Rotating-disc electrode limiting current

Figure 8. Voltammetry at rotating disc electrode. Limiting-current dependence upon rotation speed for typical reversible couple (taken from ref. 8 with permission).

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.

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. 

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. 

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). 

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

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... 

Finite diffusion — Finite (sometimes also called -> limited) diffusion situation arises when the -> diffusion layer, which otherwise might be expanded infinitely at long-term electrolysis, is restricted to a given distance, e.g., in the case of extensive stirring (- rotating disc electrode). It is the case at a thin film, in a thin layer cell, and a thin cell sandwiched with an anode and a cathode. Finite diffusion causes a decrease of the current to zero at long times in the - Cottrell plot (-> Cottrell equation, and - chronoamperometry) or for voltammetric waves (see also - electrochemical impedance spectroscopy). Finite diffusion generally occurs at -> hydrodynamic electrodes. 

Consider a series of steady state current-potential measurements with, say, a rotating disc electrode, supplemented with determination of and from the sudden jump and the following linear rise of potential with time, observed after application of a very short current-step pulse. If we consider this from the point of view of the information content, we realize that in these experiments we have, in effect, measured each quantity when its information content was unity, or very close to it. This procedure yields the best results, but it is limited to relatively slow reactions. Thus, we could say that the concept of... 

Figure 4-1. Protein film voltammetry as a technique for studying redox enzyme mechanisms. The catalytic current-potential profile provides information on the rate-defining catalytic processes occurring within the enzyme. It is important that interfacial electron transfer is facile and information is not masked by limitations due to tlie transport of substrate and product for this reason the rotating disc electrode is an important tool in these studies.

The anodic limiting current in lithium salt solutions is determined by the diffusion of the solvated electrons to the electrode. This was quantitatively established by the measurements taken on rotating disc electrodes and also by galvanostatic measurements In fact, as seen from Fig. 8, the limiting current density is proportional to the square root of the disc electrode rotation rate. This, in accordance with the rotat-... 

Fig. 10. Relationship between the logarithm of limiting anodic currents ij (curve 1) and ij (curve 2) and the equilibrium potential of a platinum rotating disc electrode (960 rpm) in hexamethylphosphotriamide solutions of solvated electrons against a background of 0.3 M NaClO. Temperature 5.5 °C (166)...

The limiting current, iL, of the rotating disc electrode is given by the Levich equation [1]... 

At the RDE, mass transport to the electrode is varied by altering the disc rotation speed (W/Hz). The consequences of this on the current-voltage relationship, for the two reactions, are shown schematically in Fig. 1. If the E step is considered to be electrochemically reversible, then the voltam-metric wave is defined by the two parameters Elj2 (the half-wave potential) and Jim (the transport-limited current), as depicted. It is the dependence of these two quantities on the disc rotation speed which allows the deduction of the mechanism, as shown in Fig. 1. For a kinetically uncomplicated reversible electrode reaction, JLIM varies as W112 [2] and EV2 is independent of W [3]. For a CE process, at fast rotation speeds the limiting current is... 

Because of the very limited escape depths of conversion electrons (about 1.8 pm in water, 0.25 pm in metallic iron), their detection is somewhat difficult. This seeming drawback provides a unique surface sensitivity. In a rotating disc electrode arrangement Kordesch et al. [539] have used a disc-shaped electrode that slowly rotates with part of the disc immersed in the electrolyte solution. As a thin electrolyte film thin enough to permit escape of conversion electrons adheres to the metal surface, potential control is always maintained. Conversion electrons were detected using a suitable gas-filled detector mounted close to the upper emersed part of the disc. In a study of passive oxide films on iron, the advantage of this approach was demonstrated beyond an unmatched surface sensitivity, the measurement time was reduced to a small fraction of that needed for transmission measurements [543]. An inherent drawback of the setup is the poor current distribution inside the very thin electrolyte film (its thickness is around 4 nm as reported by Gordon [540]). 

Figure 5.3 Current-potential dependence of a diffusion limited reduction process (deposition of Ag, c = 10 mol-dm D = 1.6 X 10 cm s ), diffusion limited currents on a rotating-disc electrode, corresponding diffusion layer thicknesses...

Figure 5.23 Dependence of the limiting current on the square root of the rotation frequency (rotation per s) on a rotating-disc electrode.

The crucial parameter for the rotating disc electrode is the collection efficiency N. This determines the fraction of stable product generated with 100% efficiency at the disc electrode that will be collected by the ring, under mass transport control, when there is a mass transport-limiting ring current. In general, under these conditions. 

The transpassive dissolution of a stainless steel rotating disc electrode in concentrated acid reveals an anodic limiting current that varies with the rotation rate ... 

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