POTENTIOMETRIC TITRATION is an Electrochemical method of analysis. Method used to determine the electrode potential of the sample solution using electrochemical cells.

1. POTENTIOMETRY
ASWINI SASIDHARAN
Assistant Professor
Nirmala College Of Pharmacy

2. POTENTIOMETRY
• Electrochemical method of analysis.
• Method used to determine the electrode potential of the sample solution
using electrochemical cells.
• Concentration of solute in a solution & potential difference btw 2
electrodes.
• Measured electrode potential – used for quantitative & qualitative
analysis.
• Electrode potential – potential difference / voltage btw reference and
indicator electrodes.
• Electrode potential depends on the concentration of sample solution
[analyte]
• Electrode potential is calculated using Nernst equation.

3. Quantitative analysis
• Determination of concentration of analyte by measuring the
changes in the potential
• Measurement of pH
• Eg- pH- metry is form of potentiometry
Qualitative analysis
• Analysis of the given sample
• To detect the presence of ions using electrodes

4. POTENTIOMETER
• It is an instrument to determine the potential differences
between a reference electrode and an indicator electrode.
• These two electrodes form an electrochemical cell that is dipped
in solution to be analyzed [SAMPLE].
• Measured potential can be used to determine the quantity of
analyte in terms of concentration.

5. PRINCIPLE
• Potentiometry is based on the fact that potential of a given sample is
directly proportional to the concentration of its ions [electroactive ions]
• When the pair of electrodes is placed in the sample solution it shows the
potential difference by the addition of titrant.
• Metal atoms may dissolve in the solution as positive ions leaving electrons
on the electrode.
• Metal ions may take up electrons from the electrode and get deposited as
neutral atoms
• So a POTENTIAL DIFFERENCE is set b/w electrode and solution.
THEORY
• It is based on the Nernst equation
• Nernst equation gives the basic relationship btw potential generated by

6. Nernst equation
The potential E (half cell potential) of any electron is given by equation
where;
E= electrode potential of half cell
Eo= standard electrode potential
R= universal gas constant
T= temperature
F= Faraday's constant
n =number of electron transferred into the half reaction
[oxidation] = concentration of oxidized species
[reduction] = concentration of reduced species

7. •By combining numerical constants [R, T, F] and
converting to logarithm

8. INSTRUMENTATION
Electrochemical cell
• Electrochemical cell is a device which converts Physical/chemical
energy into electrical energy.
• It is used to generate potential and electrical current from chemical
reaction
• It consists of 2 electrodes in contact with an electrolyte solution.
• Current is generated by the chemical reaction, which involves releasing
& accepting electrons at the two electrodes
• Each electrochemical cell is composed of 2 half cells

9. • Electrochemical cell consists of 2 electrodes:-
Copper electrode
Zinc electrode
• Both the electrodes are dipped into their respective electrolyte solution
• CuSO4- Copper electrode
• ZnSO4- Zinc electrode
• Half cell reaction contributed to the electrochemical cell are
• Net reaction

10. B

11. Half cell
• Each electrode of an electrochemical cell is referred as half cell.
• One half cell losses electrons [oxidation]
• Other half cell gain electrons [reduction]
• Half cell at which oxidation occur is anode out of two electrodes
• One is reference electrode, which has a stable and constant
potential and other is called indicator electrode.

12. Standard electrode potential
•It is also known as standard reduction potential /
oxidation-reduction potential
•It is defined as the tendency of chemical species to gain
electrons.
•Standard electrode potential of the ion participating in the
chemical reaction is measured in volts / millivolts.
•At a Standard condition - temperature [25ºC], pressure
[1atm], concentration [1Molar]

13. ELECTRODES
1. Reference Electrodes
2. Indicator Electrodes

14. REFERENCE ELECTRODES
•A- Primary reference electrodes
•Standard Hydrogen Electrodes [SHE]
•B- Secondary reference electrodes
•Saturated Calomel Electrodes [SCE]
•Mercury-mercurous sulphate electrode
•Silver-Silver chloride electrode

15. INDICATOR ELECTRODES
• Standard hydrogen electrode [SHE]
• Normal hydrogen electrodes [NHE]
• Hydrogen electrodes [HE]
• Quinhydrone electrode
• Antimony – antimony oxide electrode
• Glass electrode

16. Reference Electrode

17. Reference Electrode
•Reference electrode is defined as an electrode has stable
fixed potential
•The potential of reference electrode does not change,
dipping into any solution.
•It gives standard or known potential
•It is used as one-half cell of the electrochemical cell for
detecting potential.

