1. POTENTIOMETRY

2. Potentiometric Analysis
Based on potential measurement of electrochemical cells without any appreciable current
The use of electrodes to measure voltages from chemical reactions

3. Applications of Potentiometric Analysis

4. Components of a Potentiometric Cell
Reference electrode
Salt bridge
Analyte
Indicator electrode
RE SB A IE
– Eref + Ej Eind

5. Reference electrode Easily assembled Rugged
Half-cell with known potential (Eref)
Left hand electrode (by convention)
Easily assembled
Rugged
Insensitive to analyte concentration
Reversible and obeys Nernst equation
Constant potential
Returns to original potential
Rugged (unaffected by small variations in method parameters)

6. Indicator electrode
Generates a potential (Eind) that depends on analyte concentration
Selective
Rapid and reproducible response

7. Salt bridge Prevents mixing up of analyte components
Generates potential (Ej) = negligible

9. Reference Electrodes Standard Hydrogen Electrode
Calomel Reference Electrode
Silver/Silver Chloride Reference Electrode

10. SHE Hydrogen Gas Electrode Pt (H2 (1 atm), H+ (1M)
SHE is not often used because of fire hazard
Inconvenient to use and maintain

11. Saturated Calomel Electrode
Hg2Cl2(s)+2e-2Hg(l)+2Cl-(aq)
Aka SCE
Easy to prepare
Easy to maintain
V at 25C
Dependent on temp
Toxic
This electrode has fallen out of favor because of mercury toxicity. It offers no advantages over other electrodes but still lingers on because so many methods used it for decades

12. SCE
Chloride maintains constant ionic strength

13. Ag/AgCl Ref. Electrode E = 0.199 V AgAgCl (satd),KCl (satd)
AgCl(s) + e-Ag(s)+Cl-(aq)
E = V

14. Liquid Junction Potential
Liquid junction - interface between two solutions containing different electrolytes or different concentrations of the same electrolyte
A junction potential occurs at every liquid junction.
Caused by unequal mobilities of the + and - ions.

15. Indicator Electrodes Metallic IE Electrodes of the First Kind
Electrodes of the Second Kind
Inert Metallic Electrodes (for Redox Systems)
Membrane IE
Glass pH IE
Glass IE for other cations
Liquid Membrane IE
Crystalline-Membrane IE
Gas Sensing Probes

16. METALLIC INDICATOR ELECTRODES

17. Electrodes of the First Kind
Pure metal electrode in direct equilibrium with its cation
Metal is in contact with a solution containing its cation.
M+n(aq) + ne-  M(s)

18. Disadvantages of First Kind Electrodes
Not very selective
Ag+ interferes with Cu+2
May be pH dependent
Zn and Cd dissolve in acidic solutions
Easily oxidized (deaeration required)
Non-reproducible response

19. Electrodes of the Second Kind
Respond to anions by forming precipitates or stable complex
Examples:
Ag electrode for Cl- determination
Hg electrode for EDTA determination
Slope is positive
3rd kind
Electrode responds to different cation
Competition with ligand complex

20. Inert Metallic (Redox) Electrodes
Inert conductors that respond to redox systems
Electron source or sink
An inert metal in contact with a solution containing the soluble oxidized and reduced forms of the redox half-reaction.
May not be reversible
Examples:
Pt, Au, Pd, C

21. MEMBRANE ELECTRODES Aka p-ion electrodes
Consist of a thin membrane separating 2 solutions of different ion concentrations
Most common: pH Glass electrode

22. Glass pH Electrode

23. Properties of Glass pH electrode
Potential not affected by the presence of oxidizing or reducing agents
Operates over a wide pH range
Fast response
Functions well in physiological systems
Very selective
Long lifespan

24. Theory of the glass membrane potential
For the electrode to become operative, it must be soaked in water.
During this process, the outer surface of the membrane becomes hydrated.
When it is so, the sodium ions are exchanged for protons in the solution:
The protons are free to move and exchange with other ions.
Charge is slowly carried by migration of Na+ across glass membrane
Potential is determined by external [H+]

