Ferrocene/ferrocenium, reference redox couple

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). 

Refer to footnote a Table 23. h pa and pc represent the anodic and cathodic current peak potentials of the ferrocene/ferrocenium redox couple of the free ligand. Cathodic shifts in the ferrocene redox couples produced by the presence of anion (5 equiv) added as the tetrabutyl-ammonium salts. As the concentration of the anion increased, the ferrocene/ferrocenium redox couple began to exhibit the features of an EC mechanism. 

Both in acetonitrile and in other non-aqueous solvents, a major problem arises in terms of the manner in which the potential values are reported by various investigators. Koepp, Wendt, and Strehlow [6] noted that hydrogen ion is the poorest reference material on which to base nonaqueous potentials because of the extreme differences in its solvation in various solvents. On the basis of an investigation of the solvent dependence of 18 redox couples, these investigators concluded that ferrocene/ferrocenium ion (i.e. bis(cyclopentadienyl)iron(III/II), abbreviated as Fc+ /Fc°) and/or cobal-tocene/cobalticenium ion represented optimal potential reference materials for nonaqueous studies. On the basis of their minimal charge (+1, 0) and their symmetry (treated as though they were roughly spherical), the potentials of these two redox couples are presumed to be relatively independent of solvent properties. 

Some redox couples of organometallic complexes are used as potential references. In particular, the ferrocenium ion/ferrocene (Fc+/Fc) and bis(biphenyl)chromium(I)/ (0) (BCr+/BCr) couples have been recommended by IUPAC as the potential reference in each individual solvent (Section 6.1.3) [11]. Furthermore, these couples are often used as solvent-independent potential references for comparing the potentials in different solvents [21]. The oxidized and reduced forms of each couple have similar structures and large sizes. Moreover, the positive charge in the oxidized form is surrounded by bulky ligands. Thus, the potentials of these redox couples are expected to be fairly free of the effects of solvents and reactive impurities. However, these couples do have some problems. One problem is that in aqueous solutions Fc+ in water behaves somewhat differently to in other solvents [29] the solubility of BCr+BPhF is insufficient in aqueous solutions, although it increases somewhat at higher temperatures (>45°C) [22]. The other problem is that the potentials of these couples are influenced to some extent by solvent permittivity this was discussed in 8 of Chapter 2. The influence of solvent permittivity can be removed by... 

Figure 18.6 Comparison of the electrochemical windows of RMIm(HF)2 3F. W.E. GC disk C.E. Pt plate scanning rate 10mVs The potential is referred to the potential of ferrocene/ ferrocenium redox couple in each salt, (a) DMIm(HF)2.3F (b) EM m(HF)2 j= (c) PrM m(HF)2 (d) BMIm(HF)2.3F (e) PeMlm(HF)2 3F (f) HMIm(HF)2.3F. Vertical dotted lines denote cathode and anode limits.

Other reference electrodes have been proposed for use in the nonaqueous solvents that are widely used in coordination chemistry. Their main advantage is that they allow one to work with a single solvent. Among these electrodes, the Ag+/Ag electrode is reversible in many solvents.4 Ag+ ions are introduced as salts, such as AgCl or AgBF4. However, the inner solution has to be refreshed due to the reactivity of Ag+. Another class consists of redox electrodes in which the two components are in solution, such as ferrocenium ion/ferrocene Fc+/Fc.5 Since the potential is dependent on the concentration ratio of the redox couple, this ratio must be kept constant. An attractive solution to prevent the use of a junction lies in the preparation of a functionalized-polymer coated electrode such as poly(vinylferrocene).6 The polymer is deposited by electrooxidation in its oxidized form, polyFc+, and then partially reduced to yield poly Fc+/Fc. Their use is limited by their relative stability in the different solvents. 

The electrochemistry of a square-planar gold(III) complex with 2-(diphenylphosphino) benzenethiolate (21) was reported by Dilworth and coworkers35. Cyclic voltammetry experiments on [Au(21)2]BPh4 indicate a reversible redox couple at —0.862 V (vs the Fc/Fc+ reference couple) in 0.2 M [Bu4N]BF4/MeCN solution. Peak-to-peak separation of the redox waves was 84.2 mV and convolution methods were used to establish that the redox couple was reversible and involved the same number of electrons as the ferrocene/ferrocenium couple under identical conditions. The reductive scan was assigned... 