18. Reference Electrode
•Reference electrode consists of :-
•Inner tube fitted with mercury-mercuric chloride
or silver-silver chloride.
•Outer tube containing a suitable filling solution
[saturated cell] and devices like ground glass
sleeve, fritted disk to form liquid junction into
which the solution is immersed.

20. Reference electrode should fulfil 3 main
requirements
•STABILITY:- It should be stable over long period of time
•REVERSIBILITY:- Not show any deviation in its potential
when small current is applied.
•REPRODUCIBILITY:- It should obey Nernst equation and
should measure the potential.

21. I. Standard Hydrogen Electrodes
• It is also known as Hydrogen Electrode or Normal hydrogen
electrode
• Universal accepted reference electrode
• It is the primary standard electrode for measuring pH & potential.
• It acts as both indicator as well as reference electrode.

22. Construction
• It consists of platinum wire & platinum foil which is coated
with platinum black
• Platinum foil & platinum wire is enclosed in a glass tube which
has an inlet.

24. Working
• platinum black coating will increase the surface area of the
electrodes & catalyze the reaction.
• Glass tube which has an inlet through which pure hydrogen gas
is bubbled continuously at a pressure of 1 atm.
• Platinum black did not participate in the electrochemical
reaction but helps in transfer of electrons.
• When electrode is immersed to a standard acid solution, it acts
as reference electrode.
• Electrode dipped in unknown solution acts as indicator
electrode.

25. •Transmission of current across the interface is included in
the half cell reaction
•When coupled with a proper type of hydrogen electrode ,
it can either acts as anode / cathode
•Anode ----hydrogen gas is oxidized to hydrogen ions.
•Cathode----- reverse reaction occurs

26. ADVANTAGES
• It can be used as reference as well as indicator electrode.
• It give accurate results
• It does not show any deviation.
• It is suitable for measuring the pH over wide ranges
• It is used universally as a std for evaluation of half cell potential

27. DISADVANTAGES
•Inconvenient to construct & difficult to maintain.
•Presence of strong oxidizing / reducing agent effects the
functioning of the electrode.
•Platinum foil has to be platinized regularly
•Pressure of hydrogen gas and its purity affect the electrode
potential
•Compounds like sulphate, cyanide, arsenic, alkaloids, etc
affects the platinum surface and potential of electrodes

28. II. SATURATED CALOMEL ELECTRODE
•It is also called as Rotating platinum electrode
•Saturated Calomel Electrode is the most commonly &
widely used reference electrode.

29. Construction
• Calomel electrode consists of mercury in contact with the
solution
• It consists of 2 glass tube:
Inner tube
Outer tube
• Outer tube contains solution of Kcl
• Inner tube contains a part of mercury and mercurous
chloride [calomel].

31. Working
• Mercury in contact with the solution that is saturated with
mercury[I] chloride [calomel], that also contain a known
concentration of Kcl.
•Small orifice in the inner tube connects the two tubes.
•Fritted disk is a liquid junction [saturated Kcl] serves as
a conductive bridge b/w calomel and the sample solution
into which the electrode is immersed.

32. Working
• Content of inner tube are in contact with the outer
tube through a small orifice
• Reactions for half cell of calomel electrode

33. ADVANTAGES
• Saturated Calomel Electrode can be used in various solvent
• It can be used over wide pH range
• Easy to construct
• Stability of potential

34. • It cannot be used at high temperature
• At high temperature [>353k], disproportionation of Hg (I) & Hg
(II) ions
DISADVANTAGES

35. III. SILVER - SILVER CHLORIDE
ELECTRODE
• Electrode is almost similar to Saturated Calomel Electrode
• It is most widely used reference electrode in the glass indicator
electrode
• It is used as internal reference electrode in glass rod.

37. CONSTRUCTION
• Electrode consists of silver wire
• Coated electrolytically with a thin layer of AgCl
• It consists of a porous junction, made up of ceramic fibre
• It have an electrical lead.
• Fill hole is present at the top.

