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5 – Applications
pressure P
int
O 2 in the interior compartment to the pressure P O
ext
2
of the external
gas to be analyzed, so that P
P
int
O
O
ext
2
2
=
. This condition is satisfied when the
emf ∆E of the cell goes to zero.
2. The expression relating the oxygen partial pressure of the gas to be analyzed
to the charge Q required to equalize the pressure is
P
FV
RT
Q
4
O 2 =
#
Numerical evaluation gives
.
.
.
P
4 96 480 3 5 10
8 314 973
9 68 10
O
7
3
2
=
−
−
#
#
#
#
#
#
P
P a
580
O 2 =
3. The measurement error due to the coulometric sensor is
0.5
0.58 0.5
100%
ρ =
−
#
16%
ρ =
4. Among the advantages, we can cite the following:
2 no reference electrode and
2 miniaturization is possible.
The main drawbacks are
2 the interior volume of the compartment must be determined,
2 a risk of leaks,
2 oxygen permeation, and
2 discontinuous measurement.
Solution 5.13 – Nitrogen oxide sensor
A – Type (I) cell involving sodium ionic conducting electrolyte
(NASICON or Na-β-alumina)
1. a. Expression of emf for cell (Ia)
Na / NASICON (Na
+
) / NaNO 3 (porous layer) / Au,NO 2 ,O 2
as a function of NO 2 partial pressure