25. Alkaline error Exhibited at pH > 9
Electrodes respond to H+ and alkali cations
C,D,E and F: measured value is < true value
Electrode also responds to other cations
Higher pH at lower [Na+]

26. Acid error Exhibited at pH < 0.5
pH readings are higher (curves A and B)
Saturation effect with respect to H+

27. Selectivity Coefficient
No electrode responds exclusively to one kind of ion.
The glass pH electrode is among the most selective, but it also responds to high concentration of Na+.
When an electrode used to measure ion A, also responds to ion X, the selectivity coefficient gives the relative response of the electrode to the two different species.
The smaller the selectivity coefficient, the less interference by X.

28. Selectivity Coefficient
Measure of the response of an ISE to other ions
Eb = L’ log (a1 + kHBb1)
kHB = 0 means no interference
kHB  1 means there is interference
kHB < 1 means negligible interference

29. LIQUID MEMBRANE ELECTRODES

30. Liquid Membrane Electrodes
Potential develops across the interface between the analyte solution and a liquid ion exchanger (that bonds with analyte)
Similar to a pH electrode except that the membrane is an organic polymer saturated with a liquid ion exchanger
Used for polyvalent ions as well as some anions
Example:
Calcium dialkyl phosphate insoluble in water, but binds Ca2+ strongly

31.                                                                            
0.1 M CaCl2
Responsive to Ca2+

33. Characteristics of Ca+2 ISE
Relatively high sensitivity
Low LOD
Working pH range: 5.5 – 11
Relevant in studying physiological processes

34. A K+-selective electrode
Sensitive membrane consists of valinomycin, an antibiotic

35. CRYSTALLINE-MEMBRANE ELECTRODES

36. Crystalline-Membrane Electrodes
Solid state electrodes
Usually ionic compound
Crushed powder, melted and formed
Sometimes doped to increase conductivity
Operation similar to glass membrane

37. Crystalline-Membrane Electrodes
AgX membrane: Determination of X-
Ag2S membrane: Determination of S-2
LaF3 membrane: Determination of F-

38. F- Selective Electrode
A LaF3 is doped with EuF2.
Eu2+ has less charge than the La3+, so an anion vacancy occurs for every Eu2+.
A neighboring F- can jump into the vacancy, thereby moving the vacancy to another site.
Repetition of this process moves F- through the lattice.

39. Fluoride Electrode

40. GAS SENSING PROBES

41. Gas Sensing Probes
A galvanic cell whose potential is related to the concentration of a gas in solution
Consist of RE, ISE and electrolyte solution
A thin gas-permeable membrane (PTFE) serves as a barrier between internal and analyte solutions
Allows small gas molecules to pass and dissolve into internal solution
O2, NH3/NH4+, and CO2/HCO3-/CO32-

42. Gas Sensing Probe

43. DIRECT POTENTIOMETRY
A rapid and convenient method of determining the activity of cations/anions

44. Potentiometric Measurement
Ionic composition of standards must be the same as that of analyte to avoid discrepancies
Swamp sample and standard with inert electrolyte to keep ionic strength constant
TISAB (Total Ionic Strength Adjustment Buffer) = controls ionic strength and pH of samples and standards in ISE measurements

45. Potentiometric Measurement
Calibration Method
Standard Addition Method

46. Special Applications: Potentiometric pH Measurement using Glass electrode
One drop of solution
Tooth cavity
Sweat on skin
pH inside a living cell
Flowing liquid stream
Acidity of stomach

47. Potentiometric Titration
Involves measurement of the potential of a suitable indicator electrode as a function of titrant volume
Provides MORE RELIABLE data than the usual titration method
Useful with colored/turbid solutions
May be automated
More time consuming

48. Potentiometric Titration Curves