Rotating ring electrode, hydrodynamic

Data on the electrochemistry of the telluride ion in alkaline media are relatively limited. Mishra et al. [53] studied the oxidation of Te to Te° at solid electrodes, focusing on the intermediate step(s) of this process, and in particular, the possibility of detecting ditelluride Te via rotating ring disk electrode (RRDE) methodology. Oxidation beyond the elemental state to TeO and TeO was also studied using cyclic and hydrodynamic voltammetry. 

None of the set-ups discussed so far provides stirring of the electrolyte for bubble removal or for enhancement of the reaction rates. A standard set-up developed to study kinetic electrode processes is the rotating disc electrode [11]. The electrode is a small flat disc set in a vertical axle. The hydrodynamic flow pattern at the disc depends on rotation speed and can be calculated. An additional ring electrode set at a different potential provides information about reaction products such as, for example, hydrogen. However, because this set-up is designed to study kinetic processes and is usually equipped with a platinum disc, it becomes inconvenient if silicon samples of different geometries have to be mounted. 

Recently, Mottola [98] reported a sensor based on the disk-ring principle previously developed by Kamin and Wilson [99], and Wang and Lin [100]. Unlike Mottola s design, its forerunners involved no stationary ring electrode or rotation of the reactor part in addition, their reactor/electrode was located at the cell bottom. In Mottola s assembly, a product of an enzyme-catalysed reaction at a bioreactor rotated at a constant speed was hydrodynamically transported to a stationary ring electrode, where it was electrochemically monitored. The sample was transported to the detection imit by an tm-... 

Figure 3.39 (A) Rotating-disk electrode with hydrodynamic flow pattern. (B) Bottom view of rotating-disk electrode (RDE) and rotating ring-disk electrode (RRDE).

Fig. 9. Polarization curve of an Fe-disc Pt-split-ring electrode with hydrodynamic square wave modulation. In 1 M NaOH with anodic and cathodic scan including capacity of the Fe disc (dashed curve), modulation frequency of rotation co = 0.05 Hz (insert), simultaneous detection of Fe(II) and Fe(III) ions at Pt half rings [12].

Hydrodynamic electrodes — are electrodes where a forced convection ensures a -> steady state -> mass transport to the electrode surface, and a -> finite diffusion (subentry of -> diffusion) regime applies. The most frequently used hydrodynamic electrodes are the -> rotating disk electrode, -> rotating ring disk electrode, -> wall-jet electrode, wall-tube electrode, channel electrode, etc. See also - flow-cells, -> hydrodynamic voltammetry, -> detectors. 

If a small gas bubble is formed at the center of the rotating disc, it does not isolate the ring electrode from the solution, although it may still interfere with hydrodynamic flow at the electrode surface. 

An armoury of powerful electrochemical methods is available. Potential step techniques such as differential pulse DP or square-wave SW voltammetry offer advantages in sensitivity and resolution. Hydrodynamic techniques involving use of rotating disc or rotating ring-disc electrodes allow the chemical steps of the electrode process to be separated from mass transport. Electrochemical transformations may be monitored optically with spectroelectrochemical methods. Even the electrode interface itself is amenable to study by in situ spectroscopic techniques. Detailed descriptions of these methods are to be found in appropriate texts [1-4]. 

There are electrochemical cells of great interest where we must work with two dimensions. Examples are the rotating ring-disk electrodes, the recent microelectrodes (point- or band-) or even arrays of these. Hydrodynamic voltammetry (Levich 1962) continues to attract attention. 

HMRRDE hydrodynamically modulated rotating-ring-disk electrode... 

What sort of instrumentation would be needed for electrochemical experiments A potentiometry experiment requires little more than a pH meter. A potentiostat or galvanostat can be used for the controlling potential or current in an experiment. In a coulometric procedure, a device to integrate the current (i.e., a coulometer) would also be needed. A hydrodynamic voltammetry [e.g., a rotating disk electrode (RDE)] experiment would require an electrode rotor (to spin the electrode at a precisely known rotation speed), and the rotating ring-disk or RRDE refinement (see below) would necessitate the use of a bipotentiostat so that the disk and ring potentials can be independently controlled. An ac impedance measurement involves the use of a sine-wave oscillator and... 

Forced convection (hydrodynamic) generator - collector systems are commonly employed in rotating ring-disc or wall-jet geometries or in channel flow cells to improve collection efficiencies. For macroscopic interelectrode gap systems hydrodynamic agitation can be employed to improve feedback, but for diffusion - dominated nano-gap electrode systems hydrodynamic convection effects usually remain insignificant, whereas heating can be used to enhance the rate of diffusion processes and therefore to improve feedback currents. 

RRDE voltammetry is designed to provide inherent analysis of the reaction selectivity instead of relying on complex calculations and data plotting techniques such as K-L plot. With electrode rotation, the electrolyte will flow the same way as in RDE experiment. Thus, while the reaction is occruring on the catalyst surface, reactants will continuously be swept away by hydrodynamic flow. This flow will traverse along the electrode surface, passing over the ring electrode. 

In a typical experiment with semiconductor-liquid junctions, one of the most important experimental problems is the differentiation between reactions that involve chemical changes at the semiconductor electrode (corrosion with insoluble products) and chemical changes in the electrolyte that might be subject to mass transfer limitations. The technique of Rotating Ring Disc Electrode (RRDE) (17-19) provides an opportunity to differentiate between these two types of reactions under controlled hydrodynamic conditions. In its simplestform, the metallic ring is isolated... 

Hydrodynamic techniques include measurements with stirred solutions or electrodes (rotated electrode and rotated ring-disc electrode) and measurements in flowing streams of a carrier. 

The diffusion layer widtli is very much dependent on tire degree of agitation of tire electrolyte. Thus, via tire parameter 5, tire hydrodynamics of tire solution can be considered. Experimentally, defined hydrodynamic conditions are achieved by a rotating cylinder, disc or ring-disc electrodes, for which analytical solutions for tire diffusion equation are available [37, 4T, 42 and 43]. 

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