The ppt describes its procedure

1. POLAROGRAPHY
By:
N.Sai sree

2. CONTENTS
• Voltammetry
• Polarography
• Polarographic analysis
• Electrodes and its examples
• Principle
• Three electrode cell
• Polarographic data & Polarogram
• Half wave potential & polarographic wave equation
• Polarographic maxima
• Oxygen wave
• Applications

3. VOLTAMMETRY
• Voltammetry is the general name given to a group of
electro analytical methods in which the current is
measured as a function of applied potential where in the
polarization of the indicator or working electrode is
enhanced.
• The field has been developed from polarography.
• The word polarography first recorded in 1935 – 1940
Polaro(graph) + graphy(field of study)

4. POLAROGRAPHY
It is an electrochemical technique of
analysing solutions that measure the
current flowing between two electrodes
in the solution as well as the gradually
increasing applied
voltage to determine respectively the
concentration of solute and its
nature.
Created by: Jaroslav Heyrovsky for that
he awarded Nobel Prize in
1959
Figure 1

5. POLAROGRAPHY
Is a method of analysis based on
the measurement of current
electrolysis of an electroactive
species at a given electrode potential
under controlled conditions.
 It is the branch of voltammetry
where the working electrode is a
dropping mercury electrode (DME)
or a static mercury drop electrode
(SMDE)
Figure 2

6. Contd...
Reference electrode
Indicator electrode
In this method, a reference
electrode and an indicator electrode
are required.
 Reference electrode- it is larger
in size and non
polarized(depolarized)
 Indicator electrode- it is smaller
in size and polarized
Figure 3
Figure 4

7. Why reference electrode with larger area and
indicator electrode with smaller area???
 Indicator electrode: It is smaller in size and polarized
i.e it adopts the potential externally imposed on it.
 Reference electrode : It is larger in size and non
polarized i.e it retains to a constant potential throughout
the measurement.

8. Polarographic analysis process and the conditions
for polarographic wave formation.
1. Polarographic analysis :
Electrolytic analysis carried
out under special conditions.
specific characteristics：
A、A polarized electrode and a
depolarized electrode are used as
working electrode & referenceelectrode
B. No stirring Incomplete
electrolysis (only a small amount of
analyte is consumed)
Figure 5

9. Polarized electrode and depolarized electrode
 If the electrode potential has great changes when infinite small current
flow through the electrode, such electrode is referred to as polarized
electrode. eg. DME
 If the electrode potential does not change with current , such electrode
is called ideal depolarized electrode. eg. SCE
Figure 6

10. EXAMPLES OF MERCURY ELECTRODES
In polarography, mercury is used
as a working electrode, mercury
it is a liquid. The working
electrode is often a drop
suspended from the end of a
capillary tube. Examples:
1. HMDE (Hanging mercury
drop electrode)
2. DME (dropping mercury
electrode)
3. SMDE (static mercury drop
electrode)Figure 7

11. WHY MERCURY?
Mercury as working electrode is useful because:
 It displays a wide negative potential range
 Its surface is readily regenerated by producing a new
drop or film
 Many metal ions can be reversibly reduced into it.
Figure 8

12. PRINCIPLE:
Study of solutions or of electrode processes by means of
electrolysis with two electrodes, one polarizable and one
unpolarizable, the former formed by mercury regularly
dropping from capillary tube.
 POLARIZED ELECTRODE: Dropping Mercury
Electrode (DME)
 DEPOLARIZED ELECTRODE: Saturated Calomel
Electrode

13. Contd..
 Mercury continuously
drops from reservoir
through a capillary tube
into the solution.
 The optimum interval
between drops for most
analyses is between 2 and
5 seconds.
Figure 9

14. Contd...
Figure 10 Figure 11

15. Three electrode cell
Three electrode cell:
Working, Reference,
Counter/ auxilliary
current flows between
working and counter
electrodes. Potential
controlled by potentiostat
between working and
reference electrodes.
Figure 12

16. Contd...
Two special electrodes
 Supporting electrolyte :Usually relatively higher
concentration of strong electrolytes (alkali metal salts)
serves as supporting electrolyte
 Dissolved oxygen is usually removed by bubbling
nitrogen through the solution
 Voltage scanning Under unstirred state, recording
current-voltage curve.

