Vibrating wire electrode

K. W. Pratt and D. C. Johnson, The Vibrating Wire Electrode as am Am-perometric Detector for Flow Injection Systems. Anal. Chim. Acta, 148 (1983) 87. 

In the VEP, currents used are between 600 and 1200 A at potentials between 30 and 60 V. The vibration frequency of the wire electrode is up to 500 Hz. The materials atomized via VEP include mild steel, Cr-Ni steel, Cu-Ni alloy and tungsten. The VEP is carried out in an inert atmosphere (typically argon) for most alloys, but the arc is struck under water for tungsten wire. Wire diameter is 1-4 mm, and its feed rate is 1.7-4.3 m/min. The feed rate and current density must be determined properly according to the relationship between these two variables. At lower current densities, the wire electrode tends to stick to the rotating electrode. At higher current densities, the wire electrode becomes overheated, causing it to bend or even rupture. 

Section 10.4.2 gives a detailed description of the behaviour and elec-troanalytical use of vibrating wire and band electrodes. In this section, some general remarks are given to link this discussion with the information presented above. 

Local impedance measurements represent another form of generalized transfer-function analysis. In these experiments, a small probe is placed near tiie electrode surface. The probe uses either two small electrodes or a vibrating wire to allow measurement of potential at two positions. Under the assumption that the electrolyte conductivity between the two points of potential measurement is uniform, the current density at the probe can be estimated from the measured potential difference AVprobe by... 

The work functions of ITO, PEDT, and PAni layers were determined using a scanning Kelvin probe (SKP, UBM Messtechnik GmbH) in a chamber equipped with silica gel giving a relative humidity of 0%. A Cr or Ni wire with a tip diameter of 80 p,m was used as a vibrating reference electrode. The tip was positioned about 20 p,m above the specimen, the vibration amplitude was +10 p,m, and the vibration frequency of the needle was 1.75 kHz. As measurements could not be performed in ultrahigh vacuum, gold was used as reliable reference material. 

A rather simple interpretation of the behaviour of vibrating electrodes can be obtained by considering the response to a square-wave motion, to which a sinusoid rather crudely approximates [33]. Here, it is considered that the concentration boundary layer is periodically renewed by the instantaneous rapid motion and that in the intervals between the square-wave steps the solution is at rest. This is a reasonable approximation for most practical purposes because the hydrodynamic boundary layer relaxation time is short, (Section 10.3.3). In this simple model, the waveform would instantaneously rise to a limit during the motion, decaying as a function of t m during the static phase. This decay rate will obviously be dependent on the size and geometry of the electrode wire, microwire, band or microband. If the delay time between steps were r then the mean current would vary as (l/r,)/o f 1/2df, i.e., as t, i/2 or as fm. 

For sufficiently large electrodes with a small vibration amplitude, aid < 1, a solution of the hydrodynamic problem is possible [58, 59]. As well as the periodic flow pattern, a steady secondary flow is induced as a consequence of the interaction of viscous and inertial effects in the boundary layer [13] as shown in Fig. 10.10. It is this flow which causes the enhancement of mass-transfer. The theory developed by Schlichting [13] and Jameson [58] applies when the time of oscillation, w l is small in comparison with the time taken for a species to diffuse across the hydrodynamic boundary layer (thickness SH= (v/a>)ln diffusion timescale 8h/D), i.e., when v/D t> 1. Re needs to be sufficiently high for the calculation to converge but sufficiently low such that the flow does not become turbulent. Experiment shows that, for large diameter wires (radius, r, — 1 cm), the condition is Re 2000. The solution Sh = 0.746Re1/2 Sc1/3(a/r)1/6, where Sh (the Sherwood number) = kmr/D and km is the mass-transfer coefficient,... 

Solid sodium chloride does not conduct electricity. Its ions vibrate about fixed positions, but they are not free to move throughout the crystal. Molten (melted) NaCl, however, is an excellent conductor because its ions are freely mobile. Consider a cell in which a source of direct current is connected by wires to two inert graphite electrodes (Figure 21-2 a). They are immersed in a container of molten sodium chloride. When the current flows, we observe the following. 

The measuring cell was made of PTFE and had a four-electrode system connected to its outer cylinder. One pair of electrodes served as terminals for the measuring amplifier, and the outer pair was connected to the current supply through a 10-MO wire-wound resistor (the sample resistance was much less than 10 MQ). The measuring cell was carefully shielded from electric and magnetic fields and from mechanical vibrations. 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

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