Clay electrode

S. Kroning, F.W. Scheller, U. Wollenberger, and F. Lisdat, Myoglobin-clay electrode for nitric oxide (NO) detection in solution. Electroanalysis 16, 253—259 (2004). 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

An innovative technology called the "lasagna" process is based on the electrokinetic phenomenon called electro osmosis. The lasagna process was created to treat difficult wastes in low permeabiUty, sdt- and clay-laden soils (40). The lasagna process is so named because it consists of a number of layered subsurface electrodes and treatment zones. These layers can be constmcted either horizontally where contaminants are forced to more upward or in vertical position where lateral contaminant movement is desired. 

In 1891, a small amount of siUcon carbide was produced bypassing a strong electric current from a carbon electrode through a mixture of clay and coke contained in an iron bowl that served as the second electrode (1). The abrasive value of the crystals obtained were recognized and The Carbomndum Company was founded that year (2). About 10 years earlier tetratomic radicals of siUcon (Si2C202, Si2C2N) had been reported (3). That work also produced some SiC. 

With four-electrode measurements effected from the surface, an average soil resistivity over a larger area is obtained. The resistivity of a relatively localized layer of earth or pocket of clay can only be accurately measured by using a spike electrode. Figure 3-18 gives dimensions and shape factors, Fg, for various electrodes. 

Practical examples of using preconcentrating CMEs include the use of a mixed 2,9-dimcthyl-1.10-phenanthrolinc/carbon-pastc electrode for trace measurements of copper (55), the use of clay-containing carbon pastes for voltammetric measurements of iron (56), the use of polyelectrolyte coatings for the uptake and... 

The clay mineral bentonite (sodium montmorillonite) has an excellent ion exchange and adsorption capacity. Films can be applied to electrode surfaces from colloidal clay solutions by simple dip or spin coating that become electroactive after incorporation of electroactive cations or metal particles 136-143)... 

This paper discusses the use of specific ion electrodes for determining the anion-free water. This method is simpler and more accurate at low electrolyte concentration than ordinary chemical methods. It is potentially useful for oilfield application and laboratory automation. The mobility of this water is also examined under forced conditions with pressure gradients. It is expected that by using the methods developed in this paper, one may obtain a better understanding of the clay properties. 

Chitosan-clay bio-nanocomposites are very stable materials without significant desorption of the biopolymer when they are treated with aqueous salt solutions for long periods of time. In this way, they act as active phases of electrochemical sensors for detection of ions (Figure 1.8). The particular nanostructuration of the biopolymer in the interlayer region drives the selective uptake of monovalent versus polyvalent anions, which has been applied in electrode arrays of electronic tongues [132]. 

Clays are usually cation-exchangeable aluminosilicates, and exfoliated clay particles have a platelet shape with nanoscopic size. Cast protein-clay films on electrodes have been used to immobilize proteins. The Clay/Mb electrode has good electrocatalytic properties for the reduction of oxygen and hydrogen peroxide [236] and the biosensors can also be made based on these properties. 

Y. Sallez, P. Bianco, and E. Lojou, Electrochemical behavior of c-type cytochromes at clay-modified carbon electrodes a model for the interaction between proteins and soils. J. Electroanal. Chem. 493, 37-49 (2000). 

Wunsche An electrolytic process for liberating bromine from a bromide solution. It uses carbon electrodes and a porous clay separator. Developed in Germany in 1902. See also Kossuth. 

Fouling of the reference electrode or the reference side of a combination electrode is a common problem in soil pH measurements. Fouling can be caused by salts, organic matter, and clay. Each electrode manufacturer will provide specific cleaning procedures that help to keep electrodes functioning properly however, in many cases, no amount of cleaning is effective and the electrodes need to be replaced. 

Clay is just one example of a material used to modify the electrochemical properties of electrodes to form a chemically modified electrode (CME) (as described belovt/). A porous-clay CME has an area of 5 cm, and charging the double-layer requires a charge of 1.43 C per square centimetre. Repeat the calculations shown above in Worked Example 5.3 to determine the respective faradaic efficiencies. 

Following on from SAQ 5.8, why would anyone employ a clay-modified electrode for an electrochemical analysis ... 

At first sight, a clay-modified electrode is an unlikely choice since its capacitance is so large. However, the area of such electrodes is usually huge (see below), and hence the large capacitance. 

So why have a clay-modified electrode at all Why not just have an ordinary electrode, but one which is made of a fine wire mesh ... 

Many analyses are performed with mesh electrodes, thereby increasing the surface area. However, there is an additional reason for using a clay-modified electrode. We saw above (in Worked Example 5.2) that electrode side reactions are a common enemy of accurate electroanalytical work. 

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