Prsentation PowerPoint

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ML 4-1 ML 4-2
Potentiometric sensors for high temperature
liquids
Jacques FOULETIER Grenoble University, LEPMI,
ENSEEG, BP 75, 38402 SAINT MARTIN DHERES Cedex
(France) E-mail Jacques.Fouletier_at_lepmi.inpg.fr
Véronique GHETTA LPSC, IN2P3-CNRS, 53 Avenue des
Martyrs, 38026 GRENOBLE Cedex (France) E-mail
Veronique.Ghetta_at_lpsc.in2p3.fr
MATGEN-IV International Advanced School on
Materials for Generation-IV Nuclear
Reactors Cargèse, Corsica, September 24 - October
6, 2007
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Potentiometric measurement of activities in
molten salts and molten metals
Part 1
Activity - Activity coefficient - Activity
coefficients, reference states - Henrys and
Raoult laws
Electrochemical chains - Various types of
electrodes (1st, 2nd types, etc.) - Interface
equilibrium - Ideal Cell e.m.f. calculation
Types of cells - Formation cells (without
membranes) - Concentration cell with a porous
membrane - Concentration cells with a solid
electrolyte membrane
Electrolytes main characteristics of molten and
solid electrolytes - structure - conductivity
(ionic, mixed) - Electroactivity domains
Reference electrodes - for molten metals (Pb,
Fe, Na) - for molten salts (chlorides,
fluorides)
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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)
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From chemical potential to Electrochemical
potential
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Chemical and electrochemical potentials
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Electrochemical chains - Various types of
electrodes (1st, 2nd types, etc.) - Interface
equilibrium - Ideal cell e.m.f. calculation
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What is a potentiometric sensor?
Analysis of a component X dissolved in a molten
metal or a molten salt
The objective of this lecture is to describe the
components of this black box. These components
are referred to as electrodes, membranes,
electrolytes, etc. The whole components form an
electrochemical chain.
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Electrochemical chains
(-) Me / Electrolyte 1 // Electrolyte 2 //
Electrolyte 3 / Me / Me ()
Membranes solid electrolyte (permeable to
only one ion) porous membrane (permeable to
several ions, electrons, etc.)
Remark the analyzed component can be dissolved
in electrolyte 2 or 3 or in metal Me
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Junctions

• Junction interface between two ionic conductors

Interface
Ionic conductor
Ionic conductor
Simple ionic junction exchange of only one type
of ion Example ltltO2-gtgt / ((O2-)) stabilized
zirconia/oxide dissolved in molten chloride
Complex ionic junction solid electrolytes
conducting by different ions Examples ltltO2-gtgt /
ltltNagtgt stabilized zirconia / ?-alumina Equil
ibrium O2- 2 Na Na2O
Multiple ionic junction exchange of several
ions Example ltKClgt / ((KCl)) exchange K and
Cl- ltNASICON, Nagt / ((Na - K))
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Electrodes

• Electrode interface between an ionic conductor
  and an electronic one

Interface
Ionic conductor
Electronic conductor
Ionic conductor - aqueous solutions - molten
salts (chlorides, fluorides, nitrates,
carbonates, etc.) - solid electrolyte (anionic
or cationic conductors)
Electronic conductor - solid or liquid metals
or alloys - mixed ionic-electronic conductors
(MIEC)
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Types of electrodes (1)

• 1st kind electrode (metal/metal ion electrode)
  M / Mn
• Equilibrium Mn n e- M

