Exercises
223
3. Taking the sensor resistance to be negligible, evaluate the lifetime of the
reference (Na) if, to measure the emf, we use a voltmeter with a small input
impedance (10
7
Ω) for P CO 2 = 10
−5
bar.
Data
Sodium molar mass M Na = 23 g mol
−1
Sodium density ρ Na = 0.97 g cm
−3
4. We define the isoelectric point of the sensor as the pressure P CO 2 that renders
the emf independent of measurement temperature. What is this pressure?
Does it fall within the range of pressure of CO 2 in the ambient air?
5. We replace the sodium layer, which is difficult to fabricate, by a reference
system that involves the solid solution (AgBr) 0.99 (NaBr) 0.01 . Under the
measurement conditions, this solution is an ionic conductor.
a. Write down the electrochemical chain.
b. Give the analytic expression for the emf at the sensor terminals.
c. Discuss the evolution of the emf when we use a voltmeter with a small input
impedance and qualitatively compare the result with that of question 3.
Exercise 5.8 – CO 2 sensor (b)
We consider a carbon dioxide CO 2 potentiometric sensor based on the following
electrochemical chain:
CO 2 ,O 2 / LSM,Na 2 Ti 6 O 13 -TiO 2 / NASICON / Na 2 CO 3 ,Au / O 2 ,CO 2
reference electrode
electrolyte measurement electrode
(RE)
(ME)
where LSM is an oxide-based electrode material with dominant electronic
conduction and the formula La 0.8 Sr 0.2 MnO 3−δ and Na 2 Ti 6 O 13 -TiO 2 is a mix
of two solid phases. NASICON is a Na
+
ionic conducting solid solution with
the formula Na 3 Zr 2 Si 2 PO 12 , and Na 2 CO 3 is the sodium carbonate in the measurement electrode.
We assume that the solid phases are pure.
1. Write the equilibria that occur at the reference and measurement electrodes.
2. Give the overall operating reaction of the sensor.
223
3. Taking the sensor resistance to be negligible, evaluate the lifetime of the
reference (Na) if, to measure the emf, we use a voltmeter with a small input
impedance (10
7
Ω) for P CO 2 = 10
−5
bar.
Data
Sodium molar mass M Na = 23 g mol
−1
Sodium density ρ Na = 0.97 g cm
−3
4. We define the isoelectric point of the sensor as the pressure P CO 2 that renders
the emf independent of measurement temperature. What is this pressure?
Does it fall within the range of pressure of CO 2 in the ambient air?
5. We replace the sodium layer, which is difficult to fabricate, by a reference
system that involves the solid solution (AgBr) 0.99 (NaBr) 0.01 . Under the
measurement conditions, this solution is an ionic conductor.
a. Write down the electrochemical chain.
b. Give the analytic expression for the emf at the sensor terminals.
c. Discuss the evolution of the emf when we use a voltmeter with a small input
impedance and qualitatively compare the result with that of question 3.
Exercise 5.8 – CO 2 sensor (b)
We consider a carbon dioxide CO 2 potentiometric sensor based on the following
electrochemical chain:
CO 2 ,O 2 / LSM,Na 2 Ti 6 O 13 -TiO 2 / NASICON / Na 2 CO 3 ,Au / O 2 ,CO 2
reference electrode
electrolyte measurement electrode
(RE)
(ME)
where LSM is an oxide-based electrode material with dominant electronic
conduction and the formula La 0.8 Sr 0.2 MnO 3−δ and Na 2 Ti 6 O 13 -TiO 2 is a mix
of two solid phases. NASICON is a Na
+
ionic conducting solid solution with
the formula Na 3 Zr 2 Si 2 PO 12 , and Na 2 CO 3 is the sodium carbonate in the measurement electrode.
We assume that the solid phases are pure.
1. Write the equilibria that occur at the reference and measurement electrodes.
2. Give the overall operating reaction of the sensor.