The earliest voltammetric technique Heyrovsky invented the original polarographic method in 1922, conventional direct current polarography (DCP). It employs a dropping mercury electrode (DME) to continuously renew the electrode surface. Diffusion is the mechanism of mass transport. When an external potential is applied to a cell containing a reducing substance such as CdCl2, The following reaction will occur: Cd2+ + 2e + Hg = Cd(Hg) The technique depends on increasing the applied voltage at a steady rate and simultaneously record photographically the current-voltage curve (polarogram) The apparatus used is called a polarograph . When an external potential is applied to a cell containing a reducing substance such as CdCl2, The following reaction will occur: Cd2+ + 2e + Hg = Cd(Hg) The technique depends on increasing the applied voltage at a steady rate and simultaneously record photographically the current-voltage curve (polarogram) The apparatus used is called a polarograph . Capillary tube about 10-15cm Int. diameter of 0.05mm A vertical distance being maintained betwwen DME and the solution Drop time of 1-5 seconds Drop diameter 0.5mm The supporting electrolyte is a solution of (KNO3, NaCl, Na3PO4) in which the sample (which must be electroactive) is dissolved. Function of the supporting electrolyte It raises the conductivity of the solution. It carries the bulk of the current so prevent the migration of electroactive materials to working electrode. It may control pH It may associate with the electroactive solute as in the complexing of the metal ions by ligands.

1. POLAROGRAPHY
BY;- SANCHIT DHANKHAR

2. 2
Introduction
 The earliest voltammetric technique
 Heyrovsky invented the original polarographic
method in 1922, conventional direct current
polarography (DCP).
 It employs a dropping mercury electrode (DME)
to continuously renew the electrode surface.
 Diffusion is the mechanism of mass transport.

3. When an external potential is applied to a cell
containing a reducing substance such as CdCl2,
The following reaction will occur:
Cd2+ + 2e + Hg = Cd(Hg)
The technique depends on increasing the applied
voltage at a steady rate and simultaneously
record photographically the current-voltage
curve (polarogram)
The apparatus used is called a polarograph .
Theory of polarography

4. Inlet of inert gas
(H2 or N2 to expel
dissolved oxygen
Outlet of inert gas
A typical polarograph
Instrumentation – Three electrodes in solution containing analyte
Working electrode: microelectrode whose potential is varied with time
Reference electrode:
potential remains constant (Ag/AgCl electrode
or calomel)
Counter electrode: Hg or Pt that completes
circuit, conducts e- from signal source through
solution to the working electrode
Supporting electrolyte:excess of nonreactive
electrolyte (alkali metal) to conduct current

5. A typical polarograph

6. 6
Dropping Mercury Electrode
(Working electrode)
 Capillary tube about 10-15cm
 Int. diameter of 0.05mm
 A vertical distance being
maintained betwwen DME and
the solution
 Drop time of 1-5 seconds
 Drop diameter 0.5mm

8. The supporting electrolyte
is a solution of (KNO3, NaCl, Na3PO4) in which the
sample (which must be electroactive) is dissolved.
Function of the supporting electrolyte
It raises the conductivity of the solution.
It carries the bulk of the current so prevent the
migration of electroactive materials to working
electrode.
It may control pH
It may associate with the electroactive solute as
in the complexing of the metal ions by ligands.
Supporting electrolyte

9. 9
Polarographic measurements
Polarography measurement is governed by
ilkovic equation:
id= 708 nD1/2m2/3t1/6C
n= no. of electrons
t= droptime(second/drop)
D= diffusion coefficient of analyte (cm2/s)
m= rate of flow of Hg through capillary (mg/s)
C= analyte’s concentration in mM

10. -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
Current
i
(A)
0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte
Applied potential, V vs SCE
Working electrode is no
yet capable of reducing
Cd2+  only small
residual current
flow through the
electrode
Electrode become more and more
reducing and capable of reducing Cd2+
Cd2+ + 2e- Cd
Current starts to be registered at the
electrode (decomposition
potential)
Current at the working
electrode continue to rise as the
electrode become more
reducing and more Cd2+ around
the electrode are being reduced.
Diffusion current of Cd2+
is not limited
All Cd2+ around the electrode has
already been reduced. Limiting
Current at the electrode is reached
by limiting the diffusion rate of Cd2+
from the solution to the electrode.
Thus, current stops rising and levels
off at a plateau
id
E½
Base line
of residual
current
Half –wave potential
Polarogram

