Reversible redox couple

The close electrochemical relationship of the simple quinones, (2) and (3), with hydroquinone (1,4-benzenediol) (4) and catechol (1,2-benzenediol) (5), respectively, has proven useful in ways extending beyond their offering an attractive synthetic route. Photographic developers and dye syntheses often involve (4) or its derivatives (10). Biochemists have found much interest in the interaction of mercaptans and amino acids with various compounds related to (3). The reversible redox couple formed in many such examples and the frequendy observed quinonoid chemistry make it difficult to avoid a discussion of the aromatic reduction products of quinones (see Hydroquinone, resorcinol, and catechol). 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Analogously to eqn. 3.72 for stationary electrodes and a reversible redox couple of soluble ox and red, Kies derived for chronopotentiometry at a dme via insertion in the Nernst equation... 

For the HMDE and for a solution that contains only ox of a reversible redox couple, Reinmuth102, on the basis of Fick s second law for spherical diffusion and its initial and boundary conditions, derived the quantitative relationship (at 25° C)... 

This means that Ei lies about midway on the SV wave between Ev and Epl2 at the high, i.e. positive, side, as we have already mentioned in connection with Fig. 3.64. However, in practice and on the basis of the above arguments (1)—(3), the position of E, will deviate from this more or less theoretical approximation where the sphericity term of eqn. 3.80 has been neglected. Further, with incompletely reversible redox couples, e.g. for quasi-reversible systems (cf., pp. 125-126), there will of course be more deviations, but at any rate the linear relationship between ip and C appears to remain. 

For the MFE and a solution again that contains only ox of a reversible redox couple, De Vries and Van Dalen101,104 studied anodic SV at a plane MTFE with film thicknesses L of 4-100 pm, assuming linear diffusion on the basis of their... 

Hence the picture of the cathodic and anodic waves obtainable for a completely reversible redox couple by means of the RDE corresponds fully with that in Fig. 3.9 the value of i, i.e., the height of the sigmoidal waves, is linearly proportional to to1/2 and to C (see eqn. 3.89 and the Levich constant). If for a well chosen combination of C and E a plot of i against co1/2 deviates from a straight line passing through the origin, then in the kinetics of the electrode reaction we have to deal only with a rapid electron transfer (cf., Fig. 3.10) or even with a slow electron transfer (cf., Fig. 3.11), in which latter instance the transfer coefficient a plays an appreciable role (cf., eqns. 3.17 and 3.18). 

At the RRDE, however, so-called collection experiments, the actual purpose of its integrated construction, are often carried out, i.e., in a solution of, for instance, only ox of a reversible redox couple ox + ne red, a cathodic current at the disc iD produces red, which reaches the ring where it is completely reoxidized to ox, because the ring is maintained at a sufficiently positive potential ER an RE, generally an SCE, is used together with a bipotentiostat for precise adjustment of the potentials ED and ER required. 

The electrochemical cell can again be of the regenerative or electrosynthetic type, as with the photogalvanic cells described above. In the regenerative photovoltaic cell, the electron donor (D) and acceptor (A) (see Fig. 5.62) are two redox forms of one reversible redox couple, e.g. Fe(CN)6-/4 , I2/I , Br2/Br , S2 /S2, etc. the cell reaction is cyclic (AG = 0, cf. Eq. (5.10.24) since =A and D = A ). On the other hand, in the electrosynthetic cell, the half-cell reactions are irreversible and the products (D+ and A ) accumulate in the electrolyte. The most carefully studied reaction of this type is photoelectrolysis of water (D+ = 02 and A = H2)- Other photoelectrosynthetic studies include the preparation of S2O8-, the reduction of C02 to formic acid, N2 to NH3, etc. 

Bard AJ, Crayston JA, Kittlesen GP, Shea TV, Wrighton MS (1986) Digital simulation of the measured electrochemical response of reversible redox couples at microelectrode arrays consequences arising from closely spaced ultramicroelectrodes. Anal Chem 58 2321-2331... 

Thus, for R - 1 pm and D - 10 cm sec, a 1 nA limiting current is obtained for concentrations of electroactive species, CQX re(j — 1.6 /jM. These calculations suggest that in the presence of a reversible redox couple at micro-molar concentrations, even STM tip-sample biases of AEt < 10 mV will drive a faradaic current that is comparable to that of the tip-sample tunneling current. STM imaging under such circumstances is likely to be experimentally demanding using conventional feedback methodology. 

