Superimposed alternating potential

In ACIS, a small-amplitude sine wave is superimposed on a constant potential, and the resulting current is recorded. The current lags behind the alternating potential by a degree proportional to the impedence of the SAM. From a plot of the imaginary versus real parts of the complex impedence, both the capacitance of the SAM and the electron-transfer kinetics can be extracted. Because the bulk of the studies described in this chapter make use of cyclic voltammetric or chromo-amperometric methods, ACIS is not discussed further here. Leading references are provided for the interested reader [23, 43, 76]. 

The most common applications of ACP/ACV involve the application of a small-amplitude sinusoidal alternating potential superimposed onto a ramp voltage. The resulting current is an alternating current, the d.c. component being filtered out by use of a phase-sensitive current detector. This is possible... 

In the electroreflectance method a sinusoidal wave alternating potential (about 10—100 Hz) is superimposed on a conventional linear potential generally used in specular reflection measurements. A lock-in amplifier is used to detect resulting changes in which reflected light intensities are proportional to dRIdE. The response of l(l// o)(df /3 ) is recorded as a function of the linear potential on an XY recorder. 

Basically, the ac polarographic experiment involves measurement of the impedance of an electrolytic cell under polarographic conditions, using an alternating potential of small amplitude (<10mV) which is superimposed upon the potential of the working (indicator) electrode. This impedance may be related to the kinetics and mechanism of the electrode reaction as well as to the concentration of the electroactive species. 

This term is used to cover a range of techniques in which the mean potential is controlled potentiostatically and swept over a range while a small amplitude, relatively high frequency alternating potential superimposed on the slowly-varying sweep is used to excite a sinusoidal response in the current, which is... 

In this procedure, a constant sine wave a.c. potential of a few millivolts is superimposed upon the usual d.c. potential sweep. The applied d.c. potential is measured in the usual way and these results are coupled with measurements of the alternating current. 

The implementation of this formalism for an atomistic model does not present any difficulties, at least conceptually. However, it is not obvious how the formalism should be used for single site potentials such as the Gay-Berne model. One approach would be to superimpose a mesogenic structure on the Gay-Berne ellipsoid but this structure would necessarily have a lower symmetry. Alternatively it would be possible to place a line of atoms along... 

Datolite is caldiun borosilicate with a monoclinic structure (2/m) and space group P2i/c. It consists of superimposed complex sheets of linked oxygen and O, OH tetrahedra around silicon and boron atoms respectively the Si04 and B(0,0H)4 tetrahedra alternate, forming rings of four and eight tetrahedra. The natural datolite in our study consisted of seven samples from a variety of geologic environments. Concentrations of potential luminescence impurities in one sample are presented in Table 4.8. 

Equation (7.116) indicates that the charge-potential curves for reversible processes are only dependent on the square wave amplitude Sw and are independent of the frequency / = 1 jh and the staircase amplitude AEs. As a consequence, they are superimposable on those obtained at any differential electrochemical technique, such as DSCVC, provided that the differences between the successive potential pulses coincide (AE = 2 sw)- Moreover, when this difference is much less than RT/F (i.e., less than 25 mV at T = 198 K), the responses obtained in Cyclic Voltammetry (CV), Alternating Current Voltammetry, Potentiometric Stripping Analysis (PSA) and also in any Reciprocal Derivative Chronopotentiometry (RDCP) fulfill [5, 74, 75] ... 

From the experimental standpoint, the use of a.c. techniques offers many advantages. Sensitivity is much higher than in d.c. measurements, since phase-sensitive detection can be used and very small probe signals can be employed ( 5mV). The technique is therefore a truly equilibrium one, unlike cyclic voltammetry. An alternative approach to the commonly used sinusoidal signal superimposed on the selected d.c. potential is to use a potential step and to employ Laplace transform methods. Instrumentally, this is rather more demanding and the advantages are not clear [51]. Fourier transform methods have also been considered and their use will have advantages in terms of the time-scale for an experiment, especially at very low frequencies. 

The special case of square-wave voltammetry (SWV) is worth noting separately from other alternating current techniques because it is both more rapid and more sensitive than DPP/DPV. In SWV, the applied potential waveform is a staircase with constant step height on which is superimposed an asymmetrical forward and reverse voltage pulse of constant amplitude and very short duration, typically less than 10 ms. Thus, the entire polarogram may be run in about approximately 1 s, with the enhanced sensitivity of the method owing to sampling of the current at the end of both the forward and reverse directions of the pulse. 

Alternating-current (a.c.) polarography is sensitive to the electrochemical reversibility of a redox couple and finds use in measuring electron-transfer rates at electrodes. In this technique a small, constant a.c.-voltage component of frequency o) (rad s ) is superimposed on the usual d.c.-ramp voltage, and the resulting a.c.-current is measured. The applied potential is ... 

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