Electrode angular velocity

The elimination of transport effects is not so readily achieved. One relatively simple procedure is to measure currents (/m) as a function of electrode angular velocity (co) using a rotating disc electrode. Currents free of diffusive transport effects (/k) can then be obtained by application of the Koutecky-Levich equation,... 

Hydrodynamic voltammetric techniques have the major advantage of being steady-state techniques (see Section 1). Consequently, it is easy to measure limiting currents and half-wave potentials (see below for their definition) as a function of the convective parameter (i.e. flow rate, electrode angular velocity) in the absence of significant problems arising from capacitative charging currents. 

The crucial parameter which controls the time-scale over which electrode reactions are examined at a RDE is the electrode angular velocity, o>, which is related to the rotation frequency, /, by o> = 2tj/. 

For a CE mechanism, the electrode product of interest is formed via an initial chemical reaction. Consequently, the measured limiting current will directly correlate with the amount of electroactive product formed on the time-scale of the experiment. Thus, sufficiently slow rates of mass transport result in complete conversion of bulk material to electroactive product and under this condition the limiting current will be identical to that calculated from the expressions described in Table 5 for a simple electron transfer process (see Fig. 28a). As the electrode angular velocity (a>) or flow rate (Tf) increases, less of the material reaching the electrode will have converted into... 

Let CO be the angular velocity of rotation this is equal to Inf where/is the disk frequency or number of revolutions per second. The distance r of any point from the center of the disk is identical with the distance from the flow stagnation point. The hnear velocity of any point on the electrode is cor. We see when substituting these quantities into Eq. (4.34) that the effects of the changes in distance and hnear vefocity mutuaUy cancel, so that the resulting diffusion-layer thickness is independent of distance. 

Fig. 11. Sampled-current voltammograms recorded point-by-point at a Pt-RDE for the reduction of Co(II) in the 60.0 m/o AlCh-EtMelmCl melt. The Co(II) concentrations were ( ) 5.00, ( ) 10.0, (A) 25.0, and ( ) 50.0 mmol L-1. Also shown is a voltammogram recorded in pure melt before the addition of Co(II) ( ). The angular velocity of the electrode was 104.7 rad s-1. Adapted from Mitchell et al. [44] by permission of The Electrochemical Society.

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. 

In Fig. 5-7, the amplitude and phase shift corresponding to Eq. (5-17) for different angular velocities show that in contrast with the simple behavior of a bare electrode, the data are no longer reducible by the dimensionless frequency p. In fact 0C... 

In fact, the crucial point in the set-up, common to all variants, is the electromechanical part which must deliver at the output (i.e. at the RDE) a noise free modulation of the angular velocity indeed, regarding the theoretical requirements for some applications (e.g. partial blocked electrodes), well performing motors and servosystems are necessary. 

Rotating disc electrode of angular velocity w qaid o/fis/pY1 1 Stationary (Sect. 2.8)... 

The following data are for the cathodic reduction of thyroxine (one of the iodoamino acid derivatives) at a silver electrode. A disk electrode was rotated at various angular velocities and the current was measured [see R. A. Osteryoung et al., Anal. Chem. 56 1202-1206 (1984)]. 

The limiting reaction rate expressed as a current density depends on the electrode area, A, angular velocity, to, the kinematic viscosity, v, the concentration of reactant in solution, C and its diffusion coefficient, D0. 

If the electrode is disc-shaped, imbedded in a rod of an insulating material, and rotating with an angular velocity coT (the axis of rotation goes through the center of the disc and is perpendicular to the surface), S is independent of coordinate y (see - rotating disc electrode). 

For a -> rotating disc electrode (RDE) the - convective-diffusion equations can be solved which gives the dependence of the diffusion layer thickness on the angular velocity of the rotation (on)... 

Irreversibility — Figure. Quasireversible and irreversible behavior, a Current-potential curves and b lg j -E plots (Tafel plots) for a redox system at different angular velocities of a rotating disc electrode [Pg.374]

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