Exercises
215
The following hypotheses allow the calculation of the open-circuit emf of the
chain:
2 an equilibrium exists at each interface, taking into account the dominant
carriers in each solid phase,
2 the electric potential of each species is constant within each solid phase
that conducts via a single species (electrons or ions),
2 the stabilized zirconia (ZrO 2 −Y 2 O 3 8 mol%, YSZ) is considered a strictly
ionic conductor,
2 the ionic junction between phases (3) and (5) is implemented with porous
alumina (4). The molten salts on each side of the alumina membrane are
very similar compounds with a high concentration of ions. We can thus
consider that the potential difference (φ
5 − φ
3
) across the junction is negligible.
1. Represent schematically the electric potential within the electrochemical
chain.
2. Determine the emf of the sensor as a function of the concentration of O
2−
ions
dissolved in the molten salt and, more precisely, of pO
2−
(i.e., – log[O
2–
]).
3. We add zinc oxide to the molten salt and do the same experiment with lithium oxide Li 2 O. The emf of the sensor is given for each case in figure 83
as a function of pO
2−
.
a. Explain the origin of the two straight lines.
b. Considering only the case of ZnO, compare the slope of the line obtained
in question 2.
c. Give the solubilization reactions of the oxides, the corresponding equilibrium constants, and the expressions relating the solubilities to the
constants.
d. Evaluate the solubilities of ZnO and Li 2 O.