Detailed Theory of Modified Polarographic Analysis over Classical Polarography Includes Different Modified Methods with Applications. Medha Thakur (M.Sc Chemistry)

1. MODIFIED
POLAROGRAFIC
METHOD
Presented by : Medha Thakur
MSc Chemistry,
S.N.D.T university

2. Contents
• Drawbacks Of Classical Polarography
• What is Residual Current ?
• Differential Pulse Polarography
• Advantages
• Square Wave Polarography
• Advantages & Uses
• Cyclic Voltammetry
• CV Voltammogram
• Applications

3. Drawbacks of Classical Polarography
1. It is limited to solutions having concentration more than
10-5 M because of the residual current.
2. Potentials more positive than 0.4V w.r.t. S.C.E. are not
accessible because of oxidation of Hg above this value.
3. Quantitative determination of several elements in a
mixture is possible only if there is a difference of 0.2 - 0.3V
between succeeding halfwave potential

4. What is Residual Current ?
• In polarography, in the absence of
electrolytic species, a small current known
as the residual current ir, flows through
the electrolytic cell at most potential (curve
B).
• The residual current is the combination of
the current flowing as a result of the
reduction or oxidation of any impurities in
solution and the capacitive (or charging
current).

5. Differential Pulsed Polarography (DPP)
• A DC potential which is linearly increased with
time is applied to the polarographic cell.
• As in classical polarography the rate of
increase is perhaps 5mv/s. In contrast, however
a dc pulse of an additional 20- 100mv is applied
at regular interval of about 1-3sec.
• The length of the pulse is about 60 milli sec. &
the pulse is applied near the end of the life of a
drop & it terminates with the detachment of
the mercury drop from the electrode.

6. • Two current measurements are made alternatively- one just
prior to the D.C pulse and one near the end of the pulse.
• The difference in current per pulse (Δ i) is recorded as a function
of the linearly increasing voltage. A differential curve results
consisting of a current peak.
• A height of this peak is directly proportional to the concentration
of the analyte.
Voltage Programme

7. Advantages
• Individual peak maxima can be observed for substances with half
wave potential differing by as little as 0.04 to 0.05v, in contrast ‘CP’
requires a potential difference of at least 0.2v for resolution of
waves.
• Increase in the sensitivity of this method by about 3 orders of
magnitude over ‘CP’. [ sensitivity 10-8M of DPP, 10-5M of C.P.]
• The greater sensitivity can be attributed to two sources.
1. Increase of the Faradic current due to the application of voltage
pulse to each drop
2. Decrease of non Faradic charging current [major part of the
residual current]

8. Square Wave Polarography
• It is a type of pulse polarography that offers the
advantage of great speed and high sensitivity
with a dropping mercury electrode.
• scan is performed during the last few milli sec.
of the life of a single drop when the charging
current is essentially constant. (ir minimum).
• It has also been used with hanging drop
electrode.

9. Square wave voltammetry signals

10. Square Wave Voltammogram
• For a reversible reduction, as shown in fig. the
forward pulse produces cathodic current i1.
whereas reverse pulse gives anodic current i2.
• usually the difference in this currents Δi is
plotted to give voltammograms. This difference
is directly proportional to concentration.
• The potential of the peak corresponds to the
polarographic half wave potential.
• Detection limits are reported to be 10-7 to 10-8
M indicating the high sensitivity of the method

11. Advantages and Uses
• this technique will gain considerable use for analysis of inorganic
and organic species.
• It has also been suggested that square wave voltammetry can be
used in detector for HPLC.
• NOTE- Unlike DPP the entire scan in square wave polarography
is obtained on a single drop. Typically, a relatively long drop time
(5s or more) is used for the study.
• since the entire scan requires about 0.5s. the polarogram can be
obtained during the last 0.5s. the polarogram can be obtained
during the last 0.5s of the drop life. here the ir value is the least.

12. Cyclic Voltammetry
TriangularWaveform of CV
• The potential is scanned at a fixed rate from the initial potential to a maximum (or
minimum) potential where the scan direction is reversed and the potential is returned
at the same scan rate to the initial potential.
• The initial direction of potential scan can be either negative or positive. The indicator
electrode can be a HMDE, a solid wire or disc electrode (normally made from C, Pt or
Au) or some other solid or liquid electrode

13. Cyclic Voltammogram (CV) of Ferricyanide
• potential is varied from +0.8 to -0.15v .
• The scan rate is either direction is 50mV/s
• the potentials at which reversal takes place (here
-0.15 & +0.8v) are called switching potential.
• The direction of the initial scan may be either be
negative as shown, or positive, depending upon
the composition of the sample.
• a scan in the direction of more negative potential
is termed as forward scan, while one in the
opposite direction is