Vibrating electrode method

Fig. IV-6. Vibrating electrode method for measuring surface potentials. (From Ref 1.)...

After the charging the initial surface potential Vq of the samples was measured by the vibrating electrode method with compensation (Reedyk et al.). The measured surface potential values are presented in Table 2. The results obtained show that the charging time to 30 minutes does not influence the surface potential of the PP films. 

In contrast to the ionizing electrode method, the dynamic condenser method is based on a well-understood theory and fulfills the condition of thermodynamic equilibrium. Its practical precision is limited by noise, stray capacitances, and variation of surface potential of the air-electrode surface, i.e., the vibrating plate. At present, the precision of the dynamic condenser method may be limited severely by the nature of the surfaces of the electrode and investigated system. In common use are adsorption-... 

Fig. 4-28. Schematic layout of Kelvin s vibrating capacitor method to measure relative electrode potential of (a) electrode immersed in aqueous solution and of (b) electrode emersed from solution KP = Kelvin s probe 4 s = outer potential of aqueous solution 4>kp = outer potential of Kelvin s probe V and = applied voltages to cancel out a difference in the outer potential.

In heterogeneous systems, potential differences exist across the various phase boundaries. The surface film potential, AV, due to a monolayer is the change in the potential difference between the bulk substrate liquid and a probe placed above the liquid which results from the presence of the monolayer. Surface film potentials can be measured by air-electrode and vibrating-plate methods. 

The two metals act as the plates of a condenser, one of the plates being used for adsorption of the gas and the other being a reference electrode. Adsorption of gas causes a change in contact potential equal to the change in work function. Disappearance of the electrical field between the two plates may be detected either by the steady condenser method or the vibrating condenser method ... 

Fig. 48. Surface potential measurements by the vibrating plate method. The oscillator (Osc.) and amplifier (Audio amp.) are commercial units which drive the electrode. The oscilloscope (Scope) and high gain, low noise amplifier (Amp.) are also standard commercial instruments.

Recently, two new electrochemical mapping techniques have become available the scanning vibrating electrode technique (SVET) and the localized electrochemical impedance spectroscopy (LEIS) technique. These techniques provide the capability to identify and monitor electrochemical behavior down to the micron level. These represent significant advances over traditional electrochemical methods (cyclic voltammetry, EIS, and even EQCM), which provide data that reflect only an average over the entire sample surface. Although such data are very useful, a major drawback is that no local or spatial information is obtained. 

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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