The oxetane-derivatized hole conductors span a broad range of redox potentials between 0.0 and 0.5 V vs. the ferrocene/ferrocenium redox couple, which is a standard reference in organic electrochemistry. Thus, this class of materials is ideally suited to bridge the gap to low-lying HOMO levels of an emitter polymer. This becomes particularly important for blue-emitting polymers such as polyfluorenes. 

The ferrocene/ferrocenium couple is still the lUPAC recommended reference for reporting redox potentials in nonaque-ous solvents [37]. Recently, Connelly and Geiger strongly urged workers to report... 

Choice of reference electrodes is one of the most important points in electrochemical measurements in ILs. The reference electrodes are required to show stable electrode potentials, which are usually determined by an equilibrium between reversible redox couples. The redox reaction between silver and silver cation, Ag/Ag(I), is often used as the redox couple for reference electrode in conventional nonaqueous electrolytes. The reference electrode based on Ag/Ag(l) has been also used in various ILs. However, the potentials of Ag/Ag(l) reference electrodes are different in different ILs since the Gibbs energy for formation of Ag(I) depends on the ions composing the ILs. Therefore, it is necessary to calibrate the potentials of reference electrodes against a conunon standard redox potential. A redox couple of ferrocenium (Fc" ) and ferrocene (Fc) is often used for this purpose although its redox potential is considered slightly dependent on BLs. Platinum or silver electrodes immersed in ILs are sometimes used as quasi-reference electrodes. The potentials of these quasi-reference electrodes may seem to be stable in the ILs without any redox species. However, their potentials are unstable and unreliable since they are not determined by any redox equilibrium. Thus, use of quasireference electrodes should be avoided even when the potentials are calibrated by Fc /Fc couple. 

Cyclic voltammetry (CV) is often used to determine the electrochemical properties of D-A copolymers. Usually, the CV setup consists of a Ag/AgNOs reference electrode, a platinum wire counterelectrode and a glassy carbon electrode with a drop-cast polymer film. First, the CV curve of ferrocene/ ferrocenium redox couple (Fc/Fc ) should be measured to calibrate the reference electrode. The polymer s HOMO energy level in electron volts is calculated from the onset of the oxidation peak ( ox) according to eqn (15.4). If the reduction peak of polymer is not clearly visible, the LUMO level can be approximated from the HOMO measured by CV and the bandgap measured by UV-vis. 

The difference of 10 mV is significant since the ferrocene/ferrocenium redox couple is often used as a redox marker in non-aqueous electrochemistry (acting as an internal reference electrode), including that using room temperature ionic hquids as solvents. The correction for the difference in diffusion coefficients is necessary to give a valid reference scale. 

There are two aspects to reference redox systems. One point is the possibility of compiling electrode potentials in a variety of solvents and solvent mixtures, which are not affected by unknown liquid junction potentials. Unfortunately very frequently aqueous reference electrodes are employed in electrochemical studies in nonaqueous electrolytes. Such data, however, include an unknown, irreproducible phase boundary potential. Electrode potentials of a redox couple measured in the same electrolyte together with the reference redox system constitute reproducible, thermodynamic data. In order to stop the proliferation of—in the view of the respective authors— better and better reference redox systems, the lUPAC recommended that either ferrocenium ion/ferrocene or bw(biphenyl)chromium(l)/te(biphenyl)chromium(0) be used as a reference redox system [5]. 

The conversion to the aqueous standard hydrogen electrode as reference half-cell requires an extra-thermodynamic assumption, either the assumption of a solvent independent reference redox system or other assumptions employed in calculating single-ion transfer properties. Details about the procedure and data for univalent cationimetal systems were published [13]. The redox couple ferrocenium ion/ferrocene as reference electrode system is not very suited for such a conversion as the ferrocenium cation undergoes interactions with water and thus impairs the extra-thermodynamic assumption for aqueous solutions. This becomes apparent when... 

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