V vs. Ag/AgCl Reversible redox couple to test cell performance ... 

Then, ionic diffusion towards self-affine fractal electrode was experimentally investigated by using cyclic voltammetry in a mixture of 30 wt % glycerol and 70 wt % (0.01 M K4[Fe(CN)6] + 0.5 M Na2S04) solution. The cyclic voltammograms obtained from each electrode at various scan rates clearly showed one set of well-defined current peaks assigned to be a reversible redox couple as follows ... 

Another method used to study the reactivity of ceramic superconductors is to compare the cyclic voltammetric response of a reversible redox couple (in the present case [tcnq]0/ ) at a superconductor electrode with and without corrosion. As illustrated in Figure 14, in principle, in... 

Lund s organic nucleophiles, which are all members of perfectly reversible redox couples and might therefore have been deemed to behave as outer sphere single electron donors, in fact react in an S 2 fashion in the absence of too severe steric constraints. 

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. 

Rubredoxin is an electron-transfer protein with an Fe(IlI)/Fe(lI) redox couple at -0.31 V (SCE) in water (20). Our peptide model, [Fe( Cys-Pro-Leu-Cys-OMe)2] (Z = benzyloxycarbonyl) (21) exhibits its Fe(lll)/Fe(ll) redox couple at -0.50 V (SCE) in Mc2SO (9). This is similar to the value observed for the native protein when the difference of the solvent is taken into account. When the model complex is solubilized in water by formation of micelles with addition of the non-ionic detergent, Triton X-KX), we also observed a quasi-reversible redox couple at -0.37 V (SCE) (5). The Fe(lll) complexes of Cys-X-Y-Cys peptides also exhibit a characteristic MCD band at 350 nm due to ligand-to-metal charge transfer which has also been found in oxidized rubredoxin (4). 

The other simple peptide complex e.g. [Fe(Z-Cys-Ala-OMe)4]2- did not exhibit such a reversible redox couple under similar conditions. The Fe(lll) complexes of simple peptide thiolates or cysteine alkyl esters are found to be thermally quite unstable and decompose by oxidaticxi at the thiolate ligand by intramolecular electron transfer. Thus the macro-ring chelation of the Cys-Pro-Leu-Cys ligand appears to stabilize the Fe(in) state. The stability of the Fe(ni) form as indicated by the cyclic voltamnoogram measurements and by the visible spectra of the Fe(in) peptide complexes suggests that the peptide prevents thermal and hydrolytic decomposition of the Fe-S bond because of the hydrophobicity and steric bulk of the Pro and Leu residues (3,4). 

This sequence was found to enhance the staltility of the superoxidized state of 4Fe-cluster probably by its hydrophobic peptide side chains. Thus, the CV results indicated a quasi-reversible redox couple at +0.12 V (SCE) in DMF (see Table III). Thus the complex may be regarded as a model of HiPIP. Simple model complexes, [Fe4S4(S-r-Bu)4]2- (27), [Fe4S4(2,4,6-trimethylbenzenethiolato)4]2- (28), and [Fe4S4(2,4,6-triisopropylbenzenethiolato)4]2 (16), have been synthesized, and the latter has been crystalligraphically characterized (29). 

The kinetics of the oxidation of [Ru(bpy)3] by Tl " ", catalyzed by Ru02 a H20 dispersed in HNO3 (3M), have been studied, varying the concentrations of [Ru(bpy)3] " ", Tl +, T1+, catalyst and temperature. The kinetic data are consistent with an effect involving electron transfer between the electrochemically reversible redox couples [Ru(bpy)3] +/[Ru(bpy)3] and T1 /T1+, the transfer being mediated by Ru02 vH20. ... 

The voltammetric response of an electrodeposited film of 2 in CH2CI2 with 0.1 M TBAH is shown in Figure 6 as a representative example. A well-defined, symmetrical oxidation-reduction wave is observed, which is characteristic of surface-immobilized reversible redox couples, with the expected linear relationship of peak current with potential sweep rate A formal potential value of =+0.42... 

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