3rd kind electrode (formation of a new phase)
O2,M / ?-Alumina (Na) Equilibrium 2 Na 2
e- 1/2 O2 ltltNa2Ogtgt(?-Alumina )
Other types of electrode (not developed in this
lecture) - ideally polarisable electrodes C /
MX (no electrochemical reaction) - ion blocking
electrodes exchange of electrons, no
electrochemical reaction - electron blocking
electrodes exchange of ions, no electrochemical
reaction - intercalation electrode injection of
ions in an electron conducting phase
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Types of electrodes (2)
GAS ELECTRODE The overall reaction requires a
Three Phase Boundary (TPB) between an
electrolyte, a metal and a gas
METAL
ELECTROLYTE
Gas
Examples - Pt, O2 / stabilized
zirconia Equilibrium 1/2 O2 2 e-
O2- - Cg, Cl2 / molten chloride Equilibrium
1/2 Cl2 e- Cl-
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Equilibrium conditions between two phases same
carriers
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Equilibrium conditions between two phases
different carriers
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Objective measurement of a(Na2O) in NaCl-KCl
E.m.f. calculation of an ideal chain
Each solid electrolyte is conducting by only
one ion (the minority carriers are neglected)
The electronic conductivity of the solid
electrolytes is negligible No current is
passing through the cell Equilibrium at all the
interfaces
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E.m.f. of an ideal chain
(-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl -
KCl - Na2O / YSZ / Pt, O2 ()
Solid
Molten salt
Molten salt
Pt
O2
(-)
()
Pt
YSZ
Ag
Pyrex
NaCl - KCl - Na2O
AgCl
NaCl-KCl
Solid
Molten salt
Molten salt
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The roman catholic church
Types of cells - Cells without membrane -
Concentration cell with a porous membrane -
Concentration cells with a solid electrolyte
membrane
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CELLS WITHOUT MEMBRANE
Example measurement of a(PbO) in PbO-SiO2 mixture
(-) Pt, Fe, Pb(L) / PbO - SiO2(L) / O2(g), Pt
()
Main difficulty solubility of oxygen in lead
R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining
Met., 76 (1967) C44
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CONCENTRATION CELLS cell with membrane (1)
Cell which has identical electrodes and a
membrane inserted between solutions differing
only in concentration.
Two cases
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CONCENTRATION CELLS cell with membrane (2)
(-) Pt, Fe, Pb(L) / PbO - SiO2(L) / YSZ /
PbO(L) / Pb, Fe, Pt () ltltO2-gtgt
Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871
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CONCENTRATION CELLS cell with membrane (3)
? ? (-) Pt, Fe, Pb(L) / PbO - SiO2(L) /
YSZ / PbO(L) / Pb, Fe, Pt ()
Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871
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CONCENTRATION CELLS cell with membrane (4)
? ? (-) Pt, Fe, Pb(L) / PbO - SiO2(L) /
YSZ / PbO / Pb, Fe, Pt ()
R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining
Met., 76 (1967) C44 Z. Kozuka, C.S. Samis, Met.
Trans., 1 (1970) 871
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Electrolytes main characteristics of molten and
solid electrolytes - Structure - Conductivity
(ionic, mixed) - Electroactivity domain
Reference electrodes - for molten metals (Pb,
Fe, Na) - for molten salts (chlorides,
fluorides)
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Solid electrolytes Main characteristics
Only a few solid electrolytes are available
ZrO2-Y2O3, (ThO2-Y2O3), ?-Alumina, CaF2, AlF3,
etc.
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Examples of solid electrolytes
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Solid electrolytes (case of oxides) Main
characteristics
However, electronic species may also be present
due to equilibria between the electrolyte and the
gaseous phase
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Solid electrolytes
Requirements for an ideal potentiometric cell
Conduction by only one ion Negligible
electronic conductivity (far lower than 1 , if
possible ) Chemical stability
Not required conditions for an ideal
potentiometric cell The total conductivity can
be very low (noticeably higher than the input
impedance of the millivoltmeter) The species
exchanged at the electrodes can be different than
the majority carrier of the electrolyte (pH
electrode using a Li or Na glass, oxygen sensor
using CaF2 or ?-alumina electrolytes) The
nature of the majority carrier in the electrolyte
(anions or cations) doesnt matter (oxygen sensor
using oxide ions, fluoride ions or sodium ions)
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Molten electrolytes Main characteristics
Cf. lecture GL 11
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Reference electrodes - for molten metals (Pb,
