ML 4-1 & ML 4-2
Potentiometric sensors for high temperature liquids
PART 2
Jacques FOULETIER
Grenoble University, LEPMI, ENSEEG, BP 75, 38402 SAINT MARTIN D’HERES Cedex (France)
E-mail: Jacques.Fouletier@lepmi.inpg.fr
Véronique GHETTA
LPSC, IN2P3-CNRS, 53 Avenue des Martyrs, 38026 GRENOBLE Cedex (France)
E-mail: Veronique.Ghetta@lpsc.in2p3.fr
MATGEN-IV: International Advanced School on Materials for Generation-IV Nuclear Reactors
Cargèse, Corsica, September 24 - October 6, 2007
Part 2
Sources of errors in potentiometric cells:
- Errors ascribed to the reference electrode
- reversibility
- reactivity
- Errors due to the porous membrane
- concentration modification
- diffusion potential
- Errors due to the solid electrolyte membrane
- partial electronic conductivity
- interferences
- Errors due to the measuring electrode
- buffer capacity
- mixed potential
Case studies:
- Oxide ion activity in molten chlorides
- Oxidation potential in molten fluorides
- Monitoring of oxygen, hydrogen and carbon in molten metals (Pb, Na)
Sources of errors in potentiometric cells
(-) Pt / Ag / AgCl / NaCl - KCl / Porous / NaCl - KCl - Na2O / YSZ / Pt, O2 (+)
membrane
Sources of errors in potentiometric cells:
- Errors ascribed to the reference electrode(s)
- reversibility
- reactivity
- Errors due to the porous membrane
- concentration modification
- diffusion potential
Errors due to the solid electrolyte
membrane
- partial electronic conductivity
- interferences
- Errors due to the measuring electrode
- buffer capacity
- mixed potential
Errors ascribed to the reference electrode (1)
Types of reference electrodes:
• 2nd kind electrodes: Ag/AgCl/Cl-, Ni/NiO/YSZ
• Gas electrodes:
O2/Pt/YSZ, Cl2/Cg/ClRequirements:
-
Easy to handle
Long term stability (oxidation, miscibility within the electrolyte, etc.)
No partial reduction of the electrolyte, inducing electronic conductivity
Known thermodynamic data (calibration often necessary),
advantage of air or pure oxygen reference electrode
- Reversibility (low sensitivity to perturbations)
Errors due to the porous membrane (1)
Porous plugs or frits are used to prevent mixing of the contents of the various
compartments in a manner analogous to aqueous bridges.
Glue cap
Porous alumina tube
(5 % porosity)
Ag
Platinum wire
LiCl-KCl
+ AgCl 0.75 mol.kg-1
Flux of matter through
the porous membrane
- concentrations modification of the
analyzed medium
- contamination of the reference salt
- diffusion potential
What is “diffusion potential”?
Ionic membrane:
protonic conductor
HCl
C1
f1
H+
Jmatter
C1 > C 2
HCl
C2
f2
Non-porous
mixed conductor
HCl
C1
u(H+)
u(Cl-)
HCl
C2
Porous membrane
HCl
C1
u(H+)
u(Cl-)
HCl
C2
Jmatter
Jmatter
Porous membrane
Porous membrane
C1 > C 2
C 1 > C2
u(H+) > u(Cl-)
u(H+) > u(Cl-)
Is there only a flux of matter?
Errors due to the porous membrane (2):
Electrochemical Diffusion - Liquid Junction
Liquid Junction
J(H+)
J(H+)
HCl
C1
u(H+)
HCl
C2
u(Cl-)
HCl
C1
-
+
+
+
+
HCl
C1
HCl
C2
J(Cl-)
Porous membrane
C1 > C 2
u(H+) > u(Cl-)
Transitory State
SPACE CHARGE
E
HCl
C2
J(Cl-)
f2
f1
Stationary State
J(H+) = J(Cl-)
Electric field Junction Potential
Ej = f2 - f1
E
Errors due to the porous membrane (2’):
Electrochemical Diffusion - Liquid Junction
Porous membrane
Concentrated
solvent salt
E ~ 0
Concentrated
solvent salt
+ diluted solute
When the solvent salt is the same in both compartments and the
concentrations of solutes are low (less than 0.1 molal), the liquid
junction potentials across the compartment separator is at most
one or two mV and can be neglected.
Errors due to the solid electrolyte membrane (1)
- partial electronic conductivity
- interferences
Errors due to mixed conductivity of the electrolyte
Two situations:
- one or both interfaces are outside the electrolytic domain
Examples: oxygen monitoring in molten steel or molten sodium
- both interfaces are within the electrolytic domain (the
electronic transport number is smaller than 1%)
Errors due to interferences at the electrolyte interface(s)
- exchange of particles
Examples: exchange H+/Li+(pH electrode), K+/Na+
Errors due to the solid electrolyte membrane (2)
1. One or both interfaces are outside the electrolytic domain
Case of oxygen monitoring in molten steel and sodium
Log PO2
Temperature
Oxygen dissolved
in liquid steel
Domain of
predominant
ionic conduction
(99%)
Oxygen dissolved
in liquid sodium
