Oxygen, electrode reduction

For this or other electron transfer reactions to take place on a passive metal, the oxide film present must exhibit electronic conductivity. Depending on the prevailing potential with respect to the reversible potential of the oxygen electrode, reduction of oxygen or oxidation of water can then be achieved. 

When the tube is placed in a solution which contains oxygen, oxygen will pass through the membrane to the internal solution, and upon application of a voltage (0.6-0.8 V) to the two electrodes, the oxygen undergoes reduction at the gold cathode ... 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

An example for a compound of the perovskite type is LaNiOj. In other com-ponnds of the perovskite type, nickel may be replaced by cobalt or iron, and lan-thannm in part by alkaline-earth metals, an example being Lag 8Sro2Co03. The activity of perovskites toward cathodic oxygen reduction is low at room temperature but rises drastically with increasing temperature (particularly so above 150°C). In certain cases the activity rises so much that the equilibrium potential of the oxygen electrode is established. 

BetteUieim A, Parash R, Ozer D. 1982. Catalysis of oxygen cathodic reduction by adsorbed iron(in)-tetra(A,A,A-trimethylanilinium)porphyrin on glassy carbon electrodes. J Electrochem Soc 129 2247. 

In soil analysis, pH, specific ion, oxidation-reduction (redox), electrical conductivity (EC) cells, and oxygen electrodes are commonly used. For each of these measurements, a different specific electrode along with a separate or integral reference electrode is needed. In some cases, with extended use or long exposure to soil or soil-water suspensions, electrodes may become polarized. When this happens, erroneous results will be obtained and depolarization will need to be carried out using the electrode manufacturers directions [3],... 

Originally, a Clark oxygen electrode was used to measure a reduction in current due to the consumption of oxygen. Anodic detection of the hydrogen peroxide by oxidation at a platinum or carbon electrode was then introduced but, owing to the high electrode potential required, suffered from interference from other electroactive compounds in the sample. 

DR. DAVID STANBURY (Rice University) With regard to the problem of the electrocatalytic reduction of oxygen, I have attempted to formalize some ideas regarding the constraints of thermodynamics in order to elucidate the probable character of cobalt complexes which may catalyze the oxygen electrode via binuclear peroxo-bridged intermediates. The following gross mechanism is presupposed ... 

It is therefore almost impossible to establish the equilibrium potential of the oxygen electrode in aqueous electrolytes (+1.23 V vs rev. hydrogen electrode) and only in a very careful experiment of Bockris and Hug (112) has this been achieved. Usually at zero current density a lower potential of approximately 0.95 V is obtained, which is established by mixed reactions, with the main reaction being the reduction of oxygen to hydroperoxide. 

In the case of the ternary eutectic Li2C03-Na2C03-K2C03 at 605°C saturated with pure C02 (p02 = 6), the anodic limit is about 0.27 V vs. the oxygen electrode however, when saturated with Li20 (pO2- = 0) this melt is reported to exhibit an anodic limit of only -0.23 V [5]. The cathodic limit of ternary eutectic carbonate melts with p02 = 2 to 6 is about -1.9 to -2.1 V [5]. The reduction process produces elemental carbon according to the reaction... 

At a high cathodic potential (region II), a sharp transition is observed at the potential referred to as ET. The authors demonstrate that the sudden increase of the electrode kinetics could not be attributed to the sole electrochemical reduction of the electrode material, nor to the electrolyte reduction. They conclude that after the transition, the main electrode process is still an oxygen electrode reaction with a major change of mechanism, leading to the onset of an important electrocatalytic effect. This assertion is sustained by the analysis of ... 

Keywords Hydrogen oxidation, oxygen reduction, nanodispersed diamond, surface, heat-treated treatment, hydrogen electrode, oxygen electrode. 

Acetylcholineesterase and choline oxidase Sensors were developed consisting of a Clarke oxygen electrode modified by superposition (over the polypropylene membrane of the electrode) of a dialysis membrane on which AChE and ChO were immobilized. Both formats are based on the conversion of the substrate to choline which is oxidized in the presence of choline oxidase causing a reduction in p02 which is detected by the electrode. Free choline is measured with a similar electrode with only choline oxidase immobilized on the membrane. Calibration graphs were rectilinear upto 90 pM choline and upto 120 pM acetylcholine. Results were presented for the analysis of tissue extracts and pharmaceutical formulations. [115]... 

As long as the solution is adequately mixed so that the solution at the membrane surface has a dissolved 02 content that is representative of the bulk solution, the voltage output from an oxygen electrode is proportional to the rate of diffusion of 02 across the membrane, which is in turn proportional to the 02 concentration in the bulk solution. Changes in amounts of dissolved 02 in the sample chamber, due to 02 reduction by electron transport, alter the rate of 02 diffusion into the elec-... 

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