Exercises
233
Write an electrode reaction that is simpler than that proposed in question 7
and that accounts for the preceding observations.
9. In sensors (Ia) and (Ib), the reference electrode is formed by the pair Na
+
/Na.
Its practical implementation is delicate because of the metallic sodium (very
reductive, liquid state, cost). Propose another reference electrode that avoids
such inconveniences.
B – Type (II) cell involving solid O
2−
ionic conducting electrolyte
(ZrO 2 -Y 2 O 3 , ZrO 2 -MgO)
To measure the concentration of nitrogen oxides at high temperatures and in
aggressive conditions (and in particular to analyze automobile exhaust gas),
we use sensors such as the following type (IIa) cells:
O 2 ,NO,Pt / ZrO 2 -Y 2 O 3 / Ba(NO 3 ) 2 / Au,NO,O 2
(IIa)
(reference)
(measurement)
The barium nitrate layer and gold form a porous cermet that allows gaseous
species, and in particular nitrogen oxides, to be in direct contact with the stabilized zirconia. Figure 95 shows a schematic representation of the cell.
%D12 $X
=U2 < 2
3W
12
2
9
Figure 95 – Schematic representation of type (IIa) cell.
1. Write the electrochemical reactions that occur at each interface.
2. Show that the emf ∆E of the cell may be expressed in the form
E A 4 F
RT
ln P
3F
RT
ln P
O
N O
2
Δ = +
+
where A is a constant.