Potentiostatic step method

Chou et al. were the first to report a template-free, one-step method for synthesizing SnC>2 mesoscale tubes via an anodic deposition process.231 The anodic deposition of SnCh was conducted in a nitrate containing, chloride-based Sn bath with Pt as both cathode and anode substrate. The electrodeposition was performed under potentiostatic condition. 

This group of methods relies on potentiometric measurement at the detection side to determine the hydrogen concentration. Three basic variations of this approach have been described the step, pulse, and sinusoidal methods [97j. In the step method, the hydrogen concentration is initially homogeneous throughout the membrane. The concentration is then increased at the input side and kept constant under potentiostatic control. The change in concentration at the detection side is followed by monitoring the potential. The... 

Label-free impedance immunosensors have been developed, but in general these methods may require additional amplification to improve sensitivity [57,58]. Nevertheless, a capacitance method using a ferri/ferrocyanide probe and a potentiostatic step approach gave DL 10 pg mL (500 fM) for lL-6 in buffer [59]. Optimization of experimental protocols in flow injection impedance spectroscopy led to sensitivity in the low aM range for interferon-y in buffer [60]. Sensitivities have been enhanced using metal nanoparticle labels or AuNP labels that catalyze subsequent Ag deposition [57]. These methods may be promising for future point-of-care applications if NSB from non-analyte proteins in the patient samples can be minimized. 

E step i=m potential step method (potentiostatic transient method)... 

Other Electrochemical Approaches A two-step method has been reported to form ZnTe films on Au electrodes [148]. In the first step, Te is electrodeposited from an acidic aqueous solution in which Te02 is dissolved. The ZnTe film is then prepared via the potentiostatic or potentiodynamic reduction of the Te film in a solution containing Zn " ions. During the reduction step, Te is produced and reacts with Zn " to form ZnTe. 

Fig. 70. A schematic diagram for the potential step method P potentiostat C three electrode cell R measuring resistor O oscilloscope.

Fig. 5.18 Potentiostatic methods (A) single-pulse method, (B), (C) double-pulse methods (B for an electrocrystallization study and C for the study of products of electrolysis during the first pulse), (D) potential-sweep voltammetry, (E) triangular pulse voltammetry, (F) a series of pulses for electrode preparation, (G) cyclic voltammetry (the last pulse is recorded), (H) d.c. polarography (the electrode potential during the drop-time is considered constant this fact is expressed by the step function of time—actually the potential increases continuously), (I) a.c. polarography and (J) pulse polarography...

The principle of this method is quite simple The electrode is kept at the equilibrium potential at times t < 0 at t = 0 a potential step of magnitude r) is applied with the aid of a potentiostat (a device that keeps the potential constant at a preset value), and the current transient is recorded. Since the surface concentrations of the reactants change as the reaction proceeds, the current varies with time, and will generally decrease. Transport to and from the electrode is by diffusion. In the case of a simple redox reaction obeying the Butler-Volmer law, the diffusion equation can be solved explicitly, and the transient of the current density j(t) is (see Fig. 13.1) ... 

The mechanism of anodic dissolution of silver in cyanide solutions has been studied by Bek and coworkers [378-380]. For example, using [379] the rotating disc electrode and pulse potentiostatic method, it has been found that the limiting step involved the formation, at the electrode surface, of the adsorbed complex with two... 

For the reduction of />-nitrophenol, where the irreversible step (35a) is replaced by a reversible two-electron process (36), another equation has been derived (91, 92) for the calculation of the rate constant k of reaction (35 b). The numerical values obtained for the rate constants are claimed (92) to be in agreement with the values obtained by chrono-potentiometric and potentiostatic methods. Reaction (35 b) has also been studied as a consecutive reaction to the reverse reaction of (35c), i.e. oxidation of -aminophenol. 

Potential or current step transients seem to be more appropriate for kinetic studies since the initial and boundary conditions of the experiment are better defined unlike linear scan or cyclic voltammetry where time and potential are convoluted. The time resolution of the EQCM is limited in this case by the measurement of the resonant frequency. There are different methods to measure the crystal resonance frequency. In the simplest approach, the Miller oscillator or similar circuit tuned to one of the crystal resonance frequencies may be used and the frequency can be measured directly with a frequency meter [18]. This simple experimental device can be easily built, but has a poor resolution which is inversely proportional to the measurement time for instance for an accuracy of 1 Hz, a gate time of 1 second is needed, and for 0.1 Hz the measurement lasts as long as 10 seconds minimum to achieve the same accuracy. An advantage of the Miller oscillator is that the crystal electrode is grounded and can be used as the working electrode with a hard ground potentiostat with no conflict between the high ac circuit and the dc electrochemical circuit. 