Fe, Na) - for molten salts (chlorides,
fluorides)
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Reference electrodes (1) Molten metals (Pb, Fe,
Na)
Main criteria - known thermodynamic data
(calibration often necessary) - equilibrium
oxygen pressure within the electrolytic domain
(not always possible Cr/Cr2O3 for molten
steel monitoring) - long term stability -
constant voltage in spite of possible disturbance
(high buffer capacity) - equilibrium activity
not too far from the measured one (reduction of
the semipermeability flux use of Cr/Cr2O3 for
molten steel monitoring)
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Reference electrodes (2) Molten metals Examples
Intermediate-temperature sensors Ref. air,
Pd-PdO , Ir-Ir2O3
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Reference electrodes in molten salts
No universally accepted reference electrode is
available for electrochemical studies although
reference electrodes based on the Ag(I)/Ag(0)
couple are undoubtedly the most common.
Halogen electrode in halide melts are generally
successful, but such electrodes are inferior in
experimental convenience to those based on
Ag(I)/Ag(0).
The design of reliable reference electrodes in
molten fluorides remains a major problem, due to
the corrosive action on metal electrodes, and on
glass or ceramics used as containers or
diaphragms, and also because of the undetermined
liquid junction potentials use of quasi
reference electrode, of in-situ pulse reference
electrodes, etc. However, until yet, no totally
satisfactory designs.
G.J. Janz, in Molten Salts Handbook, Academic
Press, London, 1967.
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Reference electrodes in molten chlorides
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Reference electrodes for molten fluorides
Stability, durability, reversibility,
reproducibility and fast response ?
R. Winand, Electrochim. Acta, 17 (1972) 251
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Reference electrodes for molten fluorides
Liquid junction
Ni - NiF2 contained in a thin-walled boron
nitride envelope. The electrode was developed for
potential measurement in molten LiF-NaF-KF
(42-11.5-46.5 mol.) (FLINAK) at a working
temperature of 500-550C. Boron nitride is slowly
impregnated by the melt to provide ionic contact.
The wetting occurs in about 6 hours in molten
FLINAK. At higher temperatures, the BN appears to
deteriorate permitting mixing of the melts.
Furthermore, the boron nitride tube contained a
boric oxide binder that dissolved contaminated
the electrolyte, and changed the electrode
potential.
LiF-NaF-KF, LiF-BeF2-ZrF4 15 jours, Tmax 500
H.W. Jenkins, G. Mamantov and D.L. Manning, J.
Electroanal. Chem., 19 (1968) 385. H.W. Jenkins,
G. Mamantov and D.L. Manning, J. Electrochem.
Soc., 117 (1970) 183. P. Taxil and Zhiyu Qiao, J.
Chim. Phys., 82 (1985) 83.
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Reference electrodes for molten fluorides
? Composé ionique
Ionic membrane
The nickel-nickel fluoride reference electrode
system exhibiting a membrane from a single
crystal lanthanum trifluoride. Because of the
solubility of the LaF3 in the fluorides melts, a
nickel frit with fine porosity was used in order
to protect the crystal. The system was tested for
temperatures up to 600C. On the other hand, the
single crystal LaF3 is expensive, the assembling
of the electrode is more complicated while the
crystal cracks after few experiments.
LiF-BeF2-ZrF4 LiF-NaF-KF NaBF4 Tmax 500
H. R. Bronstein, D. L. Manning, J. Electrochem.
Soc., 119(2) (1972) 125 F. R. Clayton, G.
Mamantov, D.L. Manning, High Temp. Science, 5
(1973) 358
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Reference electrodes for molten fluorides
Pseudo-reference electrodes
Relatively stable reference point, provided no
oxidizing or reducing species come into contact
with the electrode.
According to Mamantov, Ni or Pt wires had a
constant potential within 10 mV in molten
fluorides over a period of months. G. Mamantov,
Molten Salts Characteriza- tion and Analysis,
Dekker, New York, 1969, p.537
An inert metal M in contact with a
solution Example Pt / PtOx / O2- A.D. Graves,
D. Inman, Nature, 208 (1965) 481.
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Reference electrodes for molten fluorides
Pulse reference electrode
Electrochemical generation of an in-situ redox
couple for a very short time Use this system as
an internal redox probe to check periodically a
classical reference electrode.
The amount of foreign species introduced into the
electrolyte must be very small to avoid
contamination and consequent modification of the
experimental conditions
N. Adhoum, J. Bouteillon, D. Dumas, J.C. Poignet,
J. Electroanal. Chem., 391 (1995) 63 Y.
Berghoute, A. Salmi, F. Lantelme, J. Electroanal.
Chem., 365 (1994) 171.
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End of the first part