Polarographic maxima and its interpretation

1. Dr. Y. S. THAKARE
M.Sc. (CHE) Ph D, NET, SET
Assistant Professor in Chemistry,
Shri Shivaji Science College, Amravati
Email: yogitathakare_2007@rediffmail.com
SEM-III
PAPER-X
ANALYTICAL CHEMISTRY –I
THERMAL AND ELECTROANALYTICAL METHOD
UNIT- IV
Polarographic Maxima and Its
Interpretation
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2. -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
i (A)
Potential applied on the working electrode is usually swept over (i.e. scan)
a pre-defined range of applied potential
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
A
D
C
B
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3. Limiting current (Curve CD): At limiting current (flat region or plateau ) the rate of
supply of Cd2+ ions from the bulk of the solution to the indicator electrode surface
becomes equal to the rate of their deposits that is no more diffusive force operative
of ions of Cd2+. At point C the rate of supply of Cd2+ from the bulk of the solution to
the indicator electrode surface becomes equal to the rate of their deposition. Hence
at potential greater than C the concentration of undischarged Cd2+ at the
microelectrode surface is a negligibly small as compared to the cell ions in the
solution. Therefore no further increase in the current can be expected after C but a
small steady increase in current will be results between C and D. Since is the limiting
current is now formed by rate at which Cd2+ reach the surface.
Here
the rate of deposition = to rate of travel
Dr. Yogita Sahebrao Thakare
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6. https://tenor.com/p6lv.gif
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7. DEPARTURE FROM DIFFUSION -LIMITED CURRENTS
Ilkovic equations are applicable to currents that are limited by the
diffusion of the electroactive species to the surface of the drop.
Conditions may arise when the cell current differs in magnitude from
the diffusion limited current. It has frequently been observed that the
ideal current-voltage curve obtained with the dropping mercury
cathode is distorted by the occurrence of peaks of maxima. These
maxima are reproducible and vary in shape from sharp peaks to round
hump. With the increase in applied voltage, the current rises
abnormally reaching a critically value and then rapidly decreasing to a
limiting value corresponding to normal diffusion current plateau.
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8. 1. Polarographic Maxima: The shapes of polarogram are frequently distorted by the
polarographic maxima when dropping mercury electrode is used. These maxima are
recognized by the appearance of a sharp peak or a rounded hump at the top of
wave and hence interfere with the accurate evaluation of diffusion currents and
half wave potentials. Instead of curve levelling off at the limiting current wild
oscillations occur in the polarographic waves due to some absorption
phenomenon. During the curve maxima, a which more suppressing situation exists
in which ions or molecules reach at the electrode than can occur at the electrode
surface by diffusion through the unstirred solution. A streaming of the solution
observed past the DME has also been observed during the maxima. Thus the
motion of the solution brings more electroactive species into contact with the
electrode surface than would be the case, if the solutions were unstirred.
Polarographic maxima are also called as streaming Maxima and are categorized
into two groups
First Order Maxima: First order maxima arise because of non-uniform polarization
of a mercury drop. This type of maxima appears as a continuation of the rising part
of the wave and occurs only over a small range of applied potential. The current,
after reaching the maximum value, decreases rapidly to the normal diffusion
current value (plateau). First order maxima are usually found associated with the
reduction of inorganic species. (in dilute solution only). (40 times higher than
limiting current)
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9. 2. Second Order Maxima : Second order maxima occurs over a wide
range of applied potential and are associated with organic
compounds. These maxima may be due to deformation in the Hg
drop or unusual convection phenomenon both within and in the
immediate vicinity of DME. This type of maxima takes the shape of
rounded humps on the waves and are usually found associated with
organic compounds. (In concentrated solution)
Second order maxima occur over a wider range of applied potential
than the first order maxima. No satisfactory explanation has been
given for the occurrence of these maxima. Almost all the ex
planations (such as those of J. Heyrovsky (1934). H.J. Antiweiter
(1937), and M. Von Stackelberg et al (1938) that have been put
forward suggest the cause in terms of unusual convection
phenomenon both within and in the immediate vicinity of the
dropping mercury electrode
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10. Fig: 1 First order and second order Maxima
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12. The occurrence of such maxima interferes in the measurement of true diffusion
current, so that the removal or suppression of the maxima is very important. This is
often done easily by the addition to the working solution of a very small quantity of a
surface active substance, such as sugar dyestuffs, indicators, gums, gelatin and other
colloids. These surface active substances are called maximum suppressor.
Removal of Maxima: Polarographic maxima can be removes by using suppresser like
sugar dyestuffs, indicators, gums, gelatin and other colloids Out of these maximum
suppressors, gelatin is widely used, the concentration need not exceed 0. 1%, as the
higher concentrations can lead to distortion, lowering and shifting of the positions of
the polarographic waves. Other effective and commonly used maximum suppressors
are Triton X-100, (CH3)3 C.CH.C (CH3)2 C6H4 (OC2H4)9 OH and homologues, (0.002-0.004
per cent, and methyl cellulose (0.005 per cent solution). Maximum suppressors are
capable, even in small concentrations, to remove the interference and restore the
wave to its expected shape. Maximum suppressor is believed to form an adsorbed
layer on the aqueous side of the mercury-solution interface which resists compression
this prevents the streaming movement of the diffusion layer at the interface
responsible for the current maximum.
Too much suppressor should not be used. Its excess may lead to distortion, lowering
and shifting the positions of polarographic waves.
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13. 2. Non-additive and Mixed Current: If more than one electrode
reaction occurs at DME, the products of the reaction may interfere
with the reacting species of the other reaction. For example, such an
interference is obtained when a polarogram is obtained using a
neutral, unbuffered solution, containing both cadmium and iodate
ions. When these ions are allowed to run separately, a reversible wave
at - 0.6 V is exhibited by cadmium ions and a well developed wave at -
1.1 V is shown by iodate ions. On running these ions together in the
same solution, the cadmium wave is still observed at -0.6 V but the
iodate wave at - 1.2 V does not attain a total height equal to the sum
of waves run separately. This non additivity of waves in the mixed
solution is due to the precipitation of Cd(OH)2 in the vicinity of the
mercury drop.
IO-
3+3H20+ 6e- →I- + 60H-
3Cd2+ + 60H- →3 Cd(OH)2
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14. 3. Anodic Waves and Mixed Anodic-Cathodic Waves: Anodic waves are
less common because of relatively small range of anodic potentials
that can be covered with the DME before Oxidation of electrode takes
place. The electrode reaction which involved the oxidation of Fe2+ to
Fe3+ in presence of citrate ion constitutes an example of anodic wave.
A diffusion current is obtained at zero volt versus SCE due to half
reaction.
Fe2+→ Fe3+ + e-.
At 0.2 V the current becomes zero because the oxidation of Fe2+ has
ceased. A cathodic wave results from the reduction of Fe3+ to Fe2+ ion .
The half wave potential is identical with that of anodic wave,
indicating that oxidation and reduction of Fe2+ and Fe3+ are reversible
at DME.
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15. EVALUATION OF POLAROGRAPHIC WAVES
Wave height or diffusion current can be evaluated by the following methods
1. Extrapolation Method. The point before the initital rise of wave of residual
current is extrapolated and a line parallel to it is drawn through the diffusion
current plateau. Since the slope of the charging current curve is not linear with
changing voltage and a change in drop time affects the charging current and
faradaic current differently, estimation of residual current by extrapolation
method is not accurate for solutions of low concentrations. The method is useful
when several polarographic waves are to be analyzed on the same polarogram.
Fig: 2 Measurement of Diffusion current by Extrapolation method
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16. 2. Exact Method: The measurement of wave heights ( OR Diffusion current) i.e.
Determination of E1/2 : In order to evaluate the polarogram, the wave height of each
of the wave must be measured. The measurement of diffusion current is relatively
simple with a well defined polarographic wave whose limiting current plateau is
parallels the residual current curve. Residual current can be exactly evaluated by
recording a separate polarogram for the residual current of the supporting
electrolyte alone, and is subtracted from the value of current at any particular
potential of the dropping electrode to give the diffusion current. (Both graph
measured at same applied potential
Fig: 3 Measurement of Diffusion current by Exact method
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17. 3. Point Method
Point method is used for determining the diffusion current when the wave is slightly
distorted. Two lines AB and CD perpendicular to the abscissa axis are plotted. These
are divided at point F and G so that
AF= FB and CG = GD. The intersection FG with wave gives the position of half-wave
potential. A vertical line KH passing through the point E1/2 is drawn. It gives diffusion
current or wave height. Thus the wave height represents the vertical distance at the
E1/2 between the straight lines approximating the residual and diffusion current
regions. This method gives too low value of diffusion current when the polarogram is
too much distorted.
Fig: 4 Determination of wave height by point method
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18. CONDITIONS FOR PERFORMING POLAROGRAPHIC DETERMINATIONS
Following conditions should be maintained in order to get precise, accurate and reproducible
polarogram.
1. Supporting electrolyte must be extremely pure, Trace levels of impurities can yield
interfering polarographic wave.
2. The mercury used in DME should be of highest purity.
3. Since nitrogen gas may contain reducible impurities, it must be purified.
4. Large concentrations of electroactive species which react prior to or near the species
being determined may have to be removed by precipitation, extraction,
chromatographic or controlled potential electrolyte techniques.
5. If the electrochemical reaction at DME involves H+ ions, the reduction (or oxidation)
depends upon the pH. The buffer that is used for controlling pH may also act as
supporting electrolyte.
6. Migration current should be suppressed by adding an excess of electrochemically inert
ions to the solution. The inert ions must have more negative potential than the
analysed ion. These inert electrolytes are called supporting or sometimes back ground
electrolytes.
7. Polarographic maxima or current maxima or the distortion of polarographic wave is
minimized by adding surface active agents such as gelatin, dye stuffs or agar- agar etc.

