Contamination redox electrode

In general, low level detection is masked by the noise level inherent in any measuring device. Electrochemical methods are susceptible to electrical interference from external sources, variations in reference electrode parameters resulting from aging or contamination, and interference from redox... 

A comprehensive work on the electrodeposition chemistry and characterization of anodically synthesized CdTe thin films has been presented by Ham et al. [98]. In this work, along with the electrolytic anodic synthesis of CdTe by using Cd anodes in alkaline solutions of sodium telluride, an electroless route of anodizing a Cd electrode held at open circuit in the same solution was also introduced. The anodic method was expected to produce CdTe with little contamination from Te on account of the thermodynamic properties of the system the open-circuit potential of Cd anodes in the Te electrolyte lies negative of the Te redox point, so... 

Although the detailed mechanism of electron transport and transfer involving PVF electrodes may be complex, they show remarkable stability and rapidly reversible redox behaviour in non-aqueous solvents such as acetonitrile. This has led to the suggestion69 that they might function as standard electrodes for non-aqueous solvents. Such standards are required since the SCE is unsatisfactory in a number of respects. Particularly, the liquid junction potential between aqueous and non-aqueous solutions is unknown and irreprodudble also there is a danger that the test solution will become contaminated with water and with potassium and sodium ions. 

Section 5.2 provides the thermodynamic basis for predicting whether or not a specific redox transformation can occur spontaneously in a given environment. The necessary redox halfreactions involving contaminants are usually well characterized because contaminants are the primary motivation for many studies of environmental systems. However, difficulties often arise in selecting the appropriate "environmental" half-reaction with which to balance the overall equation. When an environmental half-reaction cannot be identified, it is tempting to use traditional electrode potential measurements (127,128) as a generic measure of in situ redox conditions. These values (Emeas) then might be used as E°red or E°ox in Equation 14, to assess the thermodynamic potential of a particular contaminant transformation in a particular environment. However, a number of fundamental difficulties arise with this approach, so we do not recommend the procedure. 

Quasireference electrode (QRE) — (-> reference electrode, pseudoreference electrode). An electrode that maintains a given, but generally not well-defined, potential during the course of a series of electrochemical experiments. It has the advantage of not contaminating the test solution by solvent or ions that a conventional reference electrode might contain and transfer. Thus in studies in aprotic solvents, like acetonitrile, a silver wire can behave as a QRE. It must be calibrated with respect to a true reference electrode or reference redox couple that is added at the end of the experiments to obtain meaningful potential values. 

Munoz 1994). Furthermore the electrode is highly susceptible to contamination effects. While contaminations of a platinum electrode can be disposed of managed, thermodynamic disequilibrium and low concentrations can not. Therefore redox measurements should be aborted after 1 hour if no steady value is reached. The statement derived from the measurement in that case is, that the water is redox species are not in thermodynamical redox equilibrium with the platinum electrode. 

We have not yet studied the Influence of inhibitors on germanium electrodes. The problem is that germanium dissolves in parallel with most redox reactions at germanium electrodes. The dissolution causes a steady cleaning of the surface which should hinder the effect of contamination. The situation is very complicated in the case of cathodic polarization where the surface is covered with hydrogen atoms which act in some respects as inhibitors for others like catalysts. 

Strength on activity coefficients, since the activities, rather than the concentrations of the participating species are the quantities that determine the potential due to the redox couple. These criteria are difficult to meet and have led to the skeptical outlook that most Eh measurements are not amenable to quantitative interpretation (2,42,43). Contamination of platinum electrode surfaces by oxygen in aerated waters (, ), by sulfur in anaerobic waters (4 ) and by iron in surface sediments (45) may cause errors in the measured values. Furthermore, many Eh measurements are thought to be mixed potentials ( ). For these reasons, most Eh measurements have been used only in a qualitative sense. 

A comparison of the equilibrium (Eh) and kinetic (TEAPs) approaches to describe redox processes in a petroleum hydrocarbon-contaminated aquifer was given by Chapelle et al. (1996). In this study. Eh measurements were made with a platinum electrode, and the results plotted on a standard Eh-pH diagram (SUlen, 1952). The results of this analysis are shown in Eigure 11. Based on this analysis, it can be concluded that Fe(III) reduction is the predominant redox process, as none of the measured Eh values are sufficiently negative to indicate sulfate reduction... 

Redox potential (Eh) characterises the oxidation-reduction condition of a soil. This in turn provides a means to assess soil genesis, soil fertility and status of soil contaminants (Liu Yu 1984). Redox potential is well known to be difficult to measure precisely with conventional methods, for reasons ranging from slow electrode response to soil condition, especially for poorly poised (redox capacity) soils (Bohn 1971 Ponnamperuma 1972). Usually measurements are made under specified conditions, with values being dependent on the experimental conditions. As the measured value is conditional, it may not be sufficiently precise for some physicochemical studies. Nevertheless, there is unique information about soil condition that can be derived by measuring the redox potential. 

Additionally, electrochemical transformations may occur when the contaminants enter into the anode or the cathode, particularly chlorinated organic compounds, which are shown to undergo reductive dechlorination at the cathode and oxidative dechlorination at the anode. Such transformations should also be considered based on the redox conditions in the electrodes and the contaminant characteristics. 

As suggested by the above paragraph, other types of phenomena affect the previously described transport mechanisms of the contaminants toward the electrodes. These are the physical-chemical interactions, both between different compounds in the aqueous phase and between these aqueous species and the sohd phases of the soil system. Some of these interactions are precipitation, acid-base, complex formation and redox reactions, adsorption, and ion exchange and surface complexation reactions. 

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