• use of YDT instead of YSZ for oxygen analysis in molten sodium
diagram
• use of YSZ (or Patterson
CSZ) in molten
steel: correction required
E measured = Eth
x
ti
Errors due to the solid electrolyte membrane (3)
- E (V)
1. One or both interfaces are outside the electrolytic domain
(1) Nernst
(2) Correction of the ionic transport number
(3) Diffusion polarization correction
(1)
(2)
(3)
0.3
0.2
0
10
20
50 100 200
500 1000 2000
Mo/MoO
CSZ
Oxygen content (ppm)
Steel
(% O)interface
(% O)bulk
p(O2)
J(O2)
2
0.1
Oxygen concentration gradient at the
liquid steel-electrolyte interface
T = 1600°C
Liquid steel
Errors due to the solid electrolyte membrane (4)
2. Both interfaces are inside the electrolytic domain
O2-
Partial electronic conductivity within the electrolyte
eO2 (P1)
e- flux
J(O2)
O2 (P2)
Electroneutrality
Consequence: oxygen semipermeability flux through
the electrolyte without external current
P1 > P2
SOLID
ELECTROLYTE
Compensated by an identical ionic flux
SOLID
ELECTROLYTE
METAL
METAL
Jsp
a
Jdes
Jads
GAS
P
Equilibrium conditions
Jads = Jdes
a
Jdes
Jads
GAS
P
Stationary state
Jsp + Jads = Jdes
measured activity = oxygen pressure measured activity  oxygen pressure
Errors due to the solid electrolyte membrane (6)
Interference
When a solid electrolyte is in contact with an active species, it can penetrate
into the the bulk by exchange followed by diffusion:
iSE + jsol  jSE + isol
C ibulk
SE
fbulk
fsurf
i
C isurf
j
Solution
fmeas ftheor
fsolution
Empirical equations for the Galvani
potential have been developed
Examples:
-Alumina (NaAl11O17)
NASICON (Na3Zr2Si2PO12)
Potassium cation
Exchange Li+, Na+, K+, as a function
of the size of the channel
Errors due to the solid electrolyte membrane (7)
Interference
Case of the glass electrode
Protons do not penetrate into the membrane (their mobility is very low).
The interfering phenomenon is a surface reaction with the formation of a gel
which can be viewed as a thin protonic membrane
pH sensor
Errors due to the measuring electrode
Main source of error:
- The analyzed solutions are often complex and the cell e.m.f. is not
a thermodynamic voltage but a mixed potential
This mixed potential can be due to impurities within the analyzed
medium or can be observed after a long term exposition due to
deposition of impurities on the measuring electrode.
Mixed potential
If there is more than one redox couple in the analyzed system (solution or gas),
the voltage is not a thermodynamic potential.
It the case of a M+/M in a solution saturated with oxygen.
The following reactions take place:
M  M++ e1/2 O2 + 2 e-  O2E
Ired
E1
Em
reduction
(1)
E2
Iox
I
(2)
At the mixed potential, Em,
Ioxidation = Ireduction
oxidation
Mixed potential: generally, the voltage takes an intermediate value (E2 < Em < E1)
Mixed-potential type oxygen sensor
CO2 + + ++
7
+
12
+
10
8
6
+
HC
+
6
stoichiom. A/F 5
+
x
CO
x
4
x
x
x
x
x
x
O2 x
x
2
x
4
NO
x x
x
0
+
NO (103 ppm)
HC (102 ppm)
CO, CO2, O2
(volume %)
14
x
3
2
1
x
10
12
14
16
18
20
A/F
Exhaust gases composition
E(V)
1
RICH
MIXTURE
LEAN
MIXTURE
Lambda sensor
0
14,5
A/F
Variation of the emf vs. A/F ratio
Case studies:
Menasina beach
- Oxide ion activity in molten chlorides
- Oxidation potential in molten fluorides
- Monitoring of oxygen, hydrogen and carbon in molten metals (Pb, Na)
Media conditions:
Solid electrolyte measurements in melts
Main difficulties:
- thermal shock
- wide temperature range (200°C - 1600°C)
- time life required
- stability domain of the electrolytes
- corrosion
- reference electrode
Sodium
Copper
Steel
Glass
Temperature
(°C)
400
1150
1650
1500
Rate of T changes
5
50
5000
50
CO (ppm)
0.5 - 50
1 - 1000
1 - 2000
Time of operation
10 000 h
100 h
5 s
50 h
SE material
ThO2 - Y2O3
(cub.)
ZrO2 - CaO
(cub.)
ZrO2 - MgO
(cub.-mon.)
ZrO2 - Y2O3
(cub.)
(K.min-1)
Oxide ion activity in molten chlorides (1)
(-) Pt/Ag/AgCl/NaCl-KCl/Pyrex/NaCl-KCl-Na2O/YSZ/Pt,O2 (+)
Ref.1
Zirconia sensor
Seal
E
Air
Sensing
membrane
Ref 1
M/Ref.1 ((O2-)) YSZ Ref.2/M
Pt
fPt(+)
YSZ
Melt
Ref.2
fYSZ
fPt(-)
Pt
M/MOx
Pd/PdO, Ni/NiO, etc.
B. Tremillon, G. Picard, Proc. 1st Intern. Symp. on
Molten Salt Chem. and Techn. Kyoto (1983), p. 93.
f((O
2-
))
Emea.
ERef2
E
ERef1
E  E 
2.3 RT
.log (O 2 )
2F
Oxide ion activity in molten chlorides (2)
Ref / O2- / YSZ / Pt, O2
E  E 
2.3 RT
.log (O 2 )
2F
Measurement of oxide solubility in molten chlorides
E(mV) / Ag

