Currents due to oxygen

Fig. 10.6 Polarisation diagram showing the limited role hydrogen evolution plays at the corrosion potential of steel in aerated neutral solution, the larger role in determining cathodic protection currents and the dominant role in contributing to current requirements at very negative potenitals. The dotted line shows the total cathodic current due to oxygen reduction and...

Shannon and coworkers in 2004. Pulsed voltammetry of a solution of this POM performed in phosphate buffer at pH 8, showed a catalytic current due to oxygen generation at low potentials, approaching the thermodynamic value for water oxidation (E = 0.756 V vs. NHE). The proximity of the two ruthenium eenters was eonsidered a key factor to access oxygenic catalysis, but doubts still remains about the precise occupancy of the ruthenium sites in the belt of the POM. 

The high sensitivity of the DPP method is mainly limited by the background current due to oxygen traces in the solution. In spite of that, using computerized voltammetric pulse instrumentation the detection of... 

It is difficult in practice to use ec reduction as a method of detection in hplc. Oxygen is very easily reduced, and if it is present in the mobile phase it will create a background current thousands of times larger than the current due to the solutes. To prevent this, oxygen would have to be very carefully removed. This can be done, but it certainly is not easy in practice. So most of the ec applications are oxidations. Another important consideration with ec detectors is that the mobile phase used must have fairly high conductance, so they are used with aqueous-organic mixtures containing added salts, or with buffer solutions. 

A complication that occurs on a low at.% Ru electrode is that, owing to the low Faradaic currents (low Ru content) and hence large Rt value, currents due to other trace redox reactions, e.g. oxygen reduction, become more detectable. This reveals itself in a phase-angle of 45° as co 0 as trace oxygen reduction would be diffusion-controlled. The impedance corresponding to this situation can be shown to be the same as that in Equation 5.3, with U(p) expressed by the relationship ... 

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. 

Thirdly, oxygen need not be excluded from the analyte solution provided the voltammetric peak is more cathodic than that for the reduction of oxygen, then the magnitude of both /forward or /reverse will incorporate an equal current due to the reduction of O2. Since /forward (O2) and /reverse (02) are equal, they cancel out and /difference is Seen to be independent of the oxygen concentration in the analyte solution. ... 

Compensation for background currents due to evolution of hydrogen, reduction of oxygen, solvent oxidation, or surface processes, was made by recording the voltammogram while the sample and carrier solution followed through the cell. The difference between these was taken as the net response for the sample [161]. 

The procedure used was to determine the concentration of dissolved oxygen (02) with an 02-electrode63. A (linear) calibration curve was prepared of current (due to the response of the 02-electrode) vs. oxygen concentration in sea water ranging from fully deaerated to saturation at an 02 pressure of one atmosphere. 

The current due to trace substances in the "blank" yield solution such as traces of unremoved dissolved oxygen, impurities in the distilled or deionized water and traces of reducible or oxidizable impurities in the chemicals used to prepare the supporting electrol5de. These substances can be part of the faradaic current and provide the second component of the residual current. 

The faradaic component of residual current due to impurities can be minimized by the thorough removal of dissolved oxygen by flushing the solution with an inert gas such as nitrogen or argon by the use of double-distiUed water or distilled deionized water, and by the use of high-purity chemicals. 

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