Electrochemical time scale

The preparation and characterization of the silver(II) complex [AgL] (where H2L = tetraneopentoxyphthalocyanine) has been reported. Electrochemical studies show that oxidation to stable [AginL]+ occurs followed by further oxidation to the radical cation [AgL]2+. Reduction to [AgrL]- leads to demetallation on the electrochemical time scale.207... 

The evolution of a new set of electrochemical waves (as opposed to the gradual shifting of the redox couple) on addition of guest species may be due to a number of factors. If the complex formed has a particularly high stability constant and has a redox potential which is markedly different from that of the free ligand, a new set of waves may be observed. However, if the decomplexation kinetics of the complex formed is particularly slow on the electrochemical time scale then, as the potential is scanned between the vertex points during a cyclic voltammetric experiment, the solution complexed species will be stable over this time period and the two sets of waves will correspond to free ligand and complex. Therefore care should be taken to determine the cause of the evolution of a new set of electrochemical waves and... 

For the response illustrated in Figure 12, in which the difference between the two redox potentials is AE° = 0.34 V, one can calculate that Acom 105 - 106. This value is typical of a mixed-valent compound which, on the electrochemical time scale, is at the border between... 

The foregoing has been concerned with the application of SERS to gain information on surface electronic coupling effects for simple adsorbed redox couples that are reversible in the electrochemical as well as chemical sense, that is, exhibit Nernstian potential-dependent responses on the electrochemical time scale. As noted in the Introduction, a major hoped-for application of SERS to electrochemical processes is to gain surface molecular information regarding the kinetics and mechanisms of multiple-step electrode reactions, including the identification of reactive surface intermediates. 

DNA-dye assemblies provided a nearly reversible response at scan rates up to 500 mV s. Thus, electron transfer through the ds DNA and hexamethylenethiol linker was concluded to be fast on the electrochemical time scale. Similar conclusions were drawn from the cyclic voltametry of the electroactive intercalator daunomycin selectively bound to G C sites at varying locations within a duplex attached to the electrode surface with a 13-atom alkylamide tether [173c],... 

Note - In the above cathodic reduction, the electron transfer is slow because of the deep structural change. The cyclovoltammogram reflects this situation by the fact that the peak potentials Epc and Epa do not remain constant when the scan rate is varied (this potential peak shift is 30 mV when the scan rate is multiplied by 10), and the Epc - Epa value is much larger than 58 mV n (n = number of exchanged electrons, here 2) obtained at 20°C with redox systems for which the electron transfer is fast on the electrochemical time scale. The variation of Ep - Epa as a function of the scan rate gives access to the heterogeneous electron transfer rate constant between the cathode and the cluster in solution (see for instance chap. 2 of ref. 3.3, pp 104 and 174). 

The UPD and anodic oxidation of Pb monolayers on tellurium was investigated also in acidic aqueous solutions of Pb(II) cations and various concentrations of halides (iodide, bromide, and chloride) [103]. The Te substrate was a 0.5 xm film electrodeposited in a previous step on polycrystalline Au from an acidic Te02 solution. Particular information on the time-frequency-potential variance of the electrochemical process was obtained by potentiodynamic electrochemical impedance spectroscopy (PDEIS), as it was difficult to apply stationary techniques for accurate characterization, due to a tendency to chemical interaction between the Pb adatoms and the substrate on a time scale of minutes. The impedance... 

Phase-sensitive detection is not at all specihc for EPR spectroscopy but is used in many different types of experiments. Some readers may be familiar with the electrochemical technique of differential-pulse voltammetry. Here, the potential over the working and reference electrode, E, is varied slowly enough to be considered as essentially static on a short time scale. The disturbance is a pulse of small potential difference, AE, and the in-phase, in-frequency detection of the current affords a very low noise differential of the i-E characteristic of a redox couple. 

The electrochemical reduction of cycloheptatriene (CHT) in liquid ammonia takes place at about —2.5 V vs SCE and forms the radical anion of CHT. The radical anion is stable in ammonia on the voltammetric time scale but decays slowly by disproportionation and coupling reaction pathways to give respectively 1,3- and 1,4-cycloheptadienes (total yield 34-39%) and C14H18 (in yields of 55-58%) isomers which incorporate the bitropyl carbon skeleta20. 

The electrochemical and spectroscopic data indicates that sites on these polymers can communicate with each other, in the electron transfer sense, on a relatively short time scale and without the formation of stable mixed valence clusters. Electronic tranport via hopping or tunnelling and modulated by means of neighboring molecular group collisions would be consistent with these requirements. The relative molecular nonspecificity of this mechanism suggests that other polymeric materials would show similar effects and this has been seen for thin films of poly — (vinylferrocene) and poly — (nitrostyrene). 

These considerations demonstrate that an electrochemical experiment may be lacking in reliability if the time constant of the cell is not sufficiently low compared to the time scale of the experiment. 

The rate of dE/dt is always kept constant during a potential scan, although obtaining a series of polarograms as a function of v can be extremely informative. (The variation of the sweep rate can be regarded as being equivalent to a variation of varying the time-scale of observation of an electrochemical experiment, as will become clear later in Section 6.4.3.)... 

Molar conductivity measurements are equally applicable to both solid and liquid electrolytes. In contrast, the measurement of current flowing through an electrochemical cell on a time scale of minutes or hours while the cell is perturbed by a constant dc potential is only of value for solid solvents (Bruce and Vincent, 1987) where convection is absent. Because of the unique aspects of dc polarisation in a solid solvent this topic is treated in some detail in this chapter. Let us begin by considering a cell of the form ... 

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