Solid‐State Coulometric Titration: Critical Analysis and Application to Wüstite

Abstract

A critical analysis was performed on galvanic cell techniques which employ calcia-stabilized zirconia electrolytes. The presence of low-level electron or hole conduction in the ionic conducting range of the electrolyte was found to be responsible for oxygen permeation through the electrolyte which can either oxidize or reduce a single-phase oxide and, hence, lead to drift in the open-circuit emf with time. Taking oxygen permeation into account quantitatively, a series of isothermal titration experiments in wiistite was performed in which the cell emf was found to be linear with composition (i.e., oxygeniron molal ratio). These results are in excellent agreement with a recent composite analysis of all thermodynamic data in the Fe-O system. Previous cell studies in which deviant behavior has been reported are probably due to improper galvanic cell techniques. No evidence was found to support the existence of more than one-phase or second-order transitions within the wiistite phase.Recently, considerable interest has been generated on the use of solid-state galvanic cells which employ oxide electrolytes (e.g. CaO-ZrO2, Y~O~-ThO~) to study the thermodynamic properties of nonstoichiometric oxide systems. Since voltages may be measured with high precision emf measurements should lead to a direct evaluation of oxygen activities with much greater accuracy than is normally possible with conventional gas equilibration techniques. The precision of measurement in combination with the possibility of coulometric titration, which allows the composition of an oxide to be changed in situ, permits not only a continuous determination of oxygen activity as a function of composition, but also the composition limits of the phase itself. In spite of the potential advantages, the use of oxide electrolytes has not always led to increased accuracy in the measurement of oxygen activities. Several workers (1-4) have reported difficulties in operating cells which were stable with time, thereby necessitating the use of empirical correction factors to determine absolute compositions.Several emf measurements (1, 3, 5-8) of oxygen activities and phase boundary composition limits for wfistite exhibit rather poor agreement with each other and do not agree with a recent composite analysis of the Fe-O system by the authors (9). Included in this composite were seven independent studies, using either CO2/CO gas equilibration (10-13) or galvanic cell (3,8) techniques. The composite possessed the following properties: (i) the partial molal free energy of oxygen (aGo) in wfistite varied linearly with composition (O/Fe) 2 and temperature; a (ii) the partial molal enthalpy (~H--o) and entropy (aSo) of oxygen varied linearly with composition and were independent of temperature. EMF or gravimetric data which deviated from this linear behavior were rejected for experimental reasons (9).In the present paper, a critical analysis of galvanic cell techniques, which employ calcia-stabilized-zirconia (CSZ) electrolytes, is given to determine the cause for nonlinear...

Thermodynamic reevaluation of the Fe–O system

“…10. [15,36,37,41,42,45,[52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67] and calculated lines. The calculated oxygen isobars are shown by the dashed lines.…”

Kinetics of the Reaction of CO2/CO Gas Mixtures with Iron Oxide

“…However, various studies have been conducted that have provided no evidence for the existence of more than one phase in wu¨stite. These studies include those which provide thermogravimetetric, [31] electromotive force, [32][33][34] X-ray diffraction, [35] and work function [36] measurements.…”

“…[33] is taken into account along with Eq. [34], which gives the surface reaction of the dissociation of carbon dioxide, it is found that the proportionality given by Eq. [35] is true.…”

Oxidation entropies and enthalpies of ceria–zirconia solid solutions

“…Most of the oxygen isotherms were measured in a flow reactor, using techniques that have been described elsewhere [35] Because the range of P(O 2 ) that could be accessed in the flow reactor was limited to lower values by our ability to control the P(H 2 O)/P(H 2 ) ratio (i.e. 10 -26 atm < P(O 2 )< 10 -21 atm at 973 K), some isotherms were measured using Coulometric Titration [31,34,36,[39][40][41][42][43][44][45][46][47][48]. In Coulometric Titration, the sample is placed in a sealed container with an ion-conducting membrane (usually yttria-doped zirconia) separating the inside from the outside of the container.…”