it is about the description of Potentiometry, EMF, Types of Electrodes, Measurement of Electrode potential and applications of Potentiometry.

1. Prepared by
Dr.B.Poornima,M.Pharm, Ph.D,
Assistant professor,
Sree Vidyanikethan College of Pharmacy,
Tirupathi.
potentiometry

2. Potentiometry
• Measurement of potential (or) electrode potential
between two electrodes of electrochemical cell
without detectable current.
• Theory: the potentiometric measurements are
based on Nernest equation which relates the
potential & concentration of electroactive species.
• Nernest equation: 𝑬 = 𝑬° −
𝑹𝑻
𝒏𝑭
𝐥𝐧 𝑸
Where E --- Electrode Potential
𝑬° --- Standard electrode potential
R --- Gas constant (8.31J/mole K)
T --- Temperature
n –-- Number of electrons
F --- Faradays constant (96,500
columbs/mole)
ln -- 2.303 log 10

3. • Electrochemical cell : Device which converts
physical/chemical energy into electrical energy, or
electrical energy into chemical energy and is made up
of two half-cells.
• Half cell: it is one of the two electrodes.
ex: Zn-Cu cell (oxidizing-reducing)
• Components of Electrochemical cell
 Electrodes
 Electrolyte solution
 Voltammeter/Galvanometer
 Salt bridge

4. • Electrode – Electric conductor carries electric current
into non-metallic substances.
• Electrolyte solution – contains ions, atoms or
molecules which may be gain or loss the e-
• Galvanometer – to detect electric current.
• Salt bridge – Device used to connect the two half cells.

5. Types of electrochemical cells
Galvanic cell
(or) Voltaic cell
• Converts chemical
energy into electrical
energy.
• Produces EMF
• Anode is +ve, Cathode is
–ve.
• Oxidation takes place at
anode & reduction at
cathode.
Electrolytic cell
• Converts chemical
energy into electrical
energy.
• Consumes EMF
• Anode is +ve, Cathode is
-ve
• Oxidation takes place at
anode & reduction at
cathode.

6. Electrochemical cell
• Discharge of ions occur
only at cathode.
• Spontaneous reaction
occurs.
• These may be
reversible.
• Flow of electrons is
from anode to cathode.
• Electrons leave the cell
at anode and enter the
cell at cathode.
Electrolytic cell
• Discharge of ions
occurs at both the
electrodes.
• Non-spontaneous
reaction occurs.
• These are irreversible.
• Flow of electrons is
from anode to cathode.
• Electrons leave the cell
at anode and enter the
cell at cathode.

7. Half-cell
• It is an electrode with an electrochemical cell.
• One of the two electrodes.
• The half cell at which oxidation occurs is
known as Anode.
• The half cell at which reduction occurs is
known as Cathode.
Types of Half-cell
Metal Half cell Gas Half cell Half cell in which
metal is contact
with insoluble
salt

8. Types of Half cells
 Metal half cell – Metal is dipped into a solution of
same metal ions.
Ex: Zn rod dipped in ZnSo4 solution.
Half cell can be represented as Zn/Zn+2
 Gas Half cell – Non- reactive metal plate (Gold,
Platinum, Graphite rod…) dipped in solution. Gas is
bubbled over metal surface.
Ex: pt/H2 (1 atm), H+
 Half cell in which metal is in contact with insoluble
salt (Oxide of solution)
Ex: Saturated Calomel electrode

9. EMF
• EMF is the difference in the electric tension or the
difference in charge between two points that
causes an electric current.
• Electromotive force, abbreviation E or emf, energy
per unit electric charge that is imparted by an
energy source, such as an electric generator or a
battery.
• It is commonly measured in units of volts,
equivalent in the metre–kilogram–second system
to one joule per coulomb of electric charge.

10. Electrodes
• Two types of electrodes: i) Indicator electrode
ii) Reference electrode
Reference electrode :
• Half-cell with known potential.
• An electrode which has a stable & fixed potential.
• It always gives standard or known potential.
• It used to determine the potential of unknown
sample (or) given sample.
• Always used in combination with indicator
electrode to measure the potential.
• The potential of reference electrode – standard
against potential of the indicator electrode.

11. Types of Reference electrodes:
 Standard Hydrogen Electrode
 Calomel Electrode
 Potassium sulphate Electrode
 Ag-Agcl Electrode
 Weston Electrode

12. Characteristics of Reference electrode
• Reversible and follow Nernst equation.
• Potential should be constant with time.
• Should return to original potential after being
subjected to small currents.
• Little effect with temperature cycling.
• Should behave as ideal nonpolarized electrode.
• Made of some stable chemical species, is readily
available and usually simple to use.
• Should be non-toxic if you are going to use it in a
biological system.
• Should be rugged and portable if you are going to
use it in the field.

13. Construction of Reference Electrodes
Components of Reference electrode:
• Inner tube filled with Hg-HgCl2 (or) Ag-Agcl
• Outer tube suitable filling solution (saturated Kcl)
• Small orifice
• Fritted disc
Ideal requirements :
• Stability
• Reversibility
• Reproducibility

14. Standard hydrogen electrode (SHE)
• Its standard electrode potential is 0 at 298K.
Hence, it acts as a reference for comparison with
any other electrode.
• The redox half cell of the SHE is :
2H+ (aq) + 2e– → H2 (g)
• This reaction occurs at Platinum electrode.