19. By H KAUR

20. Unit IV Electroanalytical Techniques
Polarography: Theory, Basic principle of polarography, apparatus.
Dropping mercury electrode. Supporting electrolyte, effect of supporting
electrolyte on limiting electrode. Diffusion coefficient and its evolution.
Ilkovic equation, its derivation and its applications, Ilkovic equation-
diffusion current constant and capillary characteristics determination,
Half wave potential. Polarographic maxima. Interpretation of
polarographic curve. Role of temperature on diffusion current. Reversible,
quasi reversible and irreversible electrode reaction and evaluation of
parameter using various reaction derivative polarography, modified
polarographic techniques, AC polarography, limitations of polarography,
pulse polarography. Method of quantitative analysis: Absolute,
comparative. The PILOT ION and kinetic methods.
Voltammetry: Basic, Principles, Instrumentation, Cyclic voltammetry-
Principle, Instrumentation and applications, Voltammogram, Stripping
technique, Anodic and Cathodic voltammetry, and their applications in the
determination of metal ions and biologically important compounds.
Enzyme catalyzed reaction and applications of voltammetry in monitoring
such reaction.
Related Techniques: Amperometric titration and Chronopotentiometry,
Principle methodology, and their application in qualitative and
quantitative analysis.
Electrode – The metal rod dipped in its salt solution 20
05-August -20 Dr. Yogita Sahebrao Thakare