1. SCI3023 ELECTROCHEMISTRY Chapter 8e: Potentiometry

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

3. 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

4. Calomel electrode

6. 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

7. 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

8. 2nd kind Precipitate or stable complex of ion Ag for halides
Ag wire in AgCl saturated surface
Complexes with organic ligands
EDTA

9. 3rd kind Electrode responds to different cation
Competition with ligand complex

10. Metallic Redox Indicators
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-

11. Metallic Redox Indicators
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

12. Glass Membrane Electrode

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

14. 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

15. 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+

16. 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

17. 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

19. 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-

21. 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

22. Electrode calibration

23. NH4 electrode

24. Potentiometric titration