Morning

1. 2nd Lec
Electrochemical Methods
•Potentiometry
•Coulometry
•Voltammetry
•Conductometry

2. POTENTIOMETRY

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

4. Applications of Potentiometric Analysis

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

6. Reference electrode
• 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

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

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

10. The role of the R.E. is to
provide a fixed potential
which does not vary during
the experiment.
A good R.E. should be able to
maintain a constant potential
even if a few micro-amps are
passed through its surface.
Micropolarization test. (a) response of a good and (b) bad
reference electrode.
Aqueous
SCE Ag/AgCl Hg/HgO SHE
Nonaqueous
Ag+/Ag Pseudoreferences
Pt, Ag wires Ferrocene/ferricinium couple
Reference Electrode (RE)

11. Types of Electrodes/Reference electrodes
I
▫ Provide constant potential – not affected by sample
composition
▫ Follows Nernst equation
▫ Two common reference electrodes:
1. Saturated calomel electrode (SCE)
Mercury in contact with a solution saturated with mercury
chloride
|| KCl (saturated), Hg2Cl2(s)|Hg(s) (E0 = +0.214 vs.
SHE at 25 ºC)
2. Silver wire coated with silver chloride in a saturated KCl
solution
|| KCl (saturated), AgCl(s)|Ag(s) (E0 = +0.197 vs. SHE at 25 ºC)
E0 above is calculated based on reference to the standard

12. Reference Electrodes
1. Standard Hydrogen
Electrode(SHE)
2. Calomel Reference
Electrode(SCE)
3. Silver/Silver Chloride
Reference Electrode (Ag/Ag+)

13. The Standard Hydrogen Electrode (SHE)
 A universal reference, but is really a hypothetical
electrode (not used in practice)
– Uses a platinum electrode, which at its surface
oxidizes 2H+ to H2 gas.
– Very sensitive to temperature, pressure, and
H+ ion activity
 Because the SHE is difficult to make, the
saturated calomel electrode (SCE) is used
instead.
– Calomel = mercury (I) chloride

14. SHE
• Hydrogen Gas
Electrode
• Pt (H2 (1 atm), H+ (1M)

15. Saturated calomel electrode SCE
Advantages
Most polarographic
data referred to SCE

16. SCE

17. SCE
Saturated Calomel Electrode
Hg2Cl2(s)+2e-2Hg(l)+2Cl-(aq)
•Aka SCE
•Easy to prepare
•Easy to maintain
•0.2444 V at 25C
•Dependent on temp
•Toxic

18. Silver/silver chloride reference electrode Ag/AgCl
# Ag wire coated with AgCl(s), immersed in NaCl or KCl solution
# Ag+ + e- = Ag(s)
E0 = 0.22 V vs. SHE @ 20C
Advantages
chemical
processing
industry has
standardized on this
electrode
convenient
rugged/durable
Disadvantages
solubility of
KCl/NaCl
temperature
dependent
dE/dT = -0.73 mV/K
(must quote
temperature)

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

20. Silver/silver ion reference electrode Ag+/Ag
Ag+ + e-= Ag(s)
Requires use of internal
potential standard
Advantages
Most widely used
Easily prepared
Works well in all
aprotic solvents:
THF, AN, DMSO,
DMF
Disadvantages
Potential depends on
solvent
electrolyte (LiCl,
TBAClO4, TBAPF6,
TBABF4
Care must be taken to
minimize junction
potentials

21. Mercury-Mercuric oxide reference electrode Hg/HgO
Metal/metal oxide reference electrode
Used in particular for electrochemical studies in aqueous alkaline solution.1.0 M NaOH
usually used in inner electrolyte compartment which is separated from Test electrolyte
solution via porous polymeric frit. Hence reference electrode in like SHE system in that it
is pH independent.

22. Comparing various
Reference
Electrode potential
scales to
SHE scale.
E (vs SHE) = E (vs
REF) + EREF
(vs SHE)

23. We can initially ignore the fact that the electrode contains AgI and find E for
the silver ion reduction.
Electrode Potentials

24. 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.

26. Indicator Electrodes
I. Metallic IE
A. Electrodes of the First Kind
B. Electrodes of the Second Kind
C. Inert Metallic Electrodes (for Redox Systems)
II. Membrane IE
A. Glass pH IE
B. Glass IE for other cations
C. Liquid Membrane IE
D. Crystalline-Membrane IE
III. Gas Sensing Probes

27. METALLIC INDICATOR
ELECTRODES

28. 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)

29. 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 (de-aeration required)
• Non-reproducible response

30. Electrodes of the Second Kind
• Respond to anions by forming precipitates
or stable complex
• Examples:
1. Ag electrode for Cl- determination
2. Hg electrode for EDTA determination

31. 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

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

33. Glass pH Electrode

34. 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

35. 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+]

36. 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+]

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

38. 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.
A
to
response
X
to
response
, 
X
A
k

39. Selectivity Coefficient
• Measure of the response of an ISE to other ions
Eb = L’ + 0.0592 log (a1 + kHBb1)
• kHB = 0 means no interference
• kHB  1 means there is interference
• kHB < 1 means negligible interference

40. LIQUID MEMBRANE
ELECTRODES

41. 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

42. Responsive to Ca2+
0.1 M CaCl2

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

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

46. CRYSTALLINE-
MEMBRANE
ELECTRODES

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

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

49. 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.

50. Fluoride Electrode

51. GAS SENSING
PROBES

52. 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
-/CO3
2-

53. Gas
Sensing
Probe

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

55. 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

56. Potentiometric Measurement
1. Calibration Method
2. Standard Addition
Method

57. 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

58. 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

59. Potentiometric Titration Curves