Coulometry responses

It exhibits a single oxidation process (Ea = + 0.59 V, vs. SCE) affected by some adsorption problems. These adsorption phenomena, which typically affect the electrochemical response of these derivatives, sometimes make it difficult to ascertain by controlled potential coulometry the effective number of electrons involved in the oxidation step. In this case, the (approximate) number of electrons involved per molecule of dendrimer, nd, can be roughly calculated by comparing the cyclic voltammetric responses of the dendrimer with that of the ferrocene monomer using the following empirical equation.27,40... 

To reiterate, the amount of charge consumed during charging the double-layer is a function of potential, so as soon as coulometry commences, the charge held within the double-layer will change as ions adsorb and/or desorb in response to the change in potential. For convenience, we will discuss these processes in terms of a reduction reaction, with electrons constantly leaving the electrode. 

Cyclic voltammetry has gained widespread usage as a probe of molecular redox properties. I have indicated how this technique is typically employed to study the mechanisms and rates of some electrode processes. It must be emphasized that adherence of the CV responses to the criteria diagnostic of a certain mechanism demonstrates consistency between theory and experiment, rather than proof of the mechanism, since the fit to one mechanism may not be unique. It is incumbent upon the experimenter to bring other possible experimental probes to bear on the question. These will often include coulometry, product identification, and spectroelectrochemistry. 

In controlled-potential coulometry, the potential of the working electrode is maintained at a constant level such that only the analyte is responsible for conducting charge across the electrode/solution interface. The charge required to convert the analyte to its reaction product is then determined by recording and integrating the current-versus-time curve during the electrolysis. 

By anodic treatment in acidic solution, p-Cdg Zno g5Te becomes porous. Coulometry and chemical analysis by X-ray photoelectron spectroscopy indicate that the pore walls are covered by a more or less homogeneous layer of elemental tellurium. A passivating effect of this layer could explain why anodic etching of the p-type material yields a porous morphology, and the layer could also be responsible for changes observed in the photoelectrochemical properties. 

The theory for the different reaction schemes involves ordinary (rather than partial) differential equations, because the electrolyzed solution is assumed to be essentially homogeneous (see Section 11.3.1). The concentrations are functions of t during the bulk electrolysis, but not of x. The measured responses in coulometry are the i-t curves and the apparent number of electrons app consumed per molecule of electroactive compound. From the quantity of electricity passed during the electrolysis Q t), app can be calculated as... 

If the adsorbed molecule is electroactive and shows a voltammetric postpeak well separated from the voltammetric response of the dissolved species, it may be possible to measure Q by coulometry at a potential between the peaks for the dissolved and adsorbed species, and to measure 2 Q as the additional charge passed when the potential is changed to a value beyond the postpeak. 

Coupling reactions of the electrogenerated cation radicals can also occur. If the parent molecule is a good nucleophile, this can be considered as a nucleophilic attack by parent on its own cation radical. The nature of the electrochemical responses in CV and coulometry depend upon whether the coupled product can undergo further oxidation. For example, in the oxidation of aromatic aza-hydrocarbons such as acridine (AcH), studied by Marcoux and Adams (1974), the CV was characterized by an irreversible one-electron wave and coulometry showed an napp-value of one. An analysis of rotating disk voltammograms demonstrated that the reaction sequence (79)-(81) was more probable them that involving... 

In the conductivity, potentiometric, and voltam-metric measurements the response is correlated to concentration or activity of the analyte usually by using calibration curves. In coulometry, however, the charge measured gives directly the amount of substance and therefore no calibration is needed. However, in coulometry the sample is consumed in the measurements and the problem is that the method requires 100% current efficiency to be reliable. Conductimetry and potentiometry are sample nonconsuming methods. In voltammetry, only an insignificant amount of the sample is consumed and therefore the measurement can be repeated. Only in voltammetric stripping methods of very low concentrations of the analyte the amount consumed at the electrode reaction has to be considered if repeated measurements are to be done. 

Alternatively, the extent of oxidation or reduction of protein-active sites at various applied potentials may be determined by coulometric measurements in the presence of a suitable mediator [132]. Here, the extent of reduction or oxidation of the group is determined from the charge that is passed in response to a potential perturbation. The need for spectral monitoring is thus removed. Heineman and coworkers have reviewed [133] the use of two techniques, thin-layer pulse coulometry and thin-layer staircase coulometry, each of which are suitable for studies on biological molecules. 

The essential difference between voltammetric and other potentiodynamic techniques, such as constant current coulometry, is that in voltammetry an electrode with a small surface area (< 10mm ) is used to monitor the current produced by the species in solution reacting at this electrode in response to the potential applied. Because the electrode used in voltammetry is so small, the amount of material reacting at the electrode can be ignored. This is in contrast to the case in coulometry where large area electrodes are used so that all of a species in the cell may be oxidized or reduced. 

FIGURE 2.22. Nemst plot obtained via potential step coulometry for the metallopolymer in 1.0 Af HCIO4. The dashed line is the theoretically predicted Nemst response (Ref. 75.)... 

In the BCD, the reaction of electrons with electron-absorbing compounds is a second-order rate mechanism that can be affected by temperature as shown in Figure 6.17. It is also affected by the electronegativity of the species itself, the presence of other species, and the energy of the electrons. Even when these conditions are controlled, variations have been observed. To minimize this, suitable operating parameters and strong electron-absorbing molecules are used to force the forward rate of reaction to approach a pseudo-first-order rate for the observed coulometry. A response factor could be calculated if the kinetics of the reaction were more fully understood. 

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