Current rotating disc electrode

Beside laminar flow created by e.g. a rotating disc electrode mrbulent flow provides a means of artificially enhanced transport. A consistent mathematical description and analytical treatment of this mode of transportation is not possible. Various approximations have been proposed and tested for correctness [84Barl], an experimental setup has been described [78Ber, 83Her, 831wa]. From comparisons of measured and calculated current density vs. electrode potential relationships exchange current densities are available. (Data obtained with this method are labelled TPF.)... 

Levich124 has given the relationships between the limiting current i) and the bulk concentration C of the metal ion for plate electrodes, conical electrodes and rotated disc electrodes (RDEs) under hydrodynamic conditions anticipating his well known equations treated in Section 3.3.2.2 on hydrodynamic electrodes, we may assume the relationships concerned using the more general equation... 

Figure 2.91 Schematic representation of the rotating disc electrode response Tor the reduction and oxidation of a reversible couple. / = F/RT, /, is the limiting current, / is the current, E is the potential of the electrode and ° is the standard reduction potential of the couple.

Figure 14.1 Convection current at a rotating disc electrode.

To find that the limiting current at a rotated disc electrode (RDE) is directly proportional to the concentration of analyte, according to the Levich equation. 

Related to the rotated disc electrode (RDE) is the rotated ring-disc electrfxle (RRDE). Such an electrode is illustrated in Figure 7.9 and is seen to be, in effect, a modified RDE, insofar as the central disc is surrounded with a concentric ring electrode. The gap between the ring and the disc is filled with an insulator such as Teflon or epoxy resin. The face of the RRDE is polished flat in order to prevent viscous drag, which is itself likely to cause the induction of eddy currents. 

Convection-based systems fall into two fundamental classes, namely those using a moving electrode in a fixed bulk solution (such as the rotated disc electrode (RDE)) and fixed electrodes with a moving solution (such as flow cells and channel electrodes, and the wall-jet electrode). These convective systems can only be usefully employed if the movement of the analyte solution is reproducible over the face of the electrode. In practice, we define reproducible by ensuring that the flow is laminar. Turbulent flow leads to irreproducible conditions such as the production of eddy currents and vortices and should be avoided whenever possible. 

Provided that the flow is laminar, and the counter electrode is larger than the working electrode, convective systems yield very reproducible currents. The limiting current at a rotated disc electrode (RDE) is directly proportional to the concentration of analyte, according to the Levich equation (equation (7.1)), where the latter also describes the proportionality between the limiting current and the square root of the angular frequency at which the RDE rotates. 

The DigiSim program probably represents the current state of the art which is achievable for simulating and analysing cyclic voltammograms. This package can perform cyclic voltanunetry for a wide range of mechanisms at planar, spherical, cylindrical or rotated disc electrodes. It also computes concentration profiles. 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

We first consider the case of a rotating disc electrode, where mass transfer is particularly simple, and then go on to consider other hydro-dynamic electrodes where the situation is more complex. A summary of limiting currents calculated for various electrode geometries will be found in Table 3 (p. 384). 

One of the principal reasons for the extensive use of the rotating disc electrode is its uniform accessibility. However, if the solution conductivity is not sufficiently high, it does not have a uniform current distribution. Experimental investigations have provided criteria for the concentration of inert electrolyte necessary to add to ensure uniformity [89]. Current... 

At the rotating disc electrode, if is large, we obtain in the limiting current region [151]... 

The denominator of the hrst term of the right part represents the BV current. The denominator of the second term represents the transport-controlled current at the rotating-disc electrode this is Equation 1.41 or 1.42, where m is replaced by the right part of Equation 1.51 and all the terms, except for go1 2, are incorporated in the parameter B. 

Nature of the relation between current density and hydrogen peroxide concentration observed for different rotating-disc electrode materials with a rotation speed of 16.67 rotations per second. 

Six successively recorded current-potential curves at a platinum rotating-disc electrode with A/=16.67Hz and pH = 12.20, after pretreatment of the surface by polarisation at -0.5V vs. AglAgCI for 5 min. (Reprinted from Electrochemistry Communications, Vol 2, No 10, Gasana ef a/., Influence of changes of... pp 727-732, Copyright 2000, with permission from Elsevier.)... 

In Fig. 6.1, current-potential curves are shown of the oxidation of sodium dithionite at a platinum rotating-disc electrode in alkaline solution for dif-... 

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. 

In Section 5.9, we show how to solve the convective-diffusion equation for the rotating disc electrode in order to calculate the diffusion-limited current. When the forced convection is constant, then dc/dt = 0, which simplifies the mathematical solution. 

The method of resolution of (8.1) was indicated in Sections 5.7-5.9, showing as an example the calculation of the limiting current at the rotating disc electrode. In this chapter we discuss this and other hydrodynamic electrodes used in the study of electrode processes. The rotating disc electrode has probably been the hydrodynamic electrode... 

Hydrodynamic electrodes permit the control of the diffusion layer thickness by imposing convection. This thickess can also be modulated. Implicit functions link the current, potential and convection modulation. For the rotating disc electrode... 

Fig. 2.12. Plot of the current as a function of time for the oxidation of 4 mmol dm- 1 NADH at 0.2 V at a poly(aniline)-coated rotating disc electrode (area 0.38 cm2, deposition charge ISO mC) in 0.1 mol dm 1 citrate/phosphate buffer, pH 5. The rotation speed of the electrode was increased in the sequence I, 4, 9, 16, 25, 36 and 49Hz and reduced in sequence back to 1 Hz. The broken line connects segments of the curve corresponding to the different rotation speeds. Note The current decays more rapidly at the higher rotation speeds and responds rapidly to changes in rotation speed.

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