11. -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
i (A)
0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte
V vs SCE
Working electrode is
no yet capable of
reducing Cd2+ 
only small residual
current flow through
the electrode
Electrode become more and more
reducing and capable of reducing Cd2+
Cd2+ + 2e- Cd
Current starts to be registered at the
electrode
Current at the working
electrode continue to rise as
the electrode become more
reducing and more Cd2+
around the electrode are being
reduced. Diffusion of Cd2+
does not limit the current yet
All Cd2+ around the electrode has
already been reduced. Current at
the electrode becomes limited by
the diffusion rate of Cd2+ from the
bulk solution to the electrode.
Thus, current stops rising and
levels off at a plateau
id
E½
Base line
of residual
current

12. The Polarogram is characterized by
the following parameters:
• Residual current
• Limiting current
• Diffusion current
• Half wave potential (E1/2)
Polarogram
09980022421

13. A typical Polarogram

14. A- Mass transfer
The movement of sample from one location in solution to
another, it arises from either:
1. Migration (under the influence of electric field difference),
2. Diffusion (under influence of concentration difference),
3. Convection (under the influence of stirring),
B. Diffusion
C. Chemical reaction at
the electrode surface
Factors affecting electrode reaction rate and current

15. Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+ Pb2+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+ K+
K+
K+
K+
K+
K+
K+
K+
K+
-1.0 V vs SCE
Pb2+ + 2e- Pb
K+
K+
K+
K+
K+ K+
Layers of K+ build up around the electrode stop the
migration of Pb2+ via coulombic attraction
Concentration gradient created between
the surrounding of the electrode and the
bulk solution
Pb2+ migrate to
the electrode via
diffusion
Factors affecting electrode reaction rate and current

16. Advantages of DME
Its surface is reproducible, smooth and continuously
renewed, this eliminates the poisoning effect.
Mercury forms amalgams (solid solution) with many metals.
The diffusion current assumed a steady value immediately
after each change of applied potential and is reproducible.
The large hydrogen over-potential of mercury renders
possible deposition of substance that difficult to reduce.
The surface area can be calculated from the weight of drop.
Disadvantages of DME
At potential more positive than + 0.4 V vs SCE, mercury
dissolves producing anodic polarographic wave which
masks other waves, therefore DME can be used only for the
analysis of reducible or easily oxidizable substances.
The capillary is very small so easy to be blocked→ malfunction
of the electrode
Mercury is very toxic and easily oxidized
Advantages and disadvantages of DME

17. Application of Polarography
INORGANIC ANALYSIS
Analysis of metals
Zn
Cd
Analysis of anions as
dromate, iodate, etc.
ORGANIC ANALYSIS
Analysis of
carbonyl,peroxide,
nitro, azo group, etc.
Biochemical analysis
A. Qualitative: by using the half wave potential which is
characteristic to each substance
B. Qualitative

18. Advantages of using polarography in pharmaceutical
analysis
•1- Only small volume of sample is required.
•2- Turbid and coloured solutions can be analyzed.
•3- It can be used for the determination of
substances, which are not electrochemicall active
(indirect).
•4- Prior separation of excepients is not required.
•5- Its sensitivity is sufficient for the determination
trace elements and toxic impurities.
6- Samples of natural origin
7- high speed analysis which is important for QC
Application of Polarography

19. 19
RECENT DEVLOPEMNTS
In refinary industries.
Estimation of polyelectrolytic solution.
Biotechnology.
Microbiosensors for enviromental monitoring.

20. References
 Gurdeep.R. chatwal,sham k.anand,instrumental method
of chemical analysis,himalaya publishing
house,2008,p.no.2.482-2.497.
 Hovert H.willard,lynne L.merritt,john A.dean,frank
A.settle,jr.,instrumental method of analysis CBS
publishers 1986,p.no.732-750.
 Kenneth A. connors,e textbook of pharmaceutical
analysis,third edition,wiley india,p.no. 334.
 Danniel christein,analytical chemistry,2nd edition,wiley
india,p.no. 274.
 www.pharmapaedia.com
20

21.  www.authorstream.com
 Kissinger, P. T., AND W. W. Heineman, eds.,
Laboratory Techniques in Electroanalytical Chemistry,
Dekker, New York, 1984.
 A.H.beckett ,J.B. stenlake,practical pharmaceutical
chemistry,fourth edition ,part –two,p.no-91.
 Lingane, J.J., Electroanalytical Chemistry, 2nd ed.,
Wiley- Interscience, New York, 1958
Continued…. 21

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