Ferrocenium standard

Noviandri I, Brown KN, Fleming DS, Gulyas PT, Lay PA, Masters AF, Phillips L (1999) The decamethylferrocenium/decamethylferrocene redox couple a superior redox standard to the ferrocenium/ferrocene redox couple for studying solvent effects on the thermodynamics of electron transfer. J Phys Chem B 103 6713-6722... 

Figure 16.10 Diagram showing how values of the standard electrode potential of the couple R/R- referenced to several standard electrode potentials (SHE, Agl/Ag,l-, and ferrocenium/ferrocene) are derived. See equations 16.34 and 16.35.

The outer-sphere one-electron reduction of CO2 leads to the formation of the 02 radical anion. In dry dimethylformamide, the C02/ C02 couple has been experimentally determined to be —2.21 V vs. standard calomel electrode (SCE) or approximately —2.6 V vs. the ferrocene/ferrocenium couple [21,22]. From pulse radiolysis experiments, the reduction potential of CO2 is —1.90 V vs. the SHE in water (—2.14 V vs. SCE) [23]. Theoretical calculations have been used to calculate the contributions of various factors to the reduction potential of CO2. These include the electron affinity of CO2,... 

Figure 4 Dark currents in DSSCs with the standard I /I2 redox couple (solid line) and with a kinetically much faster redox couple, ferrocene/ferrocenium, FeCp2 + /0. A = tetrabutylammonium. The charge-transfer resistance, Rct (see Fig. 1), of the 1 2 couple is 106 times greater than that of the FeCp2+/0 couple, leading to what is sometimes mistaken as diode behavior in the dark for the cell containing the 1 2 couple. (Data from Ref. 49.)...

When dichloromethane solutions of Cp2 or FePc are impregnated on NaY or VPI-5, and heated at 423 and 523 K respectively, the application of the standard soxhlet extraction procedure removes all iron from the solids. The same is true for ferricenium-Y and ferrocenium-VPI-5. When in situ synthesis of FePc is made in both molecular sieve structures, the extraction procedure removes only part of the iron. Thus all residual iron present is associated with encaged FePc. 

Ohmic effects render Epc more negative, Epa more positive, AEp and 8Ep larger, and X smaller than the true values. Since experimental approaches to elimination of iRu errors are not foolproof (see Chap. 7), the presence of ohmic distortions should be tested by measurements on a Nernstian couple such as ferrocene/ferrocenium under conditions identical to those used to probe the test compound. In principle, errors in the measured CV parameters for a test compound can be eliminated by referencing its responses to those of the Nernstian standard. Note that this approach is accurate only if the current level of the standard, rather than its concentration, is equal to that of the test compound, since the diffusion coefficients of the two species may appreciably differ. 

Studies on the electrochemical behavior of ferrocene encapsulated in the hemi-carcerands 61 and 62, indicated that encapsulation induces substantial changes in the oxidation behavior of the ferrocene subunit [98]. In particular, encapsulated ferrocene exhibits a positive shift of the oxidation potential of c. 120 mV, probably because of the poor solvation of ferrocenium inside the apolar guest cavity. Lower apparent standard rate constants were found for the heterogeneous electron transfer reactions, compared to those found in the uncomplexed ferrocene under identical experimental conditions. This effect may be due to two main contributions (i) the increased effective molecular mass of the electroactive species and (ii) the increased distance of maximum approach of the redox active center to the electrode surface. 

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 cyclic voltammogram (CV) of (C5gN)2 showed three overlapping pairs of reversible one-electron reductions within the solvent window ( i = -997 mV, E2 = -1071 mV, 3 = -1424 mV, 4=-1485 mV, E = -1979 mV, g = -2089 mV ferrocene/ferrocenium couple, internal standard) [7]. A combination of linear sweep voltammetry and chronoamperometry estabUshed that all overlapping waves were two-electron reductions [ 120]. There was also an irreversible two-electron oxidation with a peak potential at -i- 886 mV, that is 0.2 V more negative (easier to oxidize) than Cgo [121]. The appearance of closely spaced pairs of waves in the CV was interpreted in terms of two (identical) weakly interacting electrophores, similar to the dianthrylalkanes [122]. After the third double wave, the process is irreversible, this was interpreted as irreversible cleavage of the dimer bond. 

Very recently, a significant influence of the redox state of the metallocene in ferrocene-containing cationic lipids was demonstrated. In the neutral ferrocene state, high levels of transfection with DNA coding for enhanced green fluorescent protein (EGFP) were observed similar to standard transfection reagents. In the cationic ferrocenium state. 

The basic concept of the most common form of electrochemical investigation of the redox chemistry of a coordination compound is that voltammetric data are initially collected and a mechanism for the half-cell reaction that occurs at the working electrode is postulated. A simple process, often used as a voltammetric reference potential standard, would be (Equation (1)) oxidation of ferrocene (Fc) to the ferrocenium cation (Fc ) in an organic solvent (acetonitrile, dichloromethane, etc.) containing 0.1 M of an electrolyte such as BU4NPF6 (added to lower the resistance) ... 

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. 

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). 

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