Oxygen-reaction equilibrium potential dependence

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

As described in Chapter 3, and expressed by Nemst s equation (3.9), the equilibrium potential depends on the activity, or practically on the concentration of the species that take part in the reaction. The reduction of the oxygen eoneentration that oeeurs close to the metal surface when the electrode is polarized in the eathodie direetion, leading to the concentration profile shown in Figure 4.4, causes therefore a change in the equilibrium potential. This equilibrium potential shift is regarded as an overvoltage ... 

The O2/H2O system is very slow so that the exchange current a I equilibrium is extremely low (10 /10 A cm 2) as a consequence, any other reaction at the electrode will hamper its study and that could be the reaction of impurities or other redox reactions involving the electrode itself. The so-called noble metals are not really inert and do interact with oxygen a platinum surface in contact with an O saturated solution adsorbs oxygeti as an electronically conducting monolayer but can be further oxidized to PIO, PtO . A detailed analysis of these phenomena, which falls outside the scope of the present review, can be found elsewhere [311. A platinum electrode, when a complete electronically conducting monolayer of I l—O is formed at the surface of the metal, behaves as an ideally inert electrode in such conditions, rest potentials dependent on pO2 and pH can be measured during a few hours, close to... 

Many chemical reactions in wine are characterized by electron transfers, leading to the oxidation and reduction phenomena. These reactions occur simultaneously and continue until an oxidation-reduction equilibrium is reached. The oxidation-reduction potential of a wine is an observation of the oxidation and reduction levels of the medium at a certain equilibrium. This value is quite comparable to pH as a measurement of a wine s acidity. Its value is linked to the quantity of dissolved oxygen, just as pH depends on the quantity of (H+) protons. Furthermore, it is possible to define the normal potential Eq of a given oxidiz-ing/reducing couple when half the component is oxidized and half is reduced. This characterizes the wine s oxidation capacity in the same way as p indicates the strength of an acid. 

Except for the electrochemical reaction (2.6), all other reactions depend on the pH of the solution. A number of electrochemical reactions proceed in this system, which form different electrode systems, depending on lead ion valency, solution composition and pH, and electrode potential. These reactions cover a potential range of 2.0 V. Table 2.4 summarises the electrochemical reactions involving Pb, lead oxides, PbS04 and basic lead sulfates, and the equilibrium potentials of the respective electrode systems. The reactions and the equilibrium potentials for the hydrogen and oxygen electrodes are also given in the table. Several chemical reactions in which basic lead sulfates take part are also included in Table 2.4. 

The rate of corrosion process will depend on the conductivity of electrolyte and the difference of potential between the anode and cathode. Particularly the oxygen access, necessary for the cathodic reaction, can be the factor limiting the rate of corrosion [98]. Simultaneously, as a result of corrosion current, the polarization of electrodes occurs (their potentials increase in respect to the equilibrium potential values) and the dynamically maintained potential value has the deciding effect on the corrosion rate. In the case of steel in paste environment strong polarization of anodic microareas occurs, which increase anodic potential, decreasing the difference of potential in respect to cathode therefore, as it results from the curves in E -pH system, the passivation of steel due to the oxides film occurs [98]. 

The reaction rates themselves strongly depend on the conditions under which the reactions are conducted. In particular, cathodic oxygen reduction that, at temperatures below 150°C, is far from equilibrium comes closer to the equilibrium state as the temperature is raised. The reasons why the real value of the oxygen electrode s potential at low temperatures is far from the thermodynamic value and why cathodic oxygen reduction is so slow are not clear so far, despite a large number of studies that have been conducted to examine it. 

Open Circuit Potential. Metal immersed in an aqueous solution develops an electric potential at its surface called open circuit potential (OCP) which is a characteristic of the metal solution system. The magnitude of OCP is measured with respect to reference electrode with the help of high impedance voltmeter and potentiostat is used to polarise or displace equilibrium potential of specimen in the negative (cathodic) or positive (anodic) direction with reference to OCP. This is manipulating the rates (ionic currents) of respective cathodic and anodic half-cell electrochemical reactions. The electrochemical potential of a metal in a certain solution is dependant on the type of the metal, the composition of the solution and its pH, oxygen content and temperature [104, 105]. 

Although Equation 9.5 has the lower equilibrium potential there is a significant oveipotential of at least 0.5 volts depending on the nature of the anode. This then favours the formation of Pb02 over oxygen. In the case of chloride the standard potential for chlorine formation is -1.36 volts with little overpotential and hence this reaction is favoured. If ferrous iron is present then oxidation at the anode in accordance with Equation 9.6 has a standard electrode potential of -0.7 volts and this reaction will predominate. 

PEM fuel cell characteristics are generally described with polarization curves. The thermodynamic equilibrium potential of the hydrogen/oxygen reaction is reduced by various overvoltage terms that depend on mass transport, kinetic, and ohmic phenomena within cell. In other words, the output voltage of a single cell is attributable to different current, temperature, and pressure dependant factors [1]. 

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... 

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