This document discusses potentiometry, which is an electroanalytical technique that measures the potential (voltage) of electrochemical cells containing indicator and reference electrodes. It involves using electrodes to measure voltages generated from chemical reactions. Various types of electrodes are described including metal, ion-selective, glass membrane, liquid membrane, and crystalline membrane electrodes. Applications of potentiometry include ion concentration measurements, pH measurements, and potentiometric titrations.

1. POTENTIOMETRY
By…
Prof. Sonali R. Pawar
Assistant Professor, Pharmaceutical Chemistry Department,
JSPM’s Charak College of Pharmacy and Research, Wagholi , Pune 412207

2. Potentiometric Analysis
• Based on potential measurement of
electrochemical a cells without any
appreciable current
• The use of electrodes to
measure voltages
from chemical reactions
Potentiometry
 Use of Electrodes to Measure Voltages that Provide Chemical Information
- Various electrodes have been designed to respond selectively to
specific analyte.

3.  Use a Galvanic Cell
- Unknown solution becomes a ½-cell
- Add Electrode that transfers/accepts electrons from unknown
analyte
- Connect unknown solution by salt bridge to second ½-cell at fixed
composition and potential
 Indicator Electrode: electrode that responds to analyte and donates/accepts
electrons
 Reference Electrode: second ½ cell at a constant potential
 Cell voltage is difference between the indicator and reference electrode

4. Applications of Potentiometric Analysis

5. Binding generates
potential difference.
 A Heparin Sensor
- Voltage response is proportional to heparin concentration in blood
- Sensor is selective for heparin
Negatively charged heparin
binds selectively to positively
charged membrane.
Heparin
Potential is
proportional to
[heparin]

6. Components of a Potentiometric Cell
1. Reference electrode
2. Salt bridge
3. Analyte
4. Indicator electrode
Eref + Ej + Eind

7. 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

8. Electrodes and Potentiometry
Reference Electrodes
3.) Saturated Calomel Reference Electrode (S.C.E)
 Saturated KCl maintains constant [Cl-] even
with some evaporation
 Standard hydrogen electrodes are cumbersome
- Requires H2 gas and freshly prepared
Pt surface
Eo = +0.268 V
Activity of Cl- not 1E(sat,KCl) = +0.241 V

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

10. Indicator Electrodes
1.) Two Broad Classes of Indicator Electrodes
 Metal Electrodes
- Develop an electric potential in response to a redox reaction
at the metal surface
 Ion-selective Electrodes
- Selectively bind one type of ion to a membrane to generate
an electric potential
Remember an electric potential is generated by a separation of charge

11. Indicator Electrodes
2.) Metal Electrodes
 Platinum
- Most common metal indicator electrode
- Inert: does not participate in many chemical reactions
- Simply used to transmit electrons
 Other electrodes include Gold and Carbon
 Metals (Ag, Cu, Zn, Cd, Hg) can be used to monitor their aqueous ions
- Most metals are not useable
- Equilibrium not readily established at the metal surface
Indicator Electrodes
3.) Ion-Selective Electrodes
 Responds Selectively to one ion
- Contains a thin membrane capable of only binding the desired ion
 Does not involve a redox process

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

15. Reference Electrodes
1. Standard Hydrogen Electrode
2. Calomel Reference Electrode
3. Silver/Silver Chloride Reference Electrode

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

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

18. SCE

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

20. 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.

21. 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

22. METALLIC INDICATOR
ELECTRODES

23. 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)

24. 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

25. 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

26. 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

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

28. Glass pH Electrode

29. 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

30. 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+]

31. 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+]

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

33. 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.
Atoresponse
Xtoresponse
, XAk

34. 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

35. LIQUID MEMBRANE
ELECTRODES

36. 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

37. Responsive to Ca2+
0.1 M CaCl2

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

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

41. CRYSTALLINE-
MEMBRANE
ELECTRODES

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

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

44. 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.

45. Fluoride Electrode

46. GAS SENSING PROBES

47. 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-

48. Gas
Sensing
Probe

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

50. 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

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

52. 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

53. 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

54. Potentiometric Titration Curves