Capacity-potential curves shape

The capacity of the double layer depends strongly on the solvent but there is a qualitative similarity in the shape of capacity-potential curves in all solvents including water. Early measurements seemed to indicate qualitative differences between water and such non-aqueous solvents as methanol, ethanol and ammonia which have featureless capacity curves in contrast to the characteristic humped curves found in water. However, more recent studies have shown that capacity humps occur commonly in solvents of widely differing types. They are found in solvents of high and medium dielectric constant and probably have a common origin in field reorientation of solvent dipoles. 

The shapes of the charge-potential and the capacity-potential curves follow from the Gibbs adsorption isotherm immediately. The shape of the surface-excess-potential curve is more difficult to derive. This will be the subject to be discussed next and in order to do that we shall consider the general adsorption isotherm... 

Results of the fractionation of the mixture are presented in Table 2A. Rough characterization of the fractions was made by comparing their capacity current potential curves with those of the model mixtures. The shape of the curve obtained in effluent samples after separation of the hydrophobic neutral component corresponds to that of the mixture of fulvic acid and Dextran. After removal of (he hydrophobic acid fraction, the remaining fraction resembles the... 

Analytical applications of electrochemistry, where the objectives are well defined, have fared better. There is a long list of papers going back twenty years on the applications of computers and then microprocessors. Reviews of this subject appear in the Fundamental Reviews sction of Analytical Chemistry (see refs. 8 and 9). In general, the aim in electroanalytical methods is to avoid interfering effects, such as the ohmic loss and the double layer capacity charging, and to use the Faradaic response peak current-potential curve as an analytical tool. Identification of the electroactive species is achieved by the position of the response peak on the potential axis and "pattern recognition , and quantitative analysis by peak shape and height. A recent development is squarewave voltammetry [10]. 

In the dl region, comparison of the i E) and C E) curves can be used as a first test [one should be able to deduce one from the other, the current being only capacitive, (i = CdE/dt)]. The shape of the minimum (corresponding to the capacity of the diffuse part of the electrochemical dl) on the C(E) and i(E) curves (Fig. 15) for the case of no adsorption in dilute solution, is another criterion it should not be too different than that observed for mercury, but its position in the range of potential can be completely different. The shape of the adsorption peak is another criterion (in the case of adsorption) it should be sharp and reproducible. Of course, the stability and reproducibility of the curves are important and should be observed not only in the dl region but over all the range of potential explored. 

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