1. Potentiometry Potential measurements of electrochemical cells
Ion selective methods
Reference electrode
Indicator electrode
Potential measuring device
Indicator electrodes
Ion specific electrodes
Potentiometric measurements

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

3. Calomel electrode

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. 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. 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. 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

9. Glass Membrane Electrode

10. Glass membrane structure
H+ carries current near surface
Na+ carries current in interior
Ca2+ carries no current (immobile)

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 = log(a1/a2)
a1=analyte
a2=inside ref electrode 2
If a2 is constant then
Eb = L log a1
= L pH
where L = log a2
Since Eref 1 and Eref2 are constant
Ecell = constant pH

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+, NH4+, Cs+, 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
F electrode

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. 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-/CO32-

18. 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
Fast
Very selective
Used in vivo
Expensive
Only few enzymes immobilized
Immobilization changes activity
Limited operating conditions pH
temperature
ionic strength

19. Electrode calibration

20. NH4 electrode

21. Potentiometric titration

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. 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. 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. 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