The Reversible Oxygen Electrode

The equilibrium potential of net reaction 4 in alkaline solution or reaction 9 in acid solution is generally not established because the exchange currents of the rate-determining steps are so small that other reactions interfere. However, the equilibrium potential can be determined [62,84,85] from kinetic data as demonstrated first by Hoar [84]. The intersection of the extrapolated Tafel line for the O2 evolution with the extrapolated Tafel line for the O2 reduction is determined. As illustrated by the data [6] for Ir and Pt in Fig. 82, the intersection is close to the theoretical value of the reversible potential 1.23 V in 

Hoare found [86—88] that the potential of a bright platinum electrode which had been pretreated in concentrated nitric acid became steady at 1.23 V in purified 1 M H2SO4, saturated with O2, for more than 24 hrs. Traces of hydrogen peroxide were absent. He suggested that the pretreatment in nitric acid produced a film similar to the oxygen 

It was concluded [19] that the film is unstable in sulfuric acid solution and eventually becomes incomplete, whereupon a mixed potential process becomes effective. The open-circuit potential falls to less positive values. In phosphoric acid solutions, the film is more stable [89]. A potential close to 1.24 V was established for longer periods in phosphoric acid solutions (about 72 hrs.) than in sulfuric acid solution (about 24 hrs.). It is difficult to judge whether an open-circuit potential that stays 

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 

Constant activity of H02ad was assumed. Since the coverage with H02ad is small in the potential range under consideration, the latter assumption is questionable. 

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Where fn2 is the fugacity of hydrogen. Similarly, the potential of the reversible oxygen electrode can be written as follows ... 

The electrode potential behaviour of copper in various solutions has been investigated and discussed in considerable detail by Catty and Spooner . According to these workers a large part of the surface of copper electrodes in aerated aqueous solutions is normally covered with a film of cuprous oxide and the electrode potential is usually close to the potential of these film-covered areas. The filmed metal simulates a reversible oxygen electrode at... 

Metals in practice are usually coated with an oxide film that affects the potential, and metals such as Sb, Bi, As, W and Te behave as reversible A//A/,Oy/OH electrodes whose potentials are pH dependent electrodes of this type may be used to determine the solution s pH in the same way as the reversible hydrogen electrode. According to Ives and Janz these electrodes may be regarded as a particular case of electrodes of the second kind, since the oxygen in the metal oxide participates in the self-ionisation of water. 

According to Sato et al.,6,9 the barrier-layer thickness is about 1.5 to 1.8 nm V-1, and increases to 3 nm around the oxygen-evolution potential. In Fig. 5, the scale of the electrode potential, Vrhe, is that of the reversible hydrogen electrode (RHE) in the same solution. The electrode potentials extrapolated from the linear plots of the potentials against the film thickness suggested that the potential corresponding to the barrier thickness equal to zero is almost equal to 0.0 V on the RHE scale, independent of the pH of the solution, and approximately agrees with the equilibrium potential for the oxide film formation of Fe or Fe. Therefore it is concluded that the anodic overpotential AE applied from the equilibrium potential to form the oxide film is almost entirely loaded with the barrier portion. 

A mercury-sulfate electrode served as a reference electrode. All electrode potentials are referred to the potential of the reversible hydrogen electrode in the same electrolyte and at the same temperature as the test electrode. Adsorption measurements were performed in the 0.5 M H2SO4 solution prepared using special purity B-5 sulfuric acid and water doubly-distilled. To remove oxygen dissolved in the electrolyte, pure helium or argon was bubbled through acid solution. 

Peroxide ions are unstable in acidic solutions, and this process has no essential effect on the electrode potential. However, if the gas electrode is used for measurements in basic solutions, where the stability of 02 ions increases, they become the predominant form of oxygen-containing anions. Under these conditions the slope of the. E-pO plot is twice as large as that predicted by equation (2.4.2). From equation (2.4.4) it follows that there is a linear correlation between the concentrations of peroxide and oxide ions in these melts, and it can be assumed that the gas oxygen electrode remains reversible to oxide ions. However, their equilibrium concentration will be appreciably lower than the initial one. 

However, the properties of solid electrolytes of which the membranes are made, impose some limitations on the temperature range where the membrane oxygen electrode can be used. Perfil ev and Fadeev determined the lower threshold temperature for the reversible operation of the solid electrolyte membrane to be close to 500 °C [211], owing to oxide-ion conductivity of the membrane. At higher temperatures, the membrane oxygen electrode is considered to be reversible to oxide ions, although the transport number of oxide ions, e.g. in YSZ, achieves the value of unity (1) at temperatures of the order of 1000 °C, i.e. its conductivity becomes completely ionic. 

The work of some of us (Cherginets, Banik and Rebrova) is devoted to studies of the reversibility of various oxygen electrodes such as Pt(02)lYSZ and NilNiO, in the KCl-LiCl eutectic melt at 400, 500, 600 and 700 °C. The potentiometric cell for the measurements at 400 and 500 °C consisted of the lead reference electrode (PblPbCl2, 0.05 mol kg-1) and one of the indicator oxygen electrodes [232] ... 

Fig. 2.4.11. The limits of reversible work of the membrane oxygen electrodes in chloride melts (KCl-NaCl eutectic) melt at 1000 K (pO values expressed in molar fractions), by R. Combes, J.

All the routines described for the determination of the thermodynamic (concentration) parameters in metal oxide solutions include some indirectly obtained values. For example, the equilibrium concentration of metal cations is calculated proceeding from the quantity of the oxide-ion donor consumed for titration (precipitation). Direct determination of the concentration of metal cations in the melt (if it is possible) allows one to obtain more correctly the obtained solubility product values. Our paper [332] reports a method for correction of the solubility product values for oxides on the basis of the potentiometric titration data. The modification of the standard routine consists of the simultaneous use of two indicator electrodes, one of which is the membrane oxygen electrode and the other is a metal electrode, reversible to the cations the oxide consists of. This routine was used to estimate the solubility products of copper(I) and nickel(II) oxides in the molten KCl-NaCl equimolar mixture at 700 °C. Investigation of Cu20 by the proposed method is of considerable importance since, as will be shown further, the process of dissociation/dissolution of copper(I) oxide in molten alkali-metal halides differs from the generally accepted one which was considered, e.g. in Ref. [119]. 

Calculations using this equation show that the solubility of Zr02 in that melt at 700 °C is also negligible, even in a strongly acidic medium [246]. Therefore, the use of the membrane oxygen electrodes made of stabilized Zr02 should not cause appreciable pollution by Zr(IV) compounds of the studied halide melts under these conditions. As mentioned in Part 4, the upper limit of reversible work of a YSZ-based membrane oxygen electrode is located near pO = 11, where destruction of the membrane material is noticeable. 

V.I. Minenko, S.M. Petrov and N.S. Ivanova, The Use of Reversible Oxygen Electrode in Oxygen-Containing Melts, Izv. Vysschikh Utchebn. Zav., Chem. Metallurgia N7 (1960) 10-13. 

The anomalous behavior of the electrode Pt(02) has been noted and explained by peroxide function of the gas oxygen electrodes (of the first type). On the contrary to gas electrodes, metal-oxide electrodes (of the second type) were reversible with the slope corresponding to the reaction ... 

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