The chemical reaction mechanism of electropolymerization can be described as follows. The first step in course of the oxidative electropolymerization is the formation of cation radicals. The further fate of this highly reactive species depends on the experimental conditions (composition of the solution, temperature, potential or the rate of the potential change, galvanostatic current density, material of the electrode, state of the electrode surface, etc.). In favorable case the next step is a dimerization reaction, and then stepwise chain growth proceeds via association of radical ions (RR-route) or that of cation radical with a neutral monomer (RS-route). There might even be parallel dimerization reactions leading to different products or to the polymer of a disordered structure. The inactive ions present in the solution may play a pivotal role in the stabilization of the radical ions. Potential cycling is usually more efficient than the potentiostatic method, i.e., at least a partial reduction... 

The first step in the experimental procedure consists of preparative electrolysis of the aromatic compound A to A . The preparative potentiostat is then disconnected and a UME is inserted into the cathodic compartment. The steady-state oxidation current of A is recorded as a function of time for a certain time period to ascertain that the stability of A is high. If this is indeed the case, the alkyl halide RX is added to the solution while it is stirred for a few seconds to assure that homogeneous conditions apply for the reaction of Eq. 90. The recorded current is observed to decay exponentially towards zero. A plot of In / versus t is shown in Figure 16 for four different combinations of aromatic compounds and sterically hindered alkyl halides. From the slopes of the straight lines, -2A etCrx, A et values can readily be obtained. The method is useful for the study of relatively slow reactions with kET < 10 M- s-. ... 

In the majority of cases, potentiostatic regimes were used. In [189,197], a method was developed to obtain layer-by-layer metallic precursors. This allows one to avoid the semiempirical procedure of solution composition optimization. Moreover, it makes possible deposition under kinetic or mixed control, which improves the deposit morphology. Two- or three-step potentiostatic [177] and pulsed [175] modes which ensure the optimal deposit compositions have also been developed. 

As explained earlier, in transient electrochemical methods an electrical perturbation (potential, current, charge, and so on) is imposed at the working electrode during a time period 0 (usually less than 10 s) short enough for the diffusion layer 8 (2D0) to be smaller than the convection layer (S onv imposed by natural convection. Thus the electrochemical response of the system investigated depends on the exact perturbation as well as on the elapsed time. This duality is apparent when one considers a double-pulse potentiostatic perturbation applied to the electrode as in the double-step chronoampero-metric method. 

Let us consider, for example, the simple nernstian reduction reaction in Eq. (221) and a solution containing initially only the reactant R. Before any electrochemical perturbation the electrode rest potential Ej is made largely positive to E . At time zero the potential is stepped to a value E2, sufficiently negative to E , so that the concentration of R is close to zero at the electrode surface. After a time 6, the electrode potential is stepped back to El, so that the concentration of P at the electrode surface becomes zero. When this potentiostatic perturbation, represented in Fig. 21a, is applied in a steady-state method, the R and P concentration profiles are linear and depend only on the electrode potential but not on time, as shown in Fig. 20a (for k 0). Yet when the same perturbation is applied in transient methods, the concentration profiles are curved and time dependent, as evidenced in Fig. 21b. Thus it is seen from this figure that a step duration at Ei, much longer than the step duration 0 at E2, is needed for the initial concentration profiles to be restored. This hysterisis corresponds to the propagation of the diffusion perturbation within the solution, which then keeps a memory of the past perturbation. This information is stored via the structuring of the concentrations in the space near the electrode as a function of the elapsed time. 

Yet when applied to current reversal techniques, such as double-step chronampero-metry of cyclic voltammetry, these methods require that an appreciable current be observed during the backward perturbation, that is, for t > 0, in potentiostatic methods or after the potential scan inversion in cyclic voltammetry. This requires that the characteristic time 0 of the method is adjusted to match the half-life ti/2 of the electrogenerated intermediate. Today, owing to the recent development of ultramicroelectrodes, 0 can be routinely varied from a few seconds to a few nanoseconds [102]. Yet with basic standard electrochemical equipment, 0 is usually restricted from the second to the low millisecond range. Thus for experimental situations involving faster chemical reactions, current rever-... 

The potentiostatic method is less ambiguous than the galvanostatic one. Its application, however, requires more sophisticated instrumentation. The rise time of the potentiostat should be fast enough to ensure rapid step change of the potential. Errors may arise from slow rise times as well as from current integration. With porous electrodes, all sites may not be under the same potential diffusion of reactant into or out of the pores may be slow compared with the potential change, which can lead to incorrect estimates of surface coverage and utilization. 

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