Theor. Slope: 72 mV/u. p(O2-)
LiCl-KCl-ZnCl2
ZnO
T = 723 K
-270
-280
-290
-300
Li2O
-310
-320
1
1,4
1,8
-log(O2-)
J. Shenin-King, PhD Thesis, Paris 6, 1994
The Dolmen of Paomia
Monitoring of oxygen, hydrogen and carbon
in molten metals (Na, Pb)
Monitoring of oxygen
Oxygen monitoring in molten sodium (1)
Brookhaven National Lab., USA, 1972
Interatom, Germany, 1975
Berkeley Nuclear Lab., UK, 1982
Harwell, UK, 1983
Nuclear Research Institute, Czechoslovakia, 1984
Oxygen meters have application to both primary and secondary circuits of a fast reactor.
When used in a fast reactor primary coolant circuit they have to perform in high-radiation
environment.
The corrosion of metals and alloys increases with high oxygen concentration in sodium.
Stability of the electrolytes (n-type or p-type electronic conductivity).
Electrode reaction kinetics at low temperatures.
YDT electrolyte
J. Jung, J. Nuclear Mat., 56 (1975) 213.
M.R. Hobdell, C.A. Smith, J. Nuclear Mat., 110 (1982) 125
R.G. Taylor, R. Thompson, J. Nuclear Mat., 115 (1983) 25.
D. Jakes, J. Kral, J. Burda, M. Fresl, Solid State Ionics, 13 (1984) 165.
H. Ullmann, K. Teske, Sensors and Actuators B, 4 (1991) 417.
Oxygen monitoring in molten sodium (2)
• Reference electrode Sn/SnO2 and
In/In2O3.
• Good performance over lifetimes
exceeding 400 days.
• Grain-boundary attack under highoxygen sodium.
• Tests under -radiation
R.G. Taylor, R. Thompson, J. Nuclear Mat., 115 (1983) 25
Oxygen monitoring in molten sodium (3)
Thoria-yttria electrolyte soldered
in a stainless steel tube.
Electrolyte crucible
(ThO2 - Y2O3 10-12 mol.%).
Glass brazing.
Glass brazing
D. Jakes, J. Kral, J. Burda, M. Fresl, Solid State Ionics,
13 (1984) 165.
D. Jakes, J. Sebkova, L. Kubicek, J. Nuclear Mat.,
132 (1985) 88.
Temperature dependence of the saturation
solubility in liquid sodium.
Log CO = 5.64 ± 0.22 - (2120 ± 100)/T (CO in ppm)
H. Ullmann, K. Teske, Sensors and Actuators B, 4 (1991) 417.
Oxygen monitoring in molten lead and lead-bismuth (1)
Main challenge: measurement at low temperatures (< 400°C)
Reversibility of the electrode reactions
YSZ electrolyte
Reference electrode
• Air / Pt / YSZ and Air / La0.7Sr0.3CoO3 / YSZ
mixed conductor
V. Ghetta, J. Fouletier, M. Hénault, A. Le Moulec, J. Phys. IV France, 12 (2002) 123
YSZ
• Bi / Bi2O3 / YSZ
J.L. Courouau, P. Deloffre, R. Adriano, J. Phys. IV France, 12 (2002) 141.
J. L. Courouau, J. Nucl. Mat., 335 (2004) 254.
• In / In2O3 / YSZ or Air / Pt / YSZ
J. Konys, H. Muscher, Z. Vo, O. Wedemeyer, J. Nucl. Mat., 296 (2001) 289
Oxygen monitoring in molten lead and lead-bismuth (2)
Measurement of oxygen activity in saturated molten lead
(335°C < T < 530°C)
E(mV)
Gas inlet
-9.00
Gas outlet
Air
log (ao)
-10.0
-11.0
-12.0
LEAD
ao
Oxygen sensor
with air reference
-13.0
-14.0
300
350
400
450
T (Celsius)
500
V. Ghetta et al.
550
Oxygen monitoring in molten lead and lead-bismuth (4)
Coupling of an oxygen sensor with a zirconia pump
Gas inlet
E(mV)
I(mA)
Gas outlet
Ipump
Air
• Zirconia tube
• Pt/Air internal
electrodes
• Zirconia tube
• Pt/Air internal
electrodes
Air
LEAD
ao
Oxygen sensor
The actual oxygen activity is
measured with the oxygen sensor
Oxygen pump
The oxygen content in the bath is
controlled by the oxygen pump
V. Ghetta, F. Gamaoun, M. Hénault, A. Le Moulec, J. Fouletier, J. Nucl. Materials, 296 (2001) 295-300.
V. Ghetta, J. Fouletier, M. Hénault, A. Le Moulec, J. Phys. IV France, 12 (2002) 123-140.
Oxygen monitoring in molten lead and lead-bismuth (5)
Closed system
mPb
nO 
M Pb
2F

