Other Solid Electrodes

Other solid electrode materials used are semiconductors, for example metal oxides12,13, and conducting organic salts14. These last are of much interest at present for the immobilization of organic compounds such as enzymes, given their compatibility with these macromolecules (Chapter 17). 

Electrochemical studies indicate that MB adsorbs on Hg, Au, Pt, and other solid electrodes in horizontal orientation with primary interactions with electrode coming from aromatic rings. In contrast to that, on S-modified Au electrodes, well organized MB films with vertical orientation were observed. This is attributed to the formation of disulfide bonds with S adatoms. These... 

In a recent report, it was demonstrated that adsorption of 4,4 -bipyridine on platinum led to quasi-reversible rates of electron transfer with cytochrome c as evidenced by cyclic voltammetry. However, the concentration of 4,4 -bipyridine required to produce this electrochemical response was five times that which is required at gold electrodes. This difference was ascribed to the difference in the tendency of 4,4 -bipyridine to adsorb at gold and platinum electrodes. These results indicate that the use of 4,4 -bipyridine may be applicable to other solid electrodes as well for the study of cytochrome c electron transfer reactions. 

When the source of material is a solid electrode, it is consumed under current. Therefore the lifetime of the electrode is limited. In S S E, care must be taken to press the electrode onto the SE continuously. At the other solid electrode, space must be left for the electrode to grow. In contrast, in LSE the LE stays always in good contact with the electrode. Furthermore, when the source of material is the LE, then material can be supplied indefinitely since the LE can be replenished. The geometry of LSE cells closest to all solid cells is when the LE is soaked in a porous solid. 

Other solid electrodes include tantalum and silver (for copper) and nickel (for zinc). Silver anodes prevent evolution of oxygen (which could be reduced at the cathode), and silver cathodes prevent evolution of hydrogen, with associated local pH changes. Addition of chloride allows the anode reaction (eqn. [1]) Ag -I- Cr -> AgCl -I- e [1]... 

Recipe for a silver amalgam electrode polish the Ag wire electrode (ca. 1 mm diameter) with fine alumonum oxide powder. Reduce at —1.0 V(vs. SCE) for 3 minutes ih a cell containing nitric acid (pH 1.5) and a drop of mercury. While still holding the potential, touch the electrode surface to the Hg drop. Rinse several times with deionized water immerse in deionized water for 50 minutes. Polish with aluminum oxide powder and rinse again. The electrode can be used or months, like any other solid electrode. 

With the help of solid electrodes numerous oxidation processes can be conveniently followed in voltammetry (or cyclic voltammetry) or organic compounds. This is why electrochemical detectors in chromatography often make use of platinum, carbon and other solid electrodes. 

Various in situ and ex situ methods have been used to determine the real surface area of solid electrodes. Each method10,15 32 67,73 74 218 is applicable to a limited number of electrochemical systems so that a universal method of surface area measurement is not available at present. On the other hand, a number of methods used in electrochemistry are not well founded from a physical point of view, and some of them are definitely questionable. In situ and ex situ methods used in electrochemistry have been recently reviewed by Trasatti and Petrii.73 A number of methods are listed in Table 3. 

Solid-state electronic devices such as diodes, transistors, and integrated circuits contain p-n junctions in which a p-type semiconductor is in contact with an n-type semiconductor (Fig. 3.47). The structure of a p-n junction allows an electric current to flow in only one direction. When the electrode attached to the p-type semiconductor has a negative charge, the holes in the p-type semiconductor are attracted to it, the electrons in the n-type semiconductor are attracted to the other (positive) electrode, and current does not flow. When the polarity is reversed, with the negative electrode attached to the n-type semiconductor, electrons flow from the n-type semiconductor through the p-type semiconductor toward the positive electrode. 

In a simple version of a fuel cell, a fuel such as hydrogen gas is passed over a platinum electrode, oxygen is passed over the other, similar electrode, and the electrolyte is aqueous potassium hydroxide. A porous membrane separates the two electrode compartments. Many varieties of fuel cells are possible, and in some the electrolyte is a solid polymer membrane or a ceramic (see Section 14.22). Three of the most promising fuel cells are the alkali fuel cell, the phosphoric acid fuel cell, and the methanol fuel cell. 

Several methods exist that can be used to measure changes of ESE for solid electrodes as a function of potential or other factors, but the accuracy of such measurements is much lower than that for Uquid electrodes. A plot of ESE vs. potential is called the electrocapillaty cutye (ECC). Typical ECCs measured at a mercury electrode in NaF solutions of different concentration are shown in Fig. 10.6. Also shown in this figure is a plot of values vs. potential calculated via Eq. (10.27). This plot almost coincides with that obtained from capacitance measurements (Fig. lO.lfc). This is evidence for the mutual compatibility of results obtained by these two methods of measurement. 

The reactions occurring at reacting metal electrodes are associated with structural changes lattice destruction or formation of the metal and, in certain cases, of other solid reaction components (oxides, salts, etc.). One should know the metal s original bulk and surface structure in order to analyze the influence of these structural changes. 

Johans et al. derived a model for diffusion-controlled electrodeposition at liquid-liquid interface taking into account the development of diffusion fields in both phases [91]. The current transients exhibited rising portions followed by planar diffusion-controlled decay. These features are very similar to those commonly observed in three-dimensional nucleation of metals onto solid electrodes [173-175]. The authors reduced aqueous ammonium tetrachloropalladate by butylferrocene in DCE. The experimental transients were in good agreement with the theoretical ones. The nucleation rate was considered to depend exponentially on the applied potential and a one-electron step was found to be rate determining. The results were taken to confirm the absence of preferential nucleation sites at the liquid-liquid interface. Other nucleation work at the liquid-liquid interface has described the formation of two-dimensional metallic films with rather interesting fractal shapes [176]. 

In terms of understanding the mercury/electrolyte interface, it is clear from the above discussion that the measurement of the surface free energy (in terms of the surface tension), is central. If the clectrocapillarity technique could be applied to solid electrodes, then it is capable of supplying information extremely difficult to obtain by any other technique. Sato has indeed developed a technique to measure the surface tension of a metal electrode which he terms piezoelectric surface stress measurement and is based upon the previous work of Gokhshtein (1970). 

Although a few amperometric pH sensors are reported [32], most pH electrodes are potentiometric sensors. Among various potentiometric pH sensors, conventional glass pH electrodes are widely used and the pH value measured using a glass electrode is often considered as a gold standard in the development and calibration of other novel pH sensors in vivo and in vitro [33], Other pH electrodes, such as metal/metal oxide and ISFETs have received more and more attention in recent years due to their robustness, fast response, all-solid format and capability for miniaturization. Potentiometric microelectrodes for pH measurements will be the focus of this chapter. 

---