Oxygen electrode, Nernst-equation

The complete current-voltage characteristics of the sensor can be derived from the similar consideration that was used for derivation of the i-E curve for liquid electrolytes. Because the potentials at each electrode are reversible, their difference can be expressed by the Nernst equation for the concentration of oxygen at the anode Co(0) and at the cathode Co (A). The current flowing through the layer generates a voltage drop iRb, where Rb is the bulk resistance of the ZrC>2 layer. 

Platinum catalyses the dissociation and recombination of the oxygen molecules so that O2 ions can be formed at one electrode and converted to 02 molecules at the other. The e.m.f. developed by such a cell is given by the Nernst equation (see Eq. (4.33)). In the present case z = 4 because an oxygen molecule consists of two atoms each acquiring two electrons on being ionized. Hence, the equation becomes... 

A higher polarization curve when cell operating pressure is increased can be expected on the base of the Nernst equation (3.15), but the concomitant increase of / o, due to the higher concentration of reactant gases on electrodes, with the consequent improvement of the hydrogen/oxygen reaction rate, has to be also considered [34]. 

The consequence of all these effects is that oxygen, or hydrogen, evolution reactions follow quite different kinetics than would be expected from thermodynamical considerations. For example, oxygen evolution on a platinum electrode starts at a potential significantly higher than that predicted by the Nernst equation. The reason for this is the formation of surface oxide and its associated dynamics as elucidated, besides others, by B. Conway [4]. 

An oxygen pumping cell operates by active pumping of 02 ions from the negative to the positive electrode when an electrical charge is applied. To adjust a fixed O2 concentration in a cavity, a certain electrical charge has to be applied (EMF). This electromotive force (E) is described by the Nernst equation. 

Indeed, there are no factors limiting the reversible operation of gas oxygen electrodes at high and low pO values. Theoretically, if the concentration of the potential-determining particle in the solution is equal to zero, then the absolute magnitude of the potential of the corresponding electrode approaches infinity ( oo). This conclusion has no physical sense it means that the Nemst equation is applicable to the description of the electrochemical processes in the solution, if their concentration exceeds the certain limit. In practice, the deviations from the Nernst equation arise because of the effect of the binary electric layer... 

As repeatedly mentioned in the previous parts, the correctness of the constants obtained for the Lux acid-base equilibria depends essentially on the correctness of the assumption about the completeness of dissociation under the experimental conditions of the strong Lux bases, which are used for potentiometric titration of the studied acids and for calibration of the potentiometric cell with the indicator oxygen electrode. For the latter factor, the accordance to the corresponding Nernst equation of the potentialdetermining process at the indicator electrode is less important than the completeness of the oxide-ion donor s dissociation with the formation of O2-. 

Investigations of the dependence of the stationary potential on the soot electrode with adsorbed laccase on the solution pH and on oxygen pressure have revealed that the values of the parameters 5E/d(pH) and dE/dlog Po. correspond to the values of the coefficients in the Nernst equation for the O2/H2O couple. An insignificant departure of the equilibrium potential from the theoretical value is due to a local pH rise in the adsorbed protein layer as compared to the pH in the bulk of the solution. 

If two such electrodes are separated by a thin layer of only zirconia, the application of a potential will lead to the pumping of oxygen from the cathode to the anode. This device can be used as an amperometric sensor for oxygen if a diffusion barrier restricts the flux of oxygen to the cathode. Note that similar devices are also often used as potentiometric sensors according to the Nernst equation (i.e., the lambda-probe in cars with catalytic converters). In this case one side of the cell has to act as a reference, e.g., by using ambient air. 

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