exp Esensor

RT

O
1/ 2
Pair
n O 
Theoretical
Nerst law
 tt 1 Ipump dt
0
2F
Theoretical
Faraday law
E(mV)

2 independent
measurements
Air
I(mA)
Air
ao
SENSOR
PUMP
Oxygen monitoring in molten lead and lead-bismuth (6)
Verification of the functioning of the set-up
-4
nO variations (moles)
4.0 10
Theoretical straight line
-4
3.0 10

o
slope
-4
2.0 10
-4
1.0 10
T = 527 °C
0
0.0 10
0
10
20
30
40
50
60
Qcumulative (Coulomb)
70
80
V. Ghetta et al.
Monitoring of hydrogen
Hydrogen monitoring in molten sodium (1)
Na(H) / Fe / CaH2 - CaCl2 / Fe / Li, LiH
Solid electrolyte
Iron diffusion membrane
Reference electrode
C.A. Smith, CEGB Technical Disclosure Bulletin, 227 (1974).
M.R. Hobdell, C.A. Smith, J. Nuclear Mat., 110 (1982) 125.
T. Gnanasekaran, V. Ganesan, G. Periaswami, C.K. Mathews, H.U. Borgstedt, J. Nuclear Mat. 171 (1990) 198.
Hydrogen monitoring in molten metals (2)
Use of protonic conductors:
- Yb, Nd or Gd cerates (BaCeO3)
- In doped zirconate (CaZrO3)
-20
-30

