Electrodes rotator

A number of approaches are available to improve the morphology and homogeneity of electrochemically deposited conducting polymer films. Priming of the electrode surface with a monolayer of adsorbed or covalently bonded monomer leads to more compact deposits of polyaniline,87,88 poly thiophene,80 and polypyrrole.89,90 Electrode rotation has been shown to inhibit the deposition of powdery overlayers during poly(3-methylthiophene) deposition.81... 

One also needs to be careful when using the slope of the Koutecky-Levich plot to determine av of the catalytic film. Examples of metaUoporphyrin-catalyzed ORR have been reported where, above a certain value of the electrode rotational frequency, the catalytic currents became independent of Koutecky-Levich model, either because the rate of charge or substrate transfer within the film became rate-limiting or the catalyst became partially samrated with O2 [Boulatov et al., 2002 Song et al., 1998 CoUman et al., 1980]. In other cases, the versus graphs may remain mostly linear within the experimental... 

The simplest and most commonly used convection apparatus consists of a disc electrode rotating with a constant angular velocity u [1-5]. The disc sucks the solution toward its surface, much in the way a propeller would as the solution approaches the disc, it is swept away radially and tangentially (see Fig. 14.1). The transport of the reacting species to the disc occurs both by convection and diffusion. Though the mathematics are complicated, the rate of transport can be calculated exactly for an infinite disc. A particularly nice feature of this setup is the fact that the transport is uniform so that the surface concentration of any reacting species is constant over the surface of the electrode. 

Despite the importance of the ORR and long history of study, very little is known about the reaction mechanism.126,130,131 Mechanistic information has been derived almost exclusively from rotating disk electrode (RDE)131,132 and rotating ring disk electrode (RRDE)133-136,62,128 studies. The rotating electrode minimizes mass transfer effects and allows a kinetic current density to be extracted. In the RRDE setup, the ring surrounding the disk electrode detects species weakly adsorbed to the electrode that are ejected due to electrode rotation. The ORR reaction (eqn 4) is... 

Eligh performance liquid chromatography Density functional theory Oxygen reduction reaction Rotating disk electrode Rotating ring disk electrode... 

An important parameter for observing the role of diffusion is the electrode rotation rate. Figure 5.20 gives an experimental example of... 

Part—III exclusively treats Electrochemical Methods invariably and extensively used in the analysis of pharmaceutical substances in the Official Compendia. Two important methods, namely potentiometric methods (Chapter 16) deal with various types of reference electrodes and indicator electrodes, automatic titrator besides typical examples of nitrazepam, allopurinol and clonidine hydrochloride. Amperometric methods (Chapter 17) comprise of titrations involving dropping-mercury electrode, rotating—platinum electrode and twin-polarized microelectrodes (i.e., dead-stop-end-point method). 

The characteristic shape of the anodic voltammogram of a Si electrode in aqueous fluoride media, as shown for example in Fig. 3. Id, is surprisingly stable against changes in fluoride concentration (cF) or pH. When the potential of a p-type Si electrode is swept anodic of OCP a steep current rise near 0 V is observed, followed by a sharp peak (Jj) and a narrow plateau (J2). Then a second broad maximum (Ji) is found around a positive bias of 1.5-2.5 V, followed by a broad plateau (J4) extending over several volts, as shown in Fig. 4.7. When electrode rotation is used, these curves are pen-reproducible for a given solution. The hysteresis of the curves approaches zero for slow sweeps [Ch3]. 

Two types of cell have been described. In Fig. 1 a cell with a rotating disk electrode is shown. Connections to a pH-stat and to the interface are indicated simultaneously with recording CMT measurements, the metal and reference electrode and a counter electrode (not shown in Fig. 1) can be connected to a potentiostat, so that electrochemical measurements can be recorded intermittently. The volume of solution in the cell is ca. 400 ml. What matters for safe and reliable conditions of measurement is that the disk electrode rotating at a speed of no less than 1000 rpm ensures efficient stirring, so that the effect of alkali formed at the corroding metal (or sometimes at the counter electrode located ca. 1 cm below and parallel to the metal electrode) is immediately sensed effectively by the glass electrode and also the effect of acid supplied from the autoburet is quickly detected. 

In Section 7.2, we looked at electroanalytical systems where the electrode rotates while the bulk of the solution remained still. In this present section, we will reverse this experimental concept by considering the case where it is the solution which flows - this time past a stationary electrode. Here, we shall be looking at flow ceils and channel electrodes. The principal mode of mass transport in both cases is convection, since the solution moves relative to the electrode. 

Liquid flow pattern induced by electrode rotation... 

Rotating platinum screens have been used for electrogravimetric analyses. Commercial instruments employ two concentric cylindrical platinum screen electrodes with one or both electrodes rotating to increase convection. The cell itself is usually a beaker with a sample volume of about 150 mL. Typically, no cell top is used, and when running multiple analyses, there should be adequate ventilation to prevent accumulation of hydrogen. A simple operational amplifier circuit can be constructed for the instrument. The commercial instruments available are expensive given the simplicity of this type of experiment. 

Fig. 7.3 Dependence of the current density on the overvoltage for electrodes rotating at 500 rpm (top curve) and 1,000, 2,000, and 4,000 rpm (bottom curve) under 1 atm O2 at 37°C. Scan rate 5 mV s 1. Left polished Pt in O.5MH2SO4. Right wired billirubin oxidase (BOD) on polished vitreous carbon in a pH 7.2 saline (0.15M NaCl) phosphate (0.02 M) buffer. The wired BOD film, loaded at 0.17 mg cm-2, comprised 32.3 wt% BOD, 60.2 wt% redox polymer, and 7.5 wt% PEGDGE (adapted from Mano, 2003)...

Relationship between pseudo-limiting-current of the pre-wave at = 0.45 V vs. SCE and the hydrogen peroxide concentration for various pH values, recorded at a glassy carbon electrode rotating at 16.67 Hz. The numbered curves correspond to pH values of (1) 10, (2) 11, (3) 12, (4) 13 and (5) 14 other curves correspond to each increment of 0.2 pH units. 

Finally, the pFI dependency of the current signals was investigated. Voltammetric curves were recorded obtained at a platinum rotating-disc electrode for different pFI values in the 11.65-12.95 range at constant electrode-rotation rate. These experiments were repeated at other sodium dithionite concentrations. It was found that the measured current in all three regions of the voltammetric waves did not vary with pH. 

Figure 6.11 shows the result of such a chronoamperometric experiment, where a constant potential of 0.45V vs. AglAgCl is applied to the platinum electrode rotating at 6.67Hz in a pH of 12.5. The sodium dithionite concentration was increased in ten consecutive steps. It can be seen that the limiting-current corresponds to the values obtained with linear-sweep... 

A rotating disc electrode (RDE) is a conductive disc of the material of interest embedded in an inert non-conductive polymer or resin that can be attached to an electric motor which has very fine control of the electrode s rotation rate. During the experiment, the electrode rotates in the solution under study, thus inducing a flux of redox analyte to the electrode [75]. 

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