17. POLAROGRAPHIC DATA
 Obtained from an automatic recording instrument is called a
polarogram , and the trace is called a polarographic wave.
 POLAROGRAM
It is a graph of current versus potential in a
polarographic analysis.
3 categories:
 collectively referred to as residual current
 referred to as diffusion current resulting from the reduction
of the sample
 called the limiting current
 The diffusion current of a known concentration of reference
standard are first determined followed by the determination
of the diffusion current of the unknown concentration

18. POLAROGRAM
ir (residual current) which is
the current obtained when no
electrochemical change takes
place.
 iav (average current/limiting
current)is the current obtained
by averaging current values
throughout the life time of the
drop while
 id (diffusion current) which
is the current resulting from the
diffusion of electroactive
species to the drop surface.Figure 13

19. Limiting diffusion current -- A basis of
polarographically quantitative analysis
 When the applied voltage exceeds the decomposition voltage,
diffusion-controlled current is expressed as:
i = K(C-C0)
 When the applied voltage gets more negative, C0 →0, current
becomes only diffusion limited, then
id = KC
Id reaches a limiting value proportional to ion concentration
C in bulk solution, and do not changes with applied voltage
longer

20. Half-wavepotential —polarographic qualitativeanalysis
E½ at ½ i
The potential at which current is
equal to one half the limiting current
is called half wave potential.
Figure 14

21. How it works??
o The applied voltage is gradually increased, typically by
going to a more positive( more negative decomposing
potential)
o A small residual current is observed.
o When the voltage becomes great enough, reduction occurs
at the analytical electrode causing a current.
o The electrode is rapidly saturated so current production is
limited – based on diffusion of the analyte to the small
electrode.

22. How it works??
The reduced species alters the surface of the
mercury electrode.
To prevent problems, the mercury surface is
renewed by “ knocking off ” a drop –
providing a fresh surface.
This results in an oscillation of the data as it
is collected.

23. The diffusion theory and polarographic
wave equation
We have already known:
id = KC
In above equations, K is called Ilkovic constant, it is
expressed as follows:
K = 607 n D1/2m2/3t1/6
Thus,
id = 607nD1/2m2/3t1/6C

24. id = 607nD1/2m2/3t1/6C
From above, when temperature, matrix solution and
capillary characteristic are kept constant, id is
proportional to C.
Concentration of
electroactive
analyte (mmolL-1)
Drop time(sec)
Number of
transferring
electrons in
electrode
reaction(e/mol
)
Diffusion current
(μA)
Density of analyte
in solution(Cm2.sec-
1
)
Mercury mass
flow(mg. sec-1)

25. Polarographic wave equation
E = E1/2 – RT/ nF ln(i/(id-1))
 When i = ½ id , log term in above equation is equal to
zero, corresponding potential is called halfwave
potential E1/2
 E1/2 independent on the concentration
 basis of qualitative analysis

26. Interference current in classical DC
polarograph
 Residual current
(1) redox reactions of impurities in solution
(2) charging of Hg drop (non-faradic current / non-redox
current)
 Migration current
The current produced by static attraction of the electrode
to sought-for ion

27. Polarographic Maximum (or malformed peak )
 Reproducible maxima often occur in CV curve unless
eliminated by the addition of suitable maximum
suppressor such as MC or gelatin.
 Curve a is unsuppressed oxygen maximum curve b is the
oxygen wave in presence of gelatin.
Figure 15

28. Oxygen wave
 Dissolved oxygen is easily reduced at many working
electrodes. Thus an aqueous solution saturated with air
exhibits two distinct oxygen waves.
 The first results from the reduction of oxygen to
hydrogen peroxide:
 The second wave corresponds to the further reduction of
hydrogen peroxide:

29. Contd...
Figure 16

30. Factors that affect limiting diffusion current
 Characteristics of capillary& pressure of Hg
 viscosity
 Composition of solution
 Temperature
 concentration Factors that affect Half
wave potential
Type and concentration of
supporting electrolyte
Temperature
Forming complex

31. ????
Why does Nitrogen gas pass through the solution
before elecctrolysis???
Pure Nitrogen is passed through the solution before
connecting the electrolysis so as to remove dissolved
oxygen and during purification process a current of pure
nitrogen is maintained over the surface of the solution.

32. Polarographically quantitative analytical
methods
（id) = K·C
 Direct comparison method
 Calibration curve method
 Standard addition method

33. APPLICATIONS
 Polarography is used for determination of Oxygen
content of fluids including whole body fluids ,
fermentation liquors &milk for studying the respiration
rates of microorganisms
 Several mercury containing antiseptics and insecticides
were determined polarographically
 Hormones like thyroxine,insulin,adrenaline and several
sex hormones are estimated by polarography
 It is used for the determination of antibiotics such as
pencillin,streptomycin and chloramphenicol
 Several Alkaloids can also be estimated by
polarography.

34. Contd...
 In electrochemistry polarography allows the measurement of
potentials and yields information about the rate of the
electrode process, adsorption, desorption phenomena.
 Calculation of the rate constant is possible with
polarography in this way very fast reactions of order 105 –
1010 litre mol-1sec-1 can be determined.
 Polarography prooved useful in mechanistic studies.
Elimination of Mannich bases, hydration of multiple bonds in
unsaturated ketones and aldolization are example studies.

35. Contd...
 Inorganic applications:
◦ In inorganic analysis polarography is used
predominately for trace metal analysis like copper,
zinc, iron, lead, nickel, manganese etc..
◦ Composition of alloys
◦ Purity of elements
OBJECTIVE PARAMETER
MEASURED
Identity of element Half wave potential
Quantity of element Diffusion current

36. Contd...
Organic applications:
 Electroreducible or oxidisable functional group can be
determined by polarographic technique by using dme.
The functionl group can be inferred from Half wave
potential and the quantity of the substance can be
determined from diffusion current measurement.
example functional groups like Nitro and Nitroso
groups, azo and diazo compounds, aldehydes, ketone,
organic peroxides lactons, activated C=C some acids and
organo metalic compounds.
Multi stage reduction of groups like Nitro to Nitroso to
Hydroxyl amine to Amino group can also be achieved

37. Contd...
The following table gives examples of E1/2
of some compounds.
Functional group E1/2 (V)
Benzaldehyde -1.51V
Iso propyl phenyl ketone -1.82V
Aldehydes and ketones -1.3V to -2.0V
Nitro compounds -0.1V to -0.7V

38. Contd...
 Example of reduction:

39. Contd...
 One of the easiest and most frequently encountered
organic reduction is that of the nitro group. In Nitro
furans and nitroimidazoles, for example the reaction is

40. Examples of drugs analysed by polarography
◦ prazosin
◦ Nifedipine
• Felodipine
• Amlodipine
• Spiranolactone
• Digitoxin
• p- Benzoquinone
• Vitamin K and its derivatives
• Azo and diazo compounds
• Keto steroids

41. RFERENCES
 A Textbook of pharmaceutical analysis
Third edition by A.connors pgno:154-172
 Vogel’s Textbook of Quantitative chemical analysis by
J mendham;RC denney;JD Barnes;
M Thomas, B Sivasankar
sixth edition
pgno;361-387
 Quantitative Analysis; sixth edition by
R.A.DY;Jr.A.l.underwood
Pgno:11-1 to 11-11
 Practical pharmaceutical chemistry
fourth edition-part two ; edited by:
A.H Beckett T.B Stenlake Pgno209-242