15. Construction of SHE
• It is made up of
• A platinum electrode which is covered in finely
powdered platinum black (platinized platinum
electrode).
• A hydrogen Blow.
• A solution of acid having a H+ molarity of 1 mole per
cubic decimeter.
• A hydroseal which is used to prevent the interference
of oxygen.

16. Working of SHE
• Pure and dry hydrogen gas is bubbled through Hcl
solution from the inlet at a constant pressure of 1cm.
• Hydrogen gas is absorbed on the platinum plate and
acts as a hydrogen electrode.
• An equilibrium between H2 gas and H+ ion is
established across the metal.
Electrode reaction:
• The electrode is reversible with respect to hydrogen
ions.
• During working, hydrogen gas from platinum plate
changes into hydrogen ions and electrons are set free.
• These electrons accumulate on the platinum plate.

17. • Anode:
If the electrode is serving as an anode, then the half
cell reaction is
H2 2H+(aq) + 2e- (oxidation)
• The electrons set free remains on the platinum plate and
transferred to the other electrode through Pt. wire.
• As the process is oxidation, a positive potential is
developed. It is comparatively very small, it is arbitrarily
taken as zero.
• Cathode:
If the electrode is serving as a cathode, then the half-
cell reaction is
• 2H+ (aq) + 2e- H2 (reduction)
Representation of electrode:
• When acting as anode, Pt/ H2(g) (1atm)// H+ (aq) (1M)
• When acting as cathode H+(aq) (1M)// H2 (g) (1atm)/ Pt.

18. • Advantages of SHE:
• Ultimate reference electrode against which all
electrode potential are measured.
• It has no error in strongly alkaline solution.
• It can be used over the entire pH range.
• It gives accurate results.
• It has low internal resistance negligible electric
leakage error.
Disadvantages:
• It requires Pt black surface of electrode.
• It requires a long time for the equilibrium of H2 gas &
H+ ion.
• Dissolved gases such as ammonia, Co2 e.t.c.
interfere with its use.

19. Calomel reference electrode
Construction :
• It consists of a broad glass tube having sidearm.
• The sidearm is used for coupling the calomel electrode.
• At the bottom of the glass tube, there is pure mercury and
platinum wire is sealed into it at the bottom for electrical
connections.
• The wire runs through a separator glass tube to the top of
the tube for electrical contact.
• Above pure mercury, there is a paste of mercurous
chloride (calomel Hg2Cl2) in mercury.
• The rest of the glass vessel and sidearm is filled with a
saturated KCl solution.

20. • KCl solution of 0.1M or of 1M can also be used.
• Sidearm is plugged with glass wool.
• The glass tube is closed from the top.
• Working:
• Since the calomel electrode is reversible, two types of
reactions are possible depending upon the nature of
another electrode with which it is coupled.
• When acting as negative electrode:
2Hg 2Hg+ + 2e-
2Hg+ + 2cl- Hg2Cl2
The net oxidation reaction is
2Hg + 2Cl- Hg2Cl2 + 2e-
• Thus oxidation takes place when it is coupled with
other electrode having lower oxidation potential.

21. • When acting as positive electrode:
Hg2Cl2 2Hg+ 2Cl-
2Hg+ + 2e- 2Hg
The net reduction reaction is
Hg2Cl2 + 2e- 2Hg+ + 2Cl-
Representation of Electrode:
• When acting as anode:
Pt/ Hg (l)// Hg2Cl2 / KCl
• When acting as cathode :
KCl (Sat)// Hg2Cl2 (s)/ Hg (l)/Pt

22. Oxidation potential of Calomel Electrode:
• The oxidation potential of the calomel electrode depends
upon the concentration of KCl solution used.
• The negative potentials indicate that when combined with
SHE reduction takes place at the calomel electrode.
• Advantages of Calomel electrode:
• It is easy to set up and easily reproducible.
• It is convenient and easy to transport.
Concentration of KCl Oxidation potential at 298K
0.1M or 0.1 N -0.3338K
1 M or 1 N -0.2800V
Saturated -0.2415V

23. • It is very compact and smaller in size requires little space.
• No separate salt bridge is required as it has already a side
tube containing KCl solution.
• Potential does not change appreciably with time and a
slight change in temperature.
• Disadvantages:
• When half-cell potentials are to be measured,
compensation for potential is necessary.
• The calomel electrode cannot be used in the measurement
of potentials of the cell where k+ and Cl- ions interfere in
the electrochemical reactions of the cell.

24. Silver-Silver Chloride electrode:
• This reference electrode system is analogus to the
Calomel electrode consisting of a silver electrode
immersed in a solution of Kcl, that has been saturated with
Agcl.
• The half cell may be represented as
Ag/Agcl (Std)/ Kcl//
• The half cell reaction is
Agcl (S) + e- Ag (s)+ Cl-
Normally, this electrode is prepared with a saturated
solution of Kcl. The voltage of this electrode is 0.222v at
25οC.
Advantages:
It is relatively economical.
It is an inner electrode, so contamination is less.