(h•)
domain
-10
0
(Hi•)
domain
Main specificity:
Various conductivity domains
as functions of temperature
and atmospheres
0
T =600°
-10
Log pO2
Sensors for monitoring of hydrogen in Al (ca. 973 K), Cu (ca. 1423 K)
or Zn (ca. 723 K)
Log pH2
-20
-30
(VO••) domain
-40
400°500°
N. Kurita, N. Fukatsu, K. Ito, T. Ohashi J. Electrochem. Soc., 142 (1995) 1552.
N. Fukatsu, N. Kurita, T. Yajima, K. Koide, T. Ohashi, J. Alloys and Compounds, 231 (1995) 706.
Hydrogen monitoring in molten metals (3)
Three limiting cases
CaZr0.9In0.1O3-
T=1473 K
T=973 K
T=773 K
Condition of copper
melting
Mixed conduction at low
oxygen activities
Condition of Al
melting
No direct contact between
(Al) and the electrolyte
N. Kurita, N. Fukatsu, K. Ito, T. Ohashi J. Electrochem. Soc., 142 (1995) 1552.
N. Fukatsu, N. Kurita, Ionics, 11 (2005) 54.
Condition of Na
melting
Protonic conductor
Hydrogen monitoring in molten metals (4)
((H))metal // Zirconate // Pt/gaz, fixed % H2
Sensor for liquid Al
Measurement of the hydrogen activity
in the gas phase equilibrated with the
molten Al
With Pb, Pb-Li (or Na ?) the
electrolyte could be in contact
with the melt metal.
The greek church
Monitoring of carbon
Carbon monitoring in molten sodium (1)
An optimum amount of carbon in austenitic and ferritic steels used as structural
materials is essential for maintaining good mechanical properties during the life of
the reactor.
Owing to the solubility of carbon in molten sodium, according to the temperature,
carburization or decarburization can take place.
Moreover, accidental ingress of oil from pumps or contamination from carbon dioxide
in air will lead to a build up of carbon activity in sodium.
Carbide-chloride electrolytes: not successful
Alkali molten carbonates
M.R. Hobdell, C.A. Smith, J. Nuclear Mat., 110 (1982) 125.
M.R. Hobdell, E.A. Trevillion, J.R. Gwyther, S.P. Tyfield, J. Electrochem. Soc., 129 (1982) 2746.
S. Rajendran Pillai, C.K. Mattews, J. Nuclear Mat., 137 (1986) 107.
Carbon monitoring in molten sodium (2)
Hobdell et al.
or
Fe3C / Fe / Na2CO3 - Li2CO3 / (( C ))Na
Graphite / Fe / Na2CO3 - Li2CO3 / (( C ))Na
Rajendral Pillai
Electrode reaction at both electrodes:
CO32- + 4 e- = C + 3 O2-
E 

Main difficulties:
RT  a ((C))Na 
ln 

4F 
a
 C Ref 

- use of a permeable -iron membrane (equilibrium ?)
- life time of the reference
Carbon monitoring in molten sodium (3)
Fe3C/Fe/Na2CO3-Li2CO3/(( C ))Na
Permeable thin
Fe membranes
Na2CO3-Li2CO3
E
Fe3C
Molten Na
Cgraphite/Fe/Na2CO3-Li2CO3/(( C ))Na
Permeable thin
Fe membrane
Na2CO3-Li2CO3
E Ni capsule
containing Cg
Graphite
Molten Na
Discontinuous measurement
M.R. Hobdell, C.A. Smith, J. Nuclear Mat.,
110 (1982) 125.
S. Rajendran Pillai, C.K. Mattews, J. Nuclear Mat.,
137 (1986) 107.
Thank you for your attention
Errors due to the measuring electrode
BUFFER CAPACITY OF A GAS
Buffer capacity of an acid/base mixture
Maximum buffer capacity
Buffer capacity
Titration of a weak acid
Buffer capacity : number of moles of acid
(or base) inducing ∆pH = ± 1
Errors due to the measuring electrode (3)
log  (mol.)
BUFFER CAPACITY: OXYGEN SENSORS
CO2-CO-O2
He-O2
-2
: Buffer capacity
of the gas
Number of moles
of oxygen for changing
900°C the chemical potential
1000°C of 1 kJ/mole of gas
800°C
-4
-6
-8
0
4
1 <---------> 10-6
8
12
Pressure domains
of correct utilization
of the sensor
at 900°C
16
- log pO2 (atm)
10-10 <---------------> 10-25
Oxygen pressure domain
PO2 (bar)
10-25
10-20
10-15
mixtures CO - CO2 or H2 - H2O
10-10
10-5
1
He, Ar or N 2 - O2
partial vacuum -O 2
Errors due to the measuring electrode
Monitoring of the oxygen pressure down to 10-25 bar
provided the gas exhibits a sufficient buffer capacity
D
CO2 or
Ar-H2 (5%)
or H2
PUMP
I
SENSOR
E
P(O2)
A pressure less than 10-23 bar, is it possible?
He + O2
P > 10-7 bar
P < 10-7 bar: two situations
He + O2
+ traces of CO, CO2,
H2, H2O
He + O2
CO, CO2+ H2, H2O
P < 10-7 bar
P < 10-7 bar
No oxygen monitoring
Easy oxygen monitoring