ESTIMATION OF THE RATE OF REACTION WILL BE DONE BASED ON THE POTENTIAL DIFFERENCE BETWEEN REFERENCE AND INDICATOR ELECTRODE. THE POTENTIAL OF THE REFERENCE ELECTRODE IS STABLE WHERE AS THE POTENTIAL OF THE INDICATOR ELECTRODE VARIES WITH THE POTENTIAL OF THE SOLUTION IN WHICH IT IS PLACED

2. In potentiometry, the potential of an
electrochemical cell is measured under
static conditions because no current
flows while measuring a solution’s
potential, its composition remains
unchanged.

4. Principle:
The principle involved in the Potentiometry is when
the pair of electrodes is placed in the sample
solution it shows the potential difference by the
addition of the titrant or by the change in the
concentration of the ions.

5. The electromotive force of the complete cell is given
by the following equation:
Ecell = Ereference + Eindicator + Ejunction
where Ereference is the electromotive force of the
reference electrode ,Eindicator is electromotive
force of indicator electrode, Ejunction is the
electromotive force at the junction of the liquid.

6. Theory:
when the known potential electrode immersed in the
sample solution then the potential is given by Nernst
equation:
E= E0 +(0.592/n) log c
Where E is the potential of the solution
E0 is the standard electrode potential
n is the valency of the ions
c is the concentration of the sample solution
0.592 is the value obtained from the RT/F; where R is the
gas constant, T is the temperature in Kelvin, F is the faradays
constant.

8. Components of a
Potentiometric Cell
 Reference electrode
 Salt bridge
 Analyte
 Indicator electrode

11. 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

12. The reference electrodes are classified into two
main classes they are as follows:
 Primary standard electrodes
ex: Standard hydrogen electrode
 Secondary standard electrodes
ex: Silver-silver chloride electrode
Saturated calomel electrode

13. Standard hydrogen electrode
The standard H2 electrode potential is defined as the potential that is
developed between the H2 gas adsorbed on the pt metal and H+ of
the solution when the H2 gas at a pressure of 760 mm of Hg is in
equilibrium with H+ of unit concentration
Working:
 Pt foil : coated with black pt
 1 molar HCl solution
 Pure H2 gas bubbled continuously at 1 atm.
 Pt act as conductor, inert and facilitate in attaining equilibrium
 Electrode Potential is 0.00 volts
H2 (gas) 2H↔ + (ions) + 2e-(electrons)3

15. Limitations :
1. Can’t be used in solution containing strong oxidizing
agents.
2. Difficult and expensive to maintain.
3. Excess of H2 bubbling out carries little HCl with it
and hence the H+ concentration decreases. In such a
system, it is difficult to maintain the concentration of
HCl at 1M.

16. Saturated calomel electrode
 Most commonly used electrode in potentiometry
 Tube 5-15cm long, 0.5-1 cm in diameter.
 Slurry of mercury & mercurous chloride with saturated soln
of KCl
 Connected by a small opening with saturated solution of
KCl.
 Pt metal is placed inside the slurry
 Ceramic fiber act as salt bridge
2Hg +2Cl Hg¯ → 2Cl2 + 2e¯
E = 0.2444 at 25°C

18. Advantages:
 Concentration of chloride ions don’t
change even some of the solvent get
evaporated.
 Generates small junction potential so
more accurate
Limitations :
 Mostly saturated solution of KCl is used
and that is temperature dependent.

19. Silver-silver chloride
electrode
 It consist of silver wire coated with AgCl
 Coating may be electroplating or Physical.
 This coated wire is placed in 1M solution of
AgCl.
Ag + Cl AgCl + 1e¯ ↔ ¯
E= 0.199V
Advantages:
 Easy handling, and cost effective
Limitation: sometimes show reactivity.

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

22. 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

23. METALLIC INDICATOR
ELECTRODES

24. 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)

25. 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

26. 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

27. 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

28. MEMBRANE
ELECTRODES
 Consist of a thin membrane separating 2 solutions of
different ion concentrations
 Most common: pH Glass electrode

29. Glass pH Electrode
 Its most commonly used indicator electrode
 It involves Ion Exchange reaction
 Membrane is made up of chemically bonded
Na2O, SiO2 and Al2O3
 The Glass bulb is filled with solution of HCl
& KCl, silver acetate coated with AgCl is
inserted as Electrode.

32. 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

33. 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+
]

34. First glass absorb water
Then H ions can move in the direction of
lesser concentration and replace Na ions and
others in the glass membrane.
SiO2-Na + H SiO- H + Na⁺ ⁺ → ⁺ ⁺
As a result of diffusion and exchange process a
potential develops on each side of glass
membrane

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

36. Forget say about
Applications…

37. Applications of Potentiometric
Analysis

38.  Clinical chemistry: Ion selective electrodes are present sensors
for clinical samples because of their selectivity for analyte in
complex matrices. The most common analytes are electrolytes
such as Na, k ,Ca ,H, and Cl and dissolved gases such as CO2
 Environmental chemistry: For analysis of CN- ,NH3, NO3,
F3 in water and waste water.
 Potentiometric titrations: For determining the equivalence
point of an acid base titration. possible for redox, precipitation,
acid-base, complexation as well as for all titrations in aqueous n
non aqueous solvents.
 Agriculture: NO3 ,NH4 ,I ,Ca, K ,CN, Cl in soils, plant
materials, feed stuffs, fertilizers.
 Detergent manufacturing: Ca, Ba, F for studying effects in
water quality.

40. 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

41. Potentiometric Titration
Curves

42. Acid-base reactions: Glass / calomel
electrode for determination of pH.
ex: Titration of HCl with NaOH
Titration of CH3COOH with NaOH

43. Precipitation reactions: Membrane electrodes for
the determination of the halogens using silver nitrate
reagent.
Ex: Titration of mixture of Cl- & Br- & I- with AgNO3

44. Redox titration: platinum electrode.
For example : reaction of Fe3+/ Fe2+
with Ce4+/Ce3+

45. Complex formation titration: metal and
membrane electrodes for determination of many
cations (mixture of Bi3+, Cd2+ and Ca2+ using
EDTA)

46. Can’t bare your patience
more…
I may turn as patient…