25. • It gives a potential which is reproducible in relation to
standard hydrogen electrode.
• It can be used at temperatures greater than 60οC, where
as Calomel electrode can’t.
Disadvantages:
• Silver react with fewer sample component e.g. protein in
the solution. Such reaction can lead to plugging of the
junction between the electrode and the analyte solution.

26. Indicator electrode
• The potential of this electrode is proportional to the
concentration of analyte.
• Two classes of indicator electrodes are used in
 Metallic electrode
 Glass electrode
 Quinhydrone electrode
 Antimony electrode
 Redox electrode
 Ion selective electrode

27. Metallic electrode
• This is the simplest type of electrode. Ti is merely a wire
or flat plate or cylinder of a metal which will not enter into
the reaction with the solution in which it is immersed.
• The most commonly used metal is platinum, in some
instances, silver or other metal may be used.
• At ll temperatures, where excess crystals of CdSo4.8/3
H2O are present in both arms of the H-tube.
The cell can be shown schematically as
• Cd (10% amalgum)/ CdSO4.8/3H2O, Cd SO4 (Sat.sol),
Hg2SO4/Hg

28. • The cell reactions of the electrode are
• Cd2+ + Hg+ 2e- Cd (Hg)
• Hg2
2+ + 2e- 2Hg
Advantages:
• It is easily prepared and remain stable for long period
of time and so readily reproducible.
• No permanent damage on passing on electric current.
Disadvantages:
• It shows change in voltage produced with variation
temperature.

29. Glass electrode:
• A glass electrode is used in conjunction with a
reference electrode for the determination of hydrogen
ion concentration.
Construction:
• A glass electrode consists of a bulb or covering of a
thin pH sensitive glass membrane, within which a
reference electrode is mounted.
• The contained reference electrode is usually a silver-
silver chloride electrode or a calomel electrode in
hydrochloric acid or buffered chloride solution.

30. Working :
• When a glass electrode is immersed in a solution
containing hydrogen ions, a potential is setup
between the inside & outside solutions separated by
the pH sensitive glass membrane.
• This potential setup is dependent upon the pH of the
solution outside the glass membrane.
• This potential can be measured, between the
reference electrode sealed inside the glass electrode
and a calomel reference electrode immersed in the
solution.

31. Quinhydrone electrode:
• Quinhydrone is a compound of quinone &
hydroquinone and in solution it is decomposed into
equimolecular quantities of these substances.
• It is used for the determination of pH values of a
solution.
• The quinhydrone electrode is composed of a platinum
wire dipped into a solution to which quinhydrone
crystal have been added.
• Quinhydrone is a solid compound.
• Sparingly soluble in water.
• It involves the redox reaction between
quinone and hydroquinone.

32. Advantages:
• It can be used in the presence of strong oxidizing and
reducing agent in viscous media.
• It can be used for solution having pH 2-10.
• It is immune to poisoning.
• It is simple to operate
• The equilibrium is reached quickly.
• It can be used in coloured, turbid & colloidal solution.
• It lends itself readily to the measurement of pH in a
few ml of solution.

33. • Disadvantages:
• In this electrode, the bulb is very fragile &
therefore, has to be used with great care.
• As the glass membrane has a very high electrical
resistance, the ordinary potentiometer can’t be
used for measuring the potential of glass
electrode, thus the elctronic potentiometer is
used.
• standardization has to be carried out frequently.
• It can’t be employed in pure ethyl alcohol, acetic
acid and gelatin.

34. Membrane
electrodes
Ion selective
electrodes
Crystalline
membrane
Non
crystalline
membrane
Molecular
selective
electrodes
Glasses
Liquids

36. Potentiometric titrations
• Measuring the potential difference between reference
electrode and indicator electrode witch is dipped in an
analyte whose potential to be determined.
• Measurement of potential:
Titration curve:
• Plot between potential of the sample being
determined against volume of titrant added.
First derivative curve:
• The change in potential for consecutive addition of
titrant by the change in volume.
Second derivative curve:
• The tangent of first derivative curve.

38. Types of Potentiometric titrations
• Acid-Base titrations: The neutralization of acids &
bases can be determined by change in concentration
of H+ & OH- ions.
• Complexometric titrations: in these titrations, metal
electrode whose ion is involved in complex formation.
• Oxidation& Reduction titrations: the titrations
involve transfer of e- from the substance being
oxidized to the substance being reduced.
• Precipitation titrations: these titrations involve the
formation of insoluble metal salts like Hg, Ag, Pb, Cu
etc.. Whose endpoint is determined by
potentiometrically.

39. Applications
• Estimation of pH.
• Moisture determination.
• To estimate Na, K, Ca … and dissolved gases such
as CO2.
• Analysis of CN-, NH3, NO3, F3 in water & waste water.
• To determine equivalence point of an acid-base
titrations.
• To determine NO3, NH4, I, Ca, K, CN, Cl in plant
materials, soils, feed stuff, fertilizers.
• Useful to determine coloured/ turbid/ viscous
substances.