Oxygen-reaction equilibrium potential partial pressure

Dependence of oxygen-reaction equilibrium potential on oxygen-gas partial pressure... 

At mercury and graphite electrodes the kinetics of reactions (15.21) and (15.22) can be studied separately (in different regions of potential). It follows from the experimental data (Fig. 15.6) that in acidic solutions the slope b 0.12 V. The reaction rate is proportional to the oxygen partial pressure (its solution concentration). At a given current density the electrode potential is independent of solution pH because of the shift of equilibrium potential, the electrode s polarization decreases by 0.06 V when the pH is raised by a unit. These data indicate that the rate-determining step is addition of the first electron to the oxygen molecule ... 

In an oversimplified way, it may be stated that acids of the volcanoes have reacted with the bases of the rocks the compositions of the ocean (which is at the first end point (pH = 8) of the titration of a strong acid with a carbonate) and the atmosphere (which with its 2 = 10 atm atm is neady in equilibrium with the ocean) reflect the proton balance of reaction 1. Oxidation and reduction are accompanied by proton release and proton consumption, respectively. In order to maintain chaige balance, the production of electrons, e, must eventually be balanced by the production of H. The redox potential of the steady-state system is given by the partial pressure of oxygen (0.2 atm). Furthermore, the dissolution of rocks and the precipitation of minerals are accompanied by consumption and H release, respectively. 

SCHULDINER and coworkers [90] studied the dependence of the open-circuit potential of smooth platinum upon the partial pressure of oxygen in a gas-tight system with negligible oxygen leak. The rest potential was found to increase with 0.06 V/decade of pressure between Po2=10" atm and Pq = 10" atm in agreement with earlier results [91,92]. The open-circuit potential was considered as an equilibrium potential of the reaction... 

The properties of the interface at which the formation of oxide ions occurs have been of special interest [6, 7, 28—35]. While solid electrocatalysts, Pt [28, 29, 31, 32] and C [30], were studied mainly, a molten silver cathode was employed in another type of zirconia-electrolyte fuel cell developed [34,35] at the General Electric Research and Development Center in Schenectady. Since the hindrance of the electrochemical steps of the O2 reduction at the cathode surface is small [28, 32] on platinum around 1000 °C, it is hard to elucidate the reaction mechanism beyond the net reaction 1. Analysis [33] of the potential distribution curves inside Zro 9Yo 2 02.i in contact with two platinum electrodes showed at 1380°C that the electronic hole contribution to the conductivity in the bulk of the specimen depended upon as would be expected from the equilibrium of reaction 15. The partial oxygen pressure had values between 10 and 10 atm. However, if the production of oxide ions is assumed to occur at the cathode solely by reaction 15, the rate of production is much lower than the rate of loss at the anode. A cathodic reaction of the type... 

Weaver calculated the open circuit potentials of these and other possible reactions that might occur under open circuit conditions, finding agreement between measured potentials and the potentials calculated from thermodynamic tables (Weaver et al, 1979). Hemmes and Cassir (2004) recalculated the cell open circuit potentials. They determined the equilibrium concentrations and electrode potentials in a system comprised of carbon, carbonate, CO2, CO, O ", and electrons, using the phase rule modified for electrochemical systems by Coleman and White (1996). Hemmes expressed the half-cell potentials of the anode reactions (3) and (4) referenced to an idealized cathode reaction (unit oxygen and CO2 partial pressures) ... 

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