Oxygen electrode standardization

Figure 3.3.8 Schematic illustration of the origin of activation overpotentials in a hydrogen-oxygen fuel cell. The solid curves represent exponential analytic current densities versus electrode potential of the hydrogen electrode (standard potential 0 V) and the oxygen electrode (standard potential 1.23 V). Relevant for a PEMFC fuel cell are the HOR (anode) and the ORR (cathode) branches. To satisfy a cell current (yceii), the anode potential moves more positive by riact,HOR> while the cathode potential moves more negative by iiact.oRR- As a result of this, the observed cell potential is V, which is smaller than 1.23 V. The shape of the individual characteristics is such that the cathode overpotentials are larger than those at the anode.

Similarly to aqueous electrochemistry, potentials in solid state electrochemistry utilizing YSZ are expressed in terms of the potential of a reference metal electrode exposed to P02 = 1 atm at the temperature T of interest. Thus a standard oxygen electrode scale (soe) can be defined. Similarly to equation (7.2) one has ... 

There is an important point regarding the absolute standard oxygen electrode scale defined by Eq. (7.27). The U (abs) value is defined by the equilibrium ... 

Figure 7.12 shows the relationship between the standard oxygen electrode (soe) scale of solid state electrochemistry, the corresponding standard hydrogen electrode (she) scale of solid state electrochemistry, the standard hydrogen electrode (she) scale of aqueous electrochemistry, and the physical absolute electrode scale. The first two scales refer to a standard temperature of 673.15 K, the third to 298.15 K. In constructing Figure 7.12 we have used the she aqueous electrochemical scale as presented by Trasatti.14... 

Knowledge of the value of ij (abs) makes it possible to convert all relative values of electrode potential to an absolute scale. For instance, the standard electrode potentials of the oxygen electrode, the zero charge of mercury, and the hydrated electron, in the absolute scale are equal to -5.67,. 25, and 1.57 V, recpectively. ... 

Oxygen electrode. In principle, a classical oxygen electrode in a liquid electrolyte would be possible if an electrode material were known on the surface of which the redox system 02/0H is electrochemically reversible however, Luther26 measured its standard potential from the following cell without a liquid junction ... 

Oxygen electrodes, 3 408 standard potential, 3 413t Oxygen-enriched atmospheres, silver in safety of, 22 661... 

Fig. 3.8.2 Standard polarographic device with a Clark oxygen electrode...

In addition, other processes may occur such as the reaction of the hydroperoxide ion with the conductor to form metal-oxygen bonds which in turn may be reduced. The hydroperoxide ion may itself decompose to reform oxygen, etc. The potential of an oxygen electrode is invariably a mixed potential with a value of about 1.0 V on the standard hydrogen scale, at zero current drain. 

A bioprocess system has been monitored using a multi-analyzer system with the multivariate data used to model the process.27 The fed-batch E. coli bioprocess was monitored using an electronic nose, NIR, HPLC and quadrupole mass spectrometer in addition to the standard univariate probes such as a pH, temperature and dissolved oxygen electrode. The output of the various analyzers was used to develop a multivariate statistical process control (SPC) model for use on-line. The robustness and suitability of multivariate SPC were demonstrated with a tryptophan fermentation. 

The respiratory activity of the brain tissue was determined by measuring the rate of oxygen uptake with a Clark oxygen electrode (7). The sample of tissue (a brain half) was treated exactly as if used in a calcium efflux experiment except no radioactivity or RF power was used. Following this procedure which required about 55 minutes, the tissue was placed in the oxygen electrode cell containing 1.6 ml of the standard medium (pH 7.8) at 37°C and the rate of oxygen uptake was recorded. 

The standard equilibrium potential of the oxygen electrode, corresponding to the reaction... 

Magnetic stirrers and stirring bars—Standard magnetic stirrers with stirring bars (or cut up paper clips) small enough to fit in sample chamber of the oxygen electrodes—5 required. 

The final expression is arrived at in a similar way to that for the hydrogen electrode. The concentration of water is regarded as constant in dilute aqueous solutions. The value of E9, measurable at pH = 0 with p0i = 1 atm (where Po2 is pressure of oxygen over the solution) is +1 -23 V. It is useful and easy to memorize the standard potentials of the hydrogen electrode (E9= 0 V) and of the oxygen electrode (E9 = 1 -23 V, one, two, three). The importance of these quantities will become apparent later (cf. Section 1.43). 

The oxygen electrode reaction with accompanying standard potentials at 25°C may be represented as... 

The Oxygen Electrode.—The standard potential of the oxygen electrode cannot be determined directly from e.m.f. measurements on account of the irreversible behavior of this electrode (cf. p. 353) it is possible, however, to derive the value in an indirect manner. The problem is to determine the e.m.f. of the cell... 

The so-called oxygen electrode is based on a different principle an electrolysis cell is used, and the current measured at the inert cathode is, under standardized conditions, a function of the oxygen concentration (activity). Selectivity is enhanced by covering the cathode with a membrane permeable to molecules only. 

The procedure just described essentially utilizes free energy values to calculate E.M.F. s, and another example, based on the same principle, is the determination of the standard potential of the oxygen electrode. The problem is to evaluate the E.M.F. of the cell... 

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