homogeneous solution. Typical slow couples are Cr(ni)/Cr(n) where Cr(fti) species are known to be substitution inert and Ce(iv)/Ce(m) in sulphuric acid solutions where the reduction of Cc(iv) requires loss of sulphate from the inner coordination sphere. Also substituents in a hydrocarbon which lead to a need for twisting within the molecule to form a planar anion radical cause a marked decrease in the standard rate constant. 

Tetrabutylammonium tetrafluoroborate (0.1 M) as the supporting electrolyte, scan rate 100 mV s , 20 °C potential in mV, measured with a Pt disk working electrode in degassed dichloromethane solution. The values are referenced to the ferrocene/ferrocinium couple This value is attributable to the ether group as (R)-2,2 -dimethoxy-l,l -binaphthylene gave a value of 1,030 mV... 

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