Voltammetry reduction

The redox potential for the dye can shift upon adsorption from solution due to coulombic or stronger covalent interactions with the solid substrate. This potential change can amount to several hundreds of millivolts. While n-type semiconductors cannot be used generally to measure oxidation potentials of adsorbed dye sensitizers by conventional cyclic voltammetry, reduction potential i°(S/S ) is often more accessible. Assuming oxidation and reduction potentials of the dye ground state on the surface are linked by a constant relation ... 

SanecM P, Skital P, Kaczmarski K (2006) Numerical modelling of ECE-ECE and parallel EE-EE mechanisms in cyclic voltammetry. Reduction of 1,4-benzenedisulfonyl difluoride and 1,4-naphthalenedisulfonyl difluoride. Electroanalysis 18 981-991... 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

In voltammetry a time-dependent potential is applied to an electrochemical cell, and the current flowing through the cell is measured as a function of that potential. A plot of current as a function of applied potential is called a voltammogram and is the electrochemical equivalent of a spectrum in spectroscopy, providing quantitative and qualitative information about the species involved in the oxidation or reduction reaction.The earliest voltammetric technique to be introduced was polarography, which was developed by Jaroslav Heyrovsky... 

Determining Equilibrium Constants for Coupled Chemical Reactions Another important application of voltammetry is the determination of equilibrium constants for solution reactions that are coupled to a redox reaction occurring at the electrode. The presence of the solution reaction affects the ease of electron transfer, shifting the potential to more negative or more positive potentials. Consider, for example, the reduction of O to R... 

In stripping voltammetry the analyte is first deposited on the electrode, usually as the result of an oxidation or reduction reaction. The potential is then scanned, either linearly or by using potential pulses, in a direction that removes the analyte by a reduction or oxidation reaction. 

More recent research provides reversible oxidation-reduction potential data (17). These allow the derivation of better stmcture-activity relationships in both photographic sensitization and other systems where electron-transfer sensitizers are important (see Dyes, sensitizing). Data for an extensive series of cyanine dyes are pubflshed, as obtained by second harmonic a-c voltammetry (17). A recent "quantitative stmcture-activity relationship" (QSAR) (34) shows that Brooker deviations for the heterocycHc nuclei (discussed above) can provide estimates of the oxidation potentials within 0.05 V. An oxidation potential plus a dye s absorption energy provide reduction potential estimates. Different regression equations were used for dyes with one-, three-, five-methine carbons in the chromophore. Also noted in Ref. 34 are previous correlations relating Brooker deviations for many heterocycHc nuclei to the piC (for protonation/decolorization) for carbocyanine dyes the piC is thus inversely related to oxidation potential values. 

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

Cyclic voltammetry (adsorption, monolayers) Potentiodynamic polarisation (passivation, activation) Cathodic reduction (thickness) Frequency response analysis (electrical properties, heterogeneity) Chronopotentiometry (kinetics)... 

Using dilatometry in parallel with cyclic voltammetry (CV) measurements in lmolL 1 LiC104 EC-l,2-dimethoxy-ethane (DME), Besenhard et al. [87] found that over the voltage range of about 0.8-0.3 V (vs. Li/Li+), the HOPG crystal expands by up to 150 percent. Some of this expansion seems to be reversible, as up to 50 percent contraction due to partial deintercalation of solvated lithium cations was observed on the return step of the CV. It was concluded [87] that film formation occurs via chemical reduction of a solvated graphite intercalation compound (GIC) and that the permselective film (SEI) in fact penetrates into the bulk of the HOPG. It is important to repeat the tests conducted by Besenhard et al. [87] in other EC-based electrolytes in order to determine the severity of this phenomenon. 

A related technique, reverse-pulse voltammetry, has a pulse sequence that is a mirror image of that of normal-pulse voltammetry (5). hi this case, the initial potential is on the plateau of the wave (i.e., where reduction occurs), and a series of positive-going pulses of decreasing amplitude is applied. 

In many other cases (by a change in experimental conditions, faster chemical reaction) the value of the catalytic current may be governed by the SET rate (see reaction 20). The value of k1 may be found and its variation as a function of the nature of the mediator (with several values for °j) leads by extrapolation (when k2 can be assumed to be diffusion-controlled) to the thermodynamical potential °RS02Ar which is somewhat different from the reduction potentials of overall ECE processes observed in voltammetry. 

In contrast, aromatic sulphoxides do not need extreme experimental conditions to give a well-defined step in polarography and voltammetry. Thus methyl phenyl sulphoxide (80) exhibits69 a well-defined wave in strongly acidic media at very moderate potential values. The reduction scheme assumes the transient formation of a protonated form prior to the electron transfer ... 

Figure 29. (a) Evolution of the absorption spectra of an electro-chromic polypyrrole as a function of the oxidation potential obtained during voltammetry between -900 and 400 mV from a 2.5 M LiCI04 aqueous solution. The voltammetry was performed at a scan rate of 20 mV s 1. (From Ref. 161). (b) Evolution of the absorption spectra of an electrochromic polypyrrole as a function of the reduction potential obtained during voltammetry between 400 and -900 mV from a 2.5 M LiCI04 aqueous solution. The voltammetry was performed at a scan rate of 20 mV s 1. (From Ref. 161). 

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