Ferrocenium/ferrocene reference electrode

FIGURE 1.24. Potential-dependent forward and backward rate constants of the ferrocene-ferrocenium couple attached to a gold electrode hy a long-chain alkane thiol assembled together with unsubstituted alkane thiols of similar length. Solid line use of Equations (1.37) to (1.39) with X, = 0.85 eV, ks — 1.25 s 1. Adapted from Figure 4A in reference 65, with permission from the American Association for the Advancement of Science. 

Cyclic voltammetry experiments were controlled using a Powerlab 4/20 interface and PAR model 362 scanning potentiostat with EChem software (v 1.5.2, ADlnstruments) and were carried out using a 1 mm diameter vitreous carbon working electrode, platinum counter electrode, and 2 mm silver wire reference electrode. The potential of the reference electrode was determined using the ferrocenium/ ferrocene (Fc+/Fc) couple, and all potentials are quoted relative to the SCE reference electrode. Against this reference, the Fc /Fc couple occrus at 0.38 V in acetonitrile and 0.53 V in THF [30, 31]. 

Figure 8.1 Cyclic voltammetry (a) and Osteryoung square wave voltammetry (b) of Cgo (acetonitrile/toluene + 0.1 M ( -Bu)4NPF6), using a glassy carbon electrode (GCE) working and ferrocene/ferrocenium (Fc/Fc +) couple as an internal reference. Reprinted with Permission from Ref. 4. Copyright 1992 American Chemical Society.

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

Pyrex tube that is closed on one end by a fine-porosity frit. The latter melt is preferred because changes in the composition of this melt lead to smaller changes in the reference electrode potential. The formal potential of the ferrocenium/ferrocene couple is located at 0.250 V vs. the A1(III)/A1 couple in the former melt [22]. 

Obtained in MeCN solution containing 0.1 mol dm-3 Bu°NBF4 as supporting electrolyte. Solutions were 1 x 1(T3 mol dm"3 in compound and potentials were determined with reference to an Ag/Ag+ electrode at 21 1°C, 50 mV s 1 scan rate. bEptk and represent the anodic and cathodic peak potentials. Cathodic shifts in the metallocene redox couples produced by the presence of anion (5 equiv) added as their tetrabutylammonium salts. As the concentration of the anion increased, the cathodic current peak potential of the ferrocene/ferrocenium redox couple began to exhibit the features of an EC mechanism. 

Figure C. V. of 5 (5x1 Cr3 M) and C60 (2.5x1 O 3 M) in o-dichlombenzene suporting electrolyte 0.1 M Bu4NC104 working electrode glassy carbon counter electrode Pt wire reference electrode Ag/O.OIN AgN03 in acetonitrile with 0.1 M "Bu4C104 ( E,n (ferrocene/ferrocenium) = 180V) scan rate 100 mVs-1.

The compound Re2( -H)4H4(PPhj)4 forms dark red crystals and bright red-orange powders it is slightly soluble in diethyl ether, THF, benzene, carbon disulfide, and dichloromethane. The compound is most easily characterized by H NMR spectroscopy (see above) or by cyclic voltammetry. The cyclic voltammogram in dichloromethane with 0.1 M tetrabutylam-monium hexafluorophosphate as supporting electrolyte consists of a reversible oxidation at — 0.20 V versus an Ag-AgCl reference electrode and a second irreversible oxidation at + 0.60 V. Note The ferrocenium-ferrocene... 

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. 

For ligand structures and nomenclature, see Figure 4. The potentials are reported vs. the ferrocene/ferrocenium couple and refer to the processes described by Equation (3). Conditions 0.10 M [N(Bu )]4(PF6)] in MeCN, 298 K, glassy carbon electrode, Ag/AgCl reference electrode, 200mVs f... 

In aprotic (as well as in protic and mixed) media, the two reference systems of choice are ferrocene/ferrocenium and bis (biphenil) chromium (I/O). The pen-tamethylcyclopentadienyl analog of the former was recently shown to yield higher performance [49, 50]. Among other typical electrodes, Ag/AgN03 should be mentioned. We can also mention special reference systems suitable for certain solvents, such as amalgam electrodes based... 

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. 

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... 

Determination of the energy levels is typically performed by electrochemistry, and cyclic voltammetry (CV) has been established as method of choice. The polymer is either dissolved in the supporting electrolyte or deposited on the working electrode. The measurements are usually performed with a three-electrode set-up that includes a working electrode, a reference electrode (for example Ag/AgCl), and a counter or auxiliary electrode. As electrolytes, acetonitrile (MeCN) or dichloromethane in the presence of conducting salts such as tetrabutyl-ammonium hexafluorophosphate (TBAPFe) are well-established. It is recommended to use ferrocene/ferrocenium (Fc/Fc" ) as external reference for each measurement to make electrochemical potentials comparable [31]. Detailed electrochemical characterization of P3HT films has been performed, for example, by Trznadel et al. [30] and Skompska et al. [32]. 

Table 3.1 Mid-point potentials of the ferrocene/ferrocenium redox process against different Ag-based reference electrodes...

Electrochemical measurements were performed as previously described . The working electrode was either a platinum flag (Sargent) (oxidation) or hanging mercury drop (HMDE) (reduction). The reference was a saturated calomel electrode (SCEK All potentials are reported vs. NHE by addition of 0.24 V to the SCE reduction values, while the internal ferrocene/ferrocenium couple (E = 0.40 7) was used as the standard in oxidations. 

FIGURE 5.25. Avidin-biotin construction of a monolayer glucose oxidase electrode with an attached ferrocenium cosubstrate and cyclic voltammetric response in a phosphate buffer (pH 8) at 25°C and a scan rate of 0.04 V/s. a attached ferrocene alone, h In the presence of 0.5 M glucose, c Variation of the inverse of the plateau current with the inverse of substrate concentration. Adapted from Figure 1 in reference 24, with permission from the American Chemical Society. 

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