Oxygen electrode evolution

FIGURE 15.7 Polarization curves for anodic chlorine (1) and oxygen (2) evolution at a graphite electrode, and the current yields of chlorine as a function of potential (3). 

Amphipol A8-35 was then added and the sample treated as described in 5.1 ("AP-trapped PS2, pH8"). Oxygen evolution was measured with a Clark-type oxygen electrode under continuous, saturating light. 

Wendt, H. and Plzak, V. (1990) Electrode kinetics and electrocatalysis of hydrogen and oxygen electrode reactions. 2. Electrocatalysis and electrocatalysts for cathodic evolution and anodic oxidation of hydrogen, in Electrochemical Hydrogen Technologies (ed. H. Wendt), Elsevier, Amsterdam, Chapter 1. 2. 

Second in popularity only to the pH electrode is the oxygen electrode. This device is a polarographic electrode system that can be used to measure oxygen concentration in a liquid sample or to monitor oxygen uptake or evolution by a chemical or biological system. 

Leaf discs have commonly been used for bioassays to determine if herbicides inhibit photosynthesis (Table 16.2). The simplest leaf-disc bioassay uses small discs cut from fully expanded cucumber or pumpkin cotyledons, floated in the light on a phosphate buffered medium containing suspected photosynthesis inhibitors.115 Qualitatively, if photosynthesis is inhibited, the leaf disc sinks. There are several variations of this method that can provide quantitative data. Evolution of O2 in the test solution can be measured with an oxygen electrode, CO2 induced pH changes colorimetrically determined with bromothymol-blue, or electrolyte leakage monitored with a conductivity meter. Leaf strips, algae, isolated chloroplasts, and duckweed (Lemna minor) have been used as test subjects. Although the bioassays presented in Table 16.2 are fairly easy to perform, few allelochemicals have been tested as possible inhibitors of photosynthesis. Many... 

Diamond film electrode has an inert character with weak adsorption properties (Martin et al. 1996 Swain et al. 1998 Pleskov 1999). Weak interactions of D ( OH) lead to low anode activity toward oxygen gas evolution [Reaction (3.3)] and high oxidation reactivity to the organic pollutants incineration [Reaction (3.2)]. Due to the high oxidizing power of the radicals, highly persistent pollutants, which cannot be decomposed with bioremediation method, advanced oxidation process, or even electrooxidation process with other kinds of electrodes, can be successfully degraded with the diamond film electrode. 

Furthermore, it has been proved that direct oxidation can happen on the diamond film electrode before the oxygen gas evolution potential (Zhi et al. 2003). Direct oxidation takes place, accompanying with the adsorption of chemical molecules on the diamond electrode surface. An as-grown diamond film with high hydrogen coverage is inert to adsorption of polar molecules (Vinokur, Miller et al. 1996 Yano... 

Fig. 3.8 Typical cyclic voltammogram of boron doped diamond film electrode. The result was tested in 0.5 M H2SO4 solution at 0.1 V s 1 scan rate. The standard hydrogen gas and oxygen gas evolution potentials were marked with E t and Eq2, respectively (after Guo and Chen 2007b)...

Equally intriguing is the decomposition of 4. Dioxygen was detected using an oxygen electrode quantitative GC analysis showed less than 2.2% production of methane, based on starting rhenium. The rate of 02 evolution varied inversely with [H202], whereas the final yield of 02 varied linearly with peroxide. Thus the stoichiometry in Eq. (5) was arrived at ... 

O Grady et al. [227] studied the reduction and evolution of oxygen on RuO on titanium and reported that the cathodic reaction involves the formation of peroxide in solution. From y 1/2 vs. rotating disk electrodes, the authors concluded that the reaction 02/H202 was in equilibrium at the surface [227], Miles et al. [428] also studied the oxygen electrode reactions on several metal oxides (Ir, Ru, Pd, and Rd)... 

The Tafel equation also describes the evolution of oxygen at a platinum anode. Bockris and Huq found that, with solutions carefully purified by preelectrolysis, the oxygen electrode exhibits reversible behavior (E = 1.24 V, compared with the theoretical 1.23 V). The exchange current density, however, is only of the order of 10" to 10" °A/cm in dilute sulfuric acid so polarization occurs readily, and relatively large overpotentials are observed at moderate current densities. In solutions of ordinary chemical purity the Nemst relation fails for the oxygen electrode because of mixed-potential behavior. Criddle, using platinum electrodes in highly purified 1 M KOH, obtained a rest potential of 1.59 V. The potential is reduced by peroxide, which may be formed with impurities such as metals, protein, or carbon. 

Chlorophylls in the green hairy roots were extracted with N,N -dimethylformamide and determined spectrophotometrically by the method of Porra et al. [125]. Rates of photosynthetic evolution of oxygen from green hairy roots were measured with a gas-phase oxygen-electrode system at an irradiance of 1000 pmol quanta/m /s, at a saturating concentration of CO2 (approximately 5%) and at 25 °C [126]. 

During ECM, electrochemical dissolution of anode and cathodic evolution of hydrogen proceeds on the electrodes (the WP and TE, respectively). Along with these basic reactions, parallel reactions proceed concurrently, for example, oxygen anodic evolution, cathodic reduction of nitrate ions, if NaNC>3 electrolyte is used. It is important to note that electrochemical reactions in a narrow IEG result in gas evolution. The temperature of the electrolyte in the IEG and the void fraction increase as the electrolyte flows along the gap. This leads to a variation in the electrolyte conductivity that has an effect on the distributions of current and metal dissolution rate over the WP surface. The electrode processes and the processes in... 

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