Transcript Potentiometry

Potentiometry
• Potential measurements of electrochemical cells
• Ion selective methods
 Reference electrode
 Indicator electrode
 Potential measuring device
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Reference electrode
Indicator electrodes
Ion specific electrodes
Potentiometric measurements
15-1
Reference electrode
• Known half-cell
• Insensitive to solution under examination
 Reversible and obeys Nernst equation
 Constant potential
 Returns to original potential
• Calomel electrode
 Hg in contact with Hg(I) chloride
 Ag/AgCl
15-2
Calomel electrode
15-3
15-4
Indicator electrode
• Ecell=Eindicator-Ereference
• Metallic
 1st kind, 2nd kind, 3rd kind, redox
• 1st kind
 respond directly to changing activity of
electrode ion
 Direct equilibrium with solution
15-5
Ion selective electrode
• Not very selective
• simple
• some metals easily
oxidized (deaerated
solutions)
• some metals (Zn,
Cd) dissolve in
acidic solutions
• Ag, Hg, Cu, Zn, Cd,
Bi, Tl, Pb
15-6
2nd kind
• Precipitate or stable complex of ion
 Ag for halides
 Ag wire in AgCl saturated surface
• Complexes with organic ligands
 EDTA
• 3rd kind
 Electrode responds to different cation
 Competition with ligand complex
15-7
Metallic Redox Indictors
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Inert metals
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Pt, Au, Pd
 Electron source or sink
 Redox of metal ion evaluated
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May not be reversible
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Membrane Indicator electrodes
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Non-crystalline membranes:
 Glass - silicate glasses for H+, Na+
 Liquid - liquid ion exchanger for Ca2+
 Immobilized liquid - liquid/PVC matrix for Ca2+ and NO3
Crystalline membranes:
 Single crystal - LaF3 for FPolycrystalline
 or mixed crystal - AgS for S2- and Ag+
Properties
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Low solubility - solids, semi-solids and polymers
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Some electrical conductivity - often by doping
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Selectivity - part of membrane binds/reacts with analyte
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15-8
Glass Membrane Electrode
15-9
Glass membrane structure
• H+ carries current near
surface
• Na+ carries current in
interior
• Ca2+ carries no current
(immobile)
15-10
Boundary Potential
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Difference in potentials at a
surface
Potential difference determined by
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Eref 1 - SCE (constant)
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Eref 2 - Ag/AgCl (constant)
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Eb
Eb = E1 - E2 = 0.0592 log(a1/a2)
a1=analyte
a2=inside ref electrode 2
If a2 is constant then
Eb = L + 0.0592log a1
= L - 0.0592 pH
where L = -0.0592log a2
Since Eref 1 and Eref2 are
constant
Ecell = constant - 0.0592 pH
15-11
Alkaline error
• Electrodes respond to H+ and
cation
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pH differential
• Glass Electrodes for Other
Ions:
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Maximize kH/Na for
other ions by modifying
glass surface
 Al2O3 or B2O3)
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Possible to make glass
membrane electrodes
for
 Na+, K+, NH4+, Cs+,
Rb+, Li+, Ag+
15-12
Crystalline membrane electrode
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Usually ionic compound
Single crystal
Crushed powder, melted and formed
Sometimes doped (Li+) to increase conductivity
Operation similar to glass membrane
• F electrode
15-13
Liquid membrane electrodes
• Based on potential that
develops across two
immiscible liquids with
different affinities for analyte
• Porous membrane used to
separate liquids
• Selectively bond certain ions
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Activities of different
cations
• Calcium dialkyl phosphate
insoluble in water, but binds
Ca2+ strongly
15-14
15-15
Molecular Selective electrodes
• Response towards molecules
• Gas Sensing Probes
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Simple electrochemical
cell with two reference
electrodes and gas
permeable PTFE
membrane
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allows small gas
molecules to pass and
dissolve into internal
solution
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O2, NH3/NH4+, and
CO2/HCO3-/CO32-
15-16
15-17
Biocatalytic Membrane Electrodes
• Immobilized enzyme bound to gas permeable membrane
• Catalytic enzyme reaction produces small gaseous molecule (H+,
NH3, CO2)
• gas sensing probe measures change in gas concentration in internal
solution
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Fast
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Very selective
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Used in vivo
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Expensive
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Only few enzymes immobilized
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Immobilization changes activity
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Limited operating conditions
 pH
 temperature
 ionic strength
15-18
Electrode calibration
15-19
NH4 electrode
15-20
Potentiometric titration
15-21
Coulometry
• Quantitative conversion of ion to new oxidation
state
 Constant potential coulometry
 Constant current coulometry
Coulometric titrations
* Electricity needed to complete
electrolysis measured
 Electrogravimetry
Mass of deposit on electrode
15-22
Constant voltage coulometry
• Electrolysis performed different ways
 Applied cell potential constant
 Electrolysis current constant
 Working electrode held constant
 ECell=Ecathode-Eanode +(cathode
polarization)+(anode polarization)-IR
• Constant potential, decrease in current
 1st order
 It=Ioe-kt
• Constant current change in potential
 Variation in electrochemical reaction
 Metal ion, then water
15-23
15-24
Analysis
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Measurement of electricity needed to convert ion to different oxidation state
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Coulomb (C)
 Charge transported in 1 second by current of 1 ampere
* Q=It
I= ampere, t in seconds
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Faraday (F)
 Charge in coulombs associated with mole of electrons
* 1.602E-19 C for electron
* F=96485 C/mole eQ=nFN
Find amount of Cu2+ deposited at cathode
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Current = 0.8 A, t=1000 s
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Q=0.8(1000)=800 C
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n=2
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N=800/(2*96485)=4.1 mM
15-25
Coulometric methods
• Two types of methods
• Potentiostatic coulometry
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maintains potential of working electrode at a constant so
oxidation or reduction can be quantifiably measured without
involvement of other components in the solution
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Current initially high but decreases
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Measure electricity needed for redox
 arsenic determined oxidation of arsenous acid (H3AsO3)
to arsenic acid (H3AsO4) at a platinum electrode.
• Coulometric titration
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titrant is generated electrochemically by constant current
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concentration of the titrant is equivalent to the generating
current
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volume of the titrant is equivalent to the generating time
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Indicator used to determined endpoint
15-26