1. 15-1
Potentiometry
• Potential measurements of electrochemical cells
• Ion selective methods
Reference electrode
Indicator electrode
Potential measuring device
• Reference electrode
• Indicator electrodes
• Ion specific electrodes
• Potentiometric measurements
2. 15-2
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
5. 15-5
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
6. 15-6
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
7. 15-7
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
8. 15-8
Metallic Redox Indictors
• Inert metals
Pt, Au, Pd
Electron source or sink
Redox of metal ion evaluated
May not be reversible
• Membrane Indicator electrodes
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
Low solubility - solids, semi-solids and polymers
Some electrical conductivity - often by doping
Selectivity - part of membrane binds/reacts with analyte
10. 15-10
Glass membrane structure
• H+ carries current near
surface
• Na+ carries current in
interior
• Ca2+ carries no current
(immobile)
11. 15-11
Boundary Potential
• Difference in potentials at a
surface
• Potential difference determined by
Eref 1 - SCE (constant)
Eref 2 - Ag/AgCl (constant)
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
12. 15-12
Alkaline error
• Electrodes respond to H+ and
cation
pH differential
• Glass Electrodes for Other
Ions:
Maximize kH/Na for
other ions by modifying
glass surface
Al2O3 or B2O3)
Possible to make glass
membrane electrodes
for
Na+, K+, NH+, Cs+,
4
Rb+, Li+, Ag+
13. Crystalline membrane electrode
• Usually ionic compound
• Single crystal
• Crushed powder, melted and formed
• Sometimes doped (Li+) to increase conductivity
• Operation similar to glass membrane
15-13
• F electrode
14. 15-14
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
Activities of different
cations
• Calcium dialkyl phosphate
insoluble in water, but binds
Ca2+ strongly
16. 15-16
Molecular Selective electrodes
• Response towards molecules
• Gas Sensing Probes
Simple electrochemical
cell with two reference
electrodes and gas
permeable PTFE
membrane
allows small gas
molecules to pass and
dissolve into internal
solution
O2, NH3/NH4
+, and
CO2/HCO3
-/CO3
2-
18. Biocatalytic Membrane Electrodes
• Immobilized enzyme bound to gas permeable membrane
• Catalytic enzyme reaction produces small gaseous molecule (H+,
15-18
NH3, CO2)
• gas sensing probe measures change in gas concentration in internal
solution
Fast
Very selective
Used in vivo
Expensive
Only few enzymes immobilized
Immobilization changes activity
Limited operating conditions
pH
temperature
ionic strength
22. 15-22
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
23. 15-23
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
25. 15-25
Analysis
• Measurement of electricity needed to convert ion to different oxidation state
Coulomb (C)
Charge transported in 1 second by current of 1 ampere
* Q=It
I= ampere, t in seconds
Faraday (F)
Charge in coulombs associated with mole of electrons
* 1.602E-19 C for electron
* F=96485 C/mole e-
• Q=nFN
• Find amount of Cu2+ deposited at cathode
Current = 0.8 A, t=1000 s
Q=0.8(1000)=800 C
n=2
N=800/(2*96485)=4.1 mM
26. 15-26
Coulometric methods
• Two types of methods
• Potentiostatic coulometry
maintains potential of working electrode at a constant so
oxidation or reduction can be quantifiably measured without
involvement of other components in the solution
Current initially high but decreases
Measure electricity needed for redox
arsenic determined oxidation of arsenous acid (H3AsO3)
to arsenic acid (H3AsO4) at a platinum electrode.
• Coulometric titration
titrant is generated electrochemically by constant current
concentration of the titrant is equivalent to the generating
current
volume of the titrant is equivalent to the generating time
Indicator used to determined endpoint
