Ferrocene/ferricinium

One final issue remains to be resolved Of the portion of the AEpi that is due to resistance, what part is caused by solution resistance and what part is caused by film resistance To explore this issue we examined the electrochemistry of a reversible redox couple (ferrocene/ferricinium) at a polished glassy carbon electrode in the electrolyte used for the TiS 2 electrochemistry. At a peak current density essentially identical to the peak current density for the thin film electrode in Fig. 27 (0.5 mV see ), this reversible redox couple showed a AEpi of 0.32 V (without application of positive feedback). Since this is a reversible couple (no contribution to the peak separation due to slow kinetics) and since there is no film on the electrode (no contribution to the peak separation due to film resistance), the largest portion of this 0.32 V is due to solution resistance. However, the reversible peak separation for a diffusional one-electron redox process is —0.06 V. This analysis indicates that we can anticipate a contribution of 0.32 V -0.06 V = 0.26 V from solution resistance in the 0.5 mV sec control TiS2 voltammogram in Fig. 27. 

They show that the electron transfer is reversible or quasi-reversible, the deviation from reversibility remaining quite modest for scan rates from 10 mV s to 100 mV s h Although the reversibility of the ferrocene-ferricinium in all the... 

Electroswitching of molecular aggregate formation and disruption takes place on interconversion of a lipid bearing a redox headgroup between a neutral and a charged state, as for instance in hydrophobic ferrocene-ferricinium derivatives [8.262, 8.263]. 

Like all the other acidity functions, W0(H) equals pH in dilute aqueous solution. In strong acids, this function should be a logarithmic measure of the proton activity as long as the normal potential of the redox system, ferrocene-ferricinium, is constant. This was, however, not the case in very strong acid solutions because ferrocene underwent protonation. Other electrochemical pH indicators have been proposed, such as quinine-hydroquinone or semiquinone-hydroquinone, the basicity of which can be modified by substitution on the aromatic ring. These electrochemical indicators have been used with success by Tremillon and co-workers48 for acidity measurements in anhydrous HF and HF containing superacids. 

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-... 

Of particular relevance to chemical sensor technology are the novel results of the electrochemical competition experiments. When an equimolar mixture ofNa+/K+orNa + /K + /Mg2 + cations is added to electrochemical solutions of (26), the ferrocene/ferricinium redox couple shifts anodically by an amount approximately the same as that induced by the K+ cation alone. This observation, together with the FABMS competition experimental findings, suggests that (26) is a first-generation prototype potassium-selective amperometric sensor, capable of detecting the K+ cation in the presence of Na+ and Mg2+ ions. 

A combined computational and electrochemical investigation revealed that arylated TEEs may also have potential as electrochemically driven molecular switches [27]. The electrochemical studies showed that the first reduction potentials of mono- and bis(4-nitrophenyl)-substituted TEEs occur at similar values (around —1.35 to —1.38 V versus the ferrocene/ ferricinium couple, Fc/Fc+) on steps involving one and two electrons, respectively. Moreover, the first reduction potential of nitrophenyl-substituted TEEs is hardly affected by the presence of other aryl substituents, such as the DMA donor groups, attached to the TEE frame. These findings suggest that the nitrophenyl redox centers behave as independent redox centers. However, ab initio calculations on singly-reduced trans-13 (Figure 4)... 

A similar approach has been developed independently by Pleskov 12) and Strehlow et al. 13). They assume that redox potentials for certain redox systems such as ferrocene-ferricinium ion are essentially independent of the nature of the solvent. Recent investigations have shown that the ferrocene-ferricinium ion assumption works fairly well in nonaqueous solvents, but it cannot be applied to aqueous solutions 14). 

The caesium assumption is popular with electrochemists and gives results comparable with the ferrocene-ferricinium assumption (ii) at least for solutions in water, formamide, methanol, and acetonitrile (Kolthoff, 1964 Strehlow, 1952 Koepp et al., 1960). However, we favour the tetraphenylarsonium-tetraphenylboride assumption of Grunwald et al. (1960) because it makes a more satisfactory allowance for size effects, such as making and breaking of solvent structure, while still preserving the attractive features, as to electrostatic solvation, possessed by the ferrocene-ferricinium couple and the caesium cation. 

The classical approach to a water soluble mediator is the redox pair hexacyanoferrate(II)/ hexacyanoferrate(III). A more convenient approach is the use of the redox pair ferrocene/ ferricinium immobilized to the electrode surface [14,15]. The ferrocene is oxidized at the electrode to regenerate ferricinium ion. The sensor is insensitive to oxygen and operates at relatively low potentials. This technological approach is the basis for the Exadifech glucose sensor being marketed by Baxter Travenol in the United States. 

The bis-alkenic ferrocenyl compound (61), synthesized by Beer et al., forms a 1 1 intramolecular sandwich complex with K+ and not unexpectedly a 1 2/ligand cation complex with Na". The electrochemical studies revealed some interesting features when an equimolecular mixture of Na /K or Na /K+/Mg cations is added to electrochemical solutions of (61) the anodic shift of the ferrocene/ferricinium redox couple is almost identical to the shift observed in the presence of the sole K cation, i.e. (61) is able to detect in the presence of Na and Mg <89CC183I>. 

Kolthoff and Thomas measured e.m.f. s of cells of the type (XVII) where M M+ represents a redox couple such as trisip-phenanthroline) Fe(III) rrw(o-phenanthroline)Fe(II) couple (abbreviated Fe(phen) Fe(phen)f ), or the ferrocene-ferricinium couple (abbreviated Fe(C5H5)2 Fe(C5H5)2). Measurements were made in the presence and absence of supporting electrolyte (Et4NC104). E values were obtained from... 

If medium effects on the ferrocene-ferricinium ion couple cancel, this is equivalent to... 

J. Courtet-Coupez, A. Laouenan and M. LeDemezet, Compt. Rend. C, 267,1475 (1968). Cyclic voltammetry with ferrocene-ferricinium redox couple... 

GOx (reduced) -I- ferricinium GOx (oxidized) -I- ferrocene Ferrocene ferricinium -i- electron (reaction at solid electrode surface)... 

Ferrocene may be employed in biosensors,as a non-physiological redox couple, to shuttle electrons between the biological component and the electrode. The high reversibility and low redox potentials, make It more desirable than the more common natural acceptors for the oxidases, such as oxygen, for facilitating electron transfer. Sensors based on the ferrocene/ferricinium redox couple have been previously reported [11,12,13]. 

---