38. WORKING
• Electrode is dipped into a Kcl solution of known concentration
which is saturated with AgCl.
• Potential of electrode is determined by the concentration of Cl-
ions in Kcl solution.
• Electrode potential is determined by the half cell reaction

39. Limitations
• Should not be used in solutions that contain proteins, sulphide,
iodide, bromide or any other material that would precipitate
with silver found in filling solution
• Strong reducing agents should be avoided [because they can
reduce Ag+ ions to Ag metal at liquid junction]

40. IV. MERCURY-MERCUROUS SULPHATE
• Construction is similar to Saturated Calomel Electrode.
• It consists of mercury in a solution containing sulphate ions

41. INDICATOR ELECTRODES

42. I. GLASS ELECTRODE
•Glass electrode is one of the most useful electrode
•To determine the pH of a solution

43. CONSTRUCTION
• Glass electrode consists of a glass bulb with a long neck & is made
up of special type of glass having low melting point and high
electrical conductivity which is selectively permeable to
hydrogen ions.
• It consists of AgCl electrode.
• Soda lime glass consisting of lithium silicate with barium and
lanthanum ions is generally used for preparing glass membrane
• Composition of glass membrane affects the sensitivity of
membrane towards the hydrogen ions and other cations

44. • At pH below 9 glass membrane
generally specific to hydrogen ion but
at pH above 9 it is specific to sodium
and other charged cations.
• Bulb consisting of 0.1M HCl saturated
with quinhydrone and platinum or
silver-silver chloride wire which is
used to contact with solution

45. • Glass electrode is placed in a sample solution and connected to a
reference electrode such as calomel electrode
• Then electricity is passed through it.
• Due to the passage of electrical current H+ ions of the solution enter
into the glass electrode through the semipermeable glass membrane.
• Generated electrical potential is measured using potentiometer
• Factors like size, area and composition of glass thickness of glass bulb
affects the electrical potential of the electrode.
Working

46. ADVANTAGES
• It is useful in presence of strong oxidant & reductants in the presence of
proteins also.
• It is useful to measure pH values from 0-14
• Simple to operate
• Not affected by hydrogen sulphide / cyanide poison
• Different type of solution like colored, viscous, colloidal solution can be
used

47. DISADVANTAGES
• It should be handled with care as the bulb is fragile.
• The electrode become non functional in case of minute scratches
• It requires frequent standardization
• Solvent like acetic acid, ethanol, gelatin are not suitable
• If the membrane of glass electrode is dehydrated, it will not
perform as pH indicator.
• To avoid this, glass electrode is always immersed in distilled water.
• If moisture is lost, it may be replaced by soaking the electrode in
distilled water for hours.

48. II. STANDARD HYDROGEN ELECTRODE
• Same as to reference electrode

49. III. METAL ELECTRODE
• Metal indicator electrodes develop a potential which is usually
measured by the equilibrium position of a half cell reaction at
the electrode surface.
• Classified as:-
• First order electrodes
• Second order electrodes
• Third order electrodes
• Inert electrodes

50. First order electrodes
• It consists of a metal placed in a solution of its respective ions.
• Eg:- silver wire immersed in AgNO3 solution.
• The metallic electrodes of first order is a pure metal electrode in
direct equilibrium with its cation in solution.
• Single reaction is involved
• Eg- copper indicator electrode
• Metals that display reversible half cell reactions with their
respective ions are found to be suitable for employing as first order
electrodes are:-
• Ag, Cu, Hg, Zn, Bi, Pb and Sn.

51. Second order electrodes
• Metal electrodes may be directly responsible to the
concentration of an anion, which either give rise to a complex or
a precipitate with the respective cation of the metal
• Eg:- silver-silver chloride electrode

52. Third order electrodes
•A metal electrode reacts with a different cation
•It then becomes an electrode of third kind
•Eg:- mercury electrode used for the determination of
potential of Ca of calcium containing solution.

53. •It comprises electrically inert conductors like Au, Pt which
do not take part either directly or indirectly in the redox
process.
•Potential developed at the inert electrode depends on the
nature & concentration of different redox reagents present
in the solution.
•Eg:- Pt electrode placed in a solution containing Fe3+ &
Fe2+ ions.

54. IV. Quinhydrone electrode
•Quinhydrone electrode is an
addition products of quinone [Q]
and hydroquinone [HQ]
•It is easy to build and maintain
•Electrode is constructed by adding
a small amount of quinhydrone
to the sample solution

55. ADVANTAGES
•It is simple to construct
•It gives rapid response
•It is very accurate in measurements

56. DISADVANTAGES
•It is not used for pH measurement above 8
•It is not useful in presence of strong oxidizing / reducing
agents
•It is unstable above 30°C

57. V. ANTIMONY-ANTIMONY OXIDE
ELECTRODES
•It consists of 2-3cm long
antimony rod
•It is enclosed in ebonite
tube or glass sleeve and
connected to a copper
wire

58. ADVANTAGES
•pH of solution in the range 4-12 is determined
•pH of viscous solution can be measured

59. DISADVANTAGES
•It produces the error
•It cannot used in presence of metals.
•It is sensitive to change in the temperature of solution

60. POTENTIOMETRIC TITRATION
• Potential is measured at different intervals to find out the end
point
• Titration can be applied for acid base, redox, precipitation and
complexometric
• Potential or pH measured and recorded carefully after each
addition of titrant
• From this, potentiometric curve is constructed and end point is
calculated.

62. END POINT DETERMINATION
•Potentiometric titration can be used to determine the end
point of the reaction
•When the indicator method is not suitable
•The change in emf due to the addition of known volume
[titrant] is measured using suitable electrode.
•At the end point the change is very rapid & maximum

63. 3 TYPES OF METHODS
•Normal curve / pH curve / Emf curve
•First derivative curve
•Second derivative curve

64. NORMAL CURVE
•Titration is stopped when a large
change in potential is observed.
•pH curve is plotted:- pH vs volume of
titrant
•Emf curve is plotted:- Emf vs volume
of titrant
•In both cases the end point is the mid
point of the steeply raising part of
the curve

65. FIRST DERIVATIVE
• In the 2nd
method
• A plot of change in potential or pH per
volume of titrant (ΔE/ ΔV),
(ΔpH/ΔV) and volume of titrant
• This curve is known as first derivative
curve
• End point can be measured by peak of the
curve.

66. SECOND DERIVATIVE
•In the 3rd
method
•plot of the change in potential or pH
per volume of titrant (Δ2emf/ Δ2V),
(Δ2pH/ Δ2V) and volume of titrant.
•This curve is known as second
derivative curve
•At the end point the values of
Δ2emf/Δ2V or Δ2pH/Δ2V becomes zero

67. GRANT PLOT • Instead of plotting the electrode potential against
volume, the concentration of sample remaining at
each point in the titration is plotted
• A straight plot will be obtained, in which the
concentration would decrease to zero at equivalent
points
• A series of additions of titrant are made in a
potentiometric titration and the emf [E] is noted after
each addition and then if antilog is plotted against
the volume of titrant added, a straight line is
obtained
• When extrapolated, this line cuts the volume axis at a
point corresponding to the equivalence point volume
of the reagent

68. STANDARD ADDITION PLOT
•The potential of the sample is initially recorded and then
known amounts of standard are added to the sample.
•The antilog values are plotted as a function of the
amount of standard added
•Extrapolation to horizontal axis [vol of titrant] gives
the equivalent amount of analyte in the sample

69. Dead stop end point method
• It consists of a beaker of suitable size mounted on magnetic
stirrer containing solution to be titrated
• pH electrodes are immersed in solution which is connected to
potentiometer
• Microammeter /galvanometer is incorporated in the circuit
• Small volume of titrant is added through micro burette and
current flowing through galvanometer is noted
• When current stop, volume corresponding to it is recorded

70. Advantages of Potentiometry
• Analysis of dilute, colored and turbid solutions.
• It is employed for acid-base, redox, precipitation & complexometric
reactions
• There is no need of external indicators in redox titrations
• Applicable in non aqueous media
• High Accuracy
• It is used in the titration of polyprotic acids, mixture of acids,
mixture of bases or mixture of halides

71. Applications
• For checking pH of official buffers
• For determining pH of different test solutions
• It is useful to locate equivalence point in acid base, redox,
precipitation & complexometric titration both in aqueous & non-
aqueous media
• A number of chemotherapeutic substances listed in pharmacopoeia
can be assayed
• Eg- caffeine, phenobarbital, disulfiram, nalidixic acid,
amoxicillin sodium, metoclopramide HCl, propranolol HCl, etc