Potential step method

Frumkin, 169 for zinc, 92, 100 tabulated, 101 zinc-solutions, and, 92 Potential step method, in polymer formation, 386... 

CS = coulostatic method, CV = cyclic voltamogram, FD = Faradaic distortion method, FR = Faradaic rectification, GD = galvanostatic double pulse method, IP = impedance method, PS = potential step method. See also list of... 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

Nal concentration, up to 1 X 10 They also obtained steady-state data using a potential step method. The applied potential was initially set at -0.9 V, where no iodide absorption occurs. The electrode was stepped to various positive potentials and A/(ad) was recorded until equilibrium values were obtained. Similar measurements were done in an electrolyte without Nal, and A/(no-ad) was recorded as a reference. The net change in frequency was determined as IAA/I = A/(ad) -A/(no-ad). Figure 27.22 shows I AA/I vs. E plots for various Nal concentrations. Analysis of the data showed that iodide adsorption occurred in accordance with the Temkin isotherm. 

A related technique is the current-step method The current is zero for t < 0, and then a constant current density j is applied for a certain time, and the transient of the overpotential 77(f) is recorded. The correction for the IRq drop is trivial, since I is constant, but the charging of the double layer takes longer than in the potential step method, and is never complete because 77 increases continuously. The superposition of the charge-transfer reaction and double-layer charging creates rather complex boundary conditions for the diffusion equation only for the case of a simple redox reaction and the range of small overpotentials 77 [Pg.177]

A potential-step method is advantageous in this context, but it is also unavoidable to be influenced by the previous history. Therefore, it is important to develop a standard electrochemical procedure that can cancel the history and prepare a reproducible surface for followed experiments. 

When the potential step method is employed to elucidate the mechanism of the underpotential deposition process, the current j against time t is readily found to be in the form of... 

In [119], the hydrogen adsorption and desorption reactions in thin palladium electrodes were studied using the potential step method in order to analyze the mechanism of phase transformation. Transient current responses were recorded at the onset of the potential step for 47 pm thick Pd electrodes in 1 mol dm H2SO4 at ambient temperature. A model based on a moving boundary mechanism was proposed to account for the experimental i-t curves. It was found that the hydrogen adsorption reaction shows interfacial kinetic limitations and only numerical solutions can be obtained. Such kinetic limitations were not found for the desorption reaction and a semianalytical solution that satisfactorily fits the experimental data was proposed. 

Philipp and Retter [151] have studied the formation of the first monolayer of adenine on mercury electrode in borate solutions. Applying potential-step method, they have proposed to explain the observed transients in terms of two-dimensional (2D) truncated progressive nucleation and constant growth of monolayer islands. 

Other Techniques - Other electrochemical techniques that could be employed in sensor technology would include potential-step methods (or chrono-amperometry, as current is recorded with time), current-step methods (or chronopotentiometry, as potential is recorded with time) and AC impedance. None of these techniques appear to have yet been applied to catalyst sensing in a systematic way. 

An alternative to the potential-step method is to open the electrolytic circuit after keeping the potential for a fixed time at the limiting-current region, and then measure the change in absorbance. 

Potential step method Chronoamperometry Chronocoulometry Voltage pulse method Pulse polarography Differential pulse polarography... 

Again, application of the principle to the simple potential-step method appears trivial or superfluous. However, it is of quite great important for other types of potential control, namely double potential step, cyclic potential step [73], and especially the linear potential sweep method [21, 22, 73]. In all these techniques, sets of data Jf ( ) /f (f) E can be obtained, thus enabling kt(E) to be determined from eqn. (100). For more details, the reader is referred to the quoted textbooks. 

From an experimental point of view, it may be worth mentioning that the artificial membrane is reasonably stable, though only for a short time. It is therefore also a clear example of an object that should be studied by means of a quick method. The (small-amplitude) potential step method has been applied successfully [113], as has the FFT method described in Sect. 2.5.5. With the latter, the analysis in terms of an equivalent circuit like in Fig. 29 was demonstrated quite spectacularly [114]. 

A plot of In kf vs. potential (in fact, this is a Tafel plot) will be curved in the case of a linear mechanism with more than one rate-determining step. It is of the utmost importance to cover a large potential range. However, in the potential step method, l values exceeding ca. 200 s 1/2 cannot be determined and the accessible potential range is correspondingly limited. 

In this section, a description of the state of the art is attempted by (i) a review of the most fundamental types of reaction schemes, illustrated by some examples (ii) formulation of corresponding sets of differential equations and boundary conditions and derivation of their solutions in Laplace form (iii) description of rigorous and approximate expressions for the response in the current and/or potential step methods and (iv) discussion of the faradaic impedance or admittance. Not all the underlying conditions and fundamentals will be treated in depth. The... 

A single DBP droplet is positioned in the vicinity of the microelectrode by the laser trapping technique, and the droplet-microelectrode (edge-to-edge) distance (L) is controlled arbitrarily in micrometer dimension. Knowing the oxidation potential of PPD in the water phase to be 30 mV, PPD is oxidized by a potential step method (100 mV) to induce the dye formation reaction. The anodic current relevant to oxidation of PPD reaches a steady-state value within a short electrolytic time (t) because of cylindrical diffusion of PPD to the microelectrode. The dye formation in the droplet can be easily confirmed by the color change from transparent to cyan or yellow. The dye formation reaction in a single microdroplet could be... 

As discussed earlier, the potential is scanned between selected potentials with the measurement of the corresponding electrical current. Another type of electrochemical methods is potential step methods, such as chronoamperometry. In this case, the potential of the working electrode is changed drastically and immediately from a potential Ex to a potential E2, at the... 

In potential step voltammetry, a DC potential window is covered by a DC ramp (linearly increasing applied DC potential) with discrete and symmetrical pulses superponated to this ramp. As an alternative, the pulse amplitude can increase with each pulse, superponated on a constant DC offset. Methods where pulses are involved superponated on a DC signal are generally called potential step methods. 

With potential step methods, the capacitive current is a transient, decaying with a time constant equal to RvCrii The usual procedure is to wait several of these time constants before making the current measurement, by which time the capacitive current has declined to a negligible value. It is therefore not a serious problem with potential step experiments. 

Englisch auch als, potential-step"-Methode bezeichnet. Eigentlich handelt es sich um ein echtes coulometrisches Verfahren. 

The potential step methods are called chronoamperometry and like LSV and CV can deal with only the forward process (single step) or with the reverse process, involving the primary intermediate, as well. The latter is called double potential step chronoamperometry (DPSC) and is by far the most useful in kinetic studies. The applied potential-time wave form as well as the currenttime response for a reversible electrode process are illustrated in Fig. 3. The potential is stepped from a rest value where no current flows, usually 200-300 mV from the potential where the process of interest takes place, to one... 

Potential step methods have emerged as valuable electrochemical methods due to the highly sensitive nature of the technique. The waveform employed in potential step methods, also referred to as pulsed methods, have some advantages over potential sweep methods. The main advantage is that the steplike waveform can discriminate and separate the capacitive current versus the faradaic current, the current due to the reduction or oxidation undergone by the analyte, increasing signal to noise. Capacitive versus faradaic current discrimination is the basis for all of the pulsed techniques. The rate of decay of the capacitive current and the faradaic current is not the same. The capacitive current has an exponential decay whereas the faradaic current decays as a function of t Since the rate of decay of the capacitive current is much... 

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