Hemisphere electrode

Since the Wenner formula [Eq. (24-41)] was deduced for hemispherical electrodes, measuring errors appear for spike electrodes. To avoid errors in excess of 5%, the depth of penetration must be less than a 5. Soil resistivity increases greatly under frost conditions. While electrodes can be driven through thin layers of frost, soil resistivity measurements deeper than 20 cm in frozen ground are virtually impossible. 

Theory and Experimental Aspects of the Rotating Hemispherical Electrode... 

The rotating hemispherical electrode (RHSE) was originally proposed by the author in 1971 as an analytical tool for studying high-rate corrosion and dissolution reactions [13]. Since then, much work has been published in the literature. The RHSE has a uniform primary current distribution, and its surface geometry is not easily deformed by metal deposition and dissolution reactions. These features have made the RHSE a complementary tool to the rotating disk electrode (RDE). 

The above problems may be alleviated by the use of a rotating hemispherical electrode (RHSE). In this geometry, the flat circular disk on the RDE is replaced with a metal hemisphere as shown in Fig. 1(a). The theory, experimental setup and methods of application to electrochemical studies are similar to those of the RDE. The advantages of the RHSE are ... 

This article presents a brief account of theory and practical aspects of rotating hemispherical electrodes. The fluid flow around the RHSE, mass transfer correlations, potential profile, and electrochemical application to the investigations of diffusivity, reaction rate constants, intermediate reaction products, passivity, and AC techniques are reviewed in the following sections. 

Fig. 1. Flow near a rotating hemisphere electrode. (a) Dye movement at Re = 1300. (b, c) Spiral flow patterns etched on a copper hemisphere.

To consider the convective mass transfer problem of a rotating hemisphere electrode, we assume that sufficient inert salts are present in the electrolyte that the migrational... 

In electrochemistry, spherical and hemispherical electrodes have been commonly used in the laboratory investigations. The spherical geometry has the advantage that in the absence of mass transfer effect, its primary and secondary current distributions are uniform. However, the limiting current distribution on a rotating sphere is not uniform. The limiting current density is highest at the pole, and decreases with... 

Fig. 7. Current distribution on a rotating hemispherical electrode at high rotational speed. From [47].

The experimental setup of a rotating hemispherical electrode (RHSE) is similar to that of a rotating disk electrode [50]. The basic system consists of a removable hemispherical electrode, and a variable speed rotator equipped with a provision, such as the slip-ring contact, to make electric connection to the hemispherical electrode during the experiments. 

Fig. 8(a) shows the design of a rotating hemisphere electrode used in the author s laboratory [14], It is composed of a hemisphere electrode, an arcylic support rod, and a tappered brass holder to be attached to a high speed rotator [Fig. 8(b) ]. The electrode is machined into the form of a metal screw with a hemispherical head, and is threaded into the inert acrylic support rod of a larger radius. The design has the advantages that... 

Fig. 8. Construction of a rotating hemisphere electrode (a) and cell setup (b). From [14].

The electric connection to the electrode is made by connecting a copper wire from the brass holder to the threaded portion of the hemisphere electrode. The brass holder is machined to fit snuggly into the steel shaft of a rotator. The rotation of the electrode is provided by a timing pulley connected to a variable speed DC motor. A graphite slipring contact located on the top of the shaft is used to provided electric contact to the RHSE during the experiments. 

Fig. 9. Types of rotating spherical electrodes reported in the literature, (a) Rotating micro-sphere electrode, (b, c) rotating hemisphere electodes (d) rotating ring-hemisphere electrodes (e) rotating dropping mercury electrode.

The hemispherical electrode may be coupled with a ring [20] to form a rotating ring-hemisphere electrode (RRHSE) as shown as Fig. 9(d). The ability of this combination to detect intermediate reaction products is demonstrated in Fig. 10, where a series of cathodic sweep curves for the reduction of Cu2 + in acidic cupric chloride solution are... 

The rotating ring-hemisphere electrode has been used by Chin [21] to study the dissolution of iron in neutral sulfate solutions, and by Zou and Chin [61, 62] to identify the corrosion products of iron in concentrated sodium hydroxide solutions. 

The rotating hemisphere electrode has been used to investigate the effect of AC on the electrodissolution and deposition reactions of zinc in zinc chloride [25] and copper in acid copper sulfate solutions [55], AC was found to increase the rate of nucleation and produce more uniform deposit on the zinc electrode. The corrosion of an iron rotating hemisphere in dilute sulfuric acid was investigated by Haili [31] using the AC impedance measurement. 

With its axisymmetric transport and current distribution, the rotating hemispherical electrode complements the rotating disk as a tool for studying electrode processes. Der-Tau Chin provides a valuable overview and summary of the fundamental theory and applications of this interesting device. 

Davis et. al. (64) have calculated the steady-state thin-layer current component for a series of electrode geometries. In their derivation, these authors have assumed that the flux between the electrodes is one-dimensional (perpendicular to the plane). Particularly relevant to the STM geometry are the equations for the current in a conical electrode/planar electrode TLC, Icon, and those for a hemispherical electrode/planar electrode TLC, Xhsph (64> ... 

In the above equations, a is the conical aspect ratio, r/h 7 is the ratio of the cone or hemisphere radius to the interelectrode distance, r/d and I, the dimensionless faradaic current (either Icon or hsph > t e rati° between the one-dimensional current contribution, ifLC ant t le limiting current for an isolated hemispherical electrode, i gph (see Eq. 5)(64) ... 

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