Pulsed current measurement

From the idle current measurement, the temperature Tq value is calculated. From the pulsed current measurement, the Tj value is then calculated. The difference in femperafures is ... 

From the difficulties encountered with interpretation of CVs which the discussions above amply show, it would appear that other voltammetric methods, especially differential methods, would have found wider application to CPs. This has unfortunately not been the case. The results in Figs. 4-17-a.b.c and 4-18 represent some of the few studies of this nature. In Fig. 4-17. the results of CV and of Differential Pulse Voltammetry (DPV) are compared. The latter is a technique in which a small potential pulse is superimposed on a staircase potential function with the difference between the post-pulse and pre-pulse current measured (inset in Fig. 4-171. The differential method yields peak-shaped curves unencumbered by residual current tails, as in CVs, and thus a clearer identification of peaks and their widths. Fig. 4-19 then shows DPV of Poly(phenylene vinylene) used to compute the bandgap, as described earlier. Normal Pulse Voltammetry (NPV), in which a sort of digital pulse-ramp is applied in place of the analog ramp of CV and the current sampled at the end of the pulse [50], has been applied to poly(l-amino pyrene) [48], yielding redox potentials as well as diffusion coefficients (Fig. 4-181. Other differential methods such as Square Wave Voltammetry have been applied to poly(aromatic amines) in the author s laboratories. There is however little other extant work with pulse voltammetry of CPs, although the very brief results above clearly provide a strong indication for it. 

Measurement by quasi - constant current (steady - state value of pulse current) providing a compete tuning out from the effect of not only electric but also magnetic material properties. 

Ion chromatography (see Section 7.4). Conductivity cells can be coupled to ion chromatographic systems to provide a sensitive method for measuring ionic concentrations in the eluate. To achieve this end, special micro-conductivity cells have been developed of a flow-through pattern and placed in a thermostatted enclosure a typical cell may contain a volume of about 1.5 /iL and have a cell constant of approximately 15 cm-1. It is claimed15 that sensitivity is improved by use of a bipolar square-wave pulsed current which reduces polarisation and capacitance effects, and the changes in conductivity caused by the heating effect of the current (see Refs 16, 17). 

Rather high charging currents cross the electrode when a variable potential component is applied. Therefore, to reduce the influence of these currents in the case of rectangular pulses, the measurements are made at a specific time after the potential change, when the charging current has decreased drastically. In the case of sinusoidal superimposed currents, one uses another device based on the fact that the... 

Flow-Type Ionization Chamber PFC flow rate 1-2 /min, continuously measured, current measurement and a-pulse counting... 

FIGURE 4.10 Current-time traces (top) and potential-time traces (bottom) for the pulsed galvanostatic measurement of lOmg l-1 protamine in aqueous sample containing 0.1 M NaCl and 50mM TRIS (pH 7.40) [54]. An applied cathodic current of — 3 pA leads to the extraction of protamine into the membrane, and the observed potential is significantly different for samples with and without protamine (bottom). The membrane is renewed potentiostatically at 0 V for 10 sec before the next current pulse. 

Binding of metal ions by amorphous silica. 300 mg Si02 dm-3, [Mej = 10 5 MJ. The fraction of metal ion bound was calculated from peak currents measured with differential pulse polarography. 

Unlike in CV, pairs of current measurements are made on each period of the square wave. These are at time forward late in the forward pulse, named /forward, and frevere in the reverse pulse, named Iteveix- Both tformad and Reverse are much greater than the time for fully charging the electrode capacitance, so that only the Faradaic current is recorded. With calculation of /net, the difference between /forward and /reverse, SWV presents three types of peak-shaped I-E relations. Figure 65 displays the SWV of a reversible one-electron reduction process. 

An alternative and more recent electroanalytical tool is square-wave voltammetry (which is probably now employed more often than normal or differential pulse voltammetry). In this technique, a potential waveform (see Figure 6.26) is applied to the working electrode. Pairs of current measurements are then made (depicted on the figure as t and f2) these measurements are made for each wave period ( cycle ), which is why they are drawn as times after to (when the cycle started). The current associated with the forward part of the pulse is called /forward, while the current associated with the reverse part is /reverse- A square-wave voltammogram is then just a graph of the difference between these two... 

The response of a reversible reaction (2.146) depends on two dimensionless adsorption parameters, Pr and po. When pR = po the adsorbed species accomplish instantaneously a redox equilibrium after application of each potential pulse, thus no current remains to be sampled at the end of the potential pulses. The only current measured is due to the flux of the dissolved forms of both reactant and product of the reaction. For these reasons, the response of a reversible reaction of an adsorbed redox couple is identical to the response of the simple reaction of a dissolved redox couple (2.157), provided Pr = po- As a consequence, the real net peak current depends linearly on /J, and the peak potential is independent of the frequency. If the adsorption strength of the product decreases, i.e., the ratio increases, the net peak current starts to increase (Fig. 2.73). Under these conditions, the establishment of equilibrium between the adsorbed redox forms is prevented by the mass transfer of the product from the electrode surface. Thus, the redox reaction of adsorbed species contributes to the overall response, causing an increase of the current. In the hmiting case, when ]8o —0, the reaction (2.146) simplifies to reaction (2.144). 

Figure 8 Pulsed current technique for the measurement of critical current. From Jones, T., McGinnis, W., Boss, R., Jacobs, E., Schindler, J., Rees, C., Tech. Doc. 1306 July 1988 Naval Ocean Systems Center, San Diego, CA.

Measurements on ceramic samples of both YBajCugOy and Bi-Sr-Ca-Cu-O have been made using a pulsed current technique by McGinnis et al. (18) and the temperature variation compared with the BCS value as obtained by Ambegaokar and Baratoff (52) Jc (TC-T)3/2. The data have yielded different interpretations that may be dependent on sample preparation. 

Pulsed-current techniques can furnish electrochemical kinetic information and have been used at the RDE. With a pulse duration of 10-4 s and a cycle time of 10-3 s, good agreement was found with steady-state results [144] for the kinetic determination of the ferri-ferrocyanide system [260, 261], Reduction of the pulse duration and cycle time would allow the measurement of larger rate constants. Kinetic parameter extraction has also been discussed for first-order irreversible reactions with two-step cathodic current pulses [262], A generalised theory describing the effect of pulsed current electrolysis on current—potential relations has appeared [263],... 

Figure 17.6 (a) Reverse normal pulse voltammogram, (b) current measured prior to the analysis pulse in (a), and (c) normal large-amplitude pulse voltammogram at a platinum electrode for a 13.4 mM solution of Ti(IV) in the 60-40 mol% AlCl3-l-methyl-3-ethylimidazolium chloride melt at 25°C. [From Ref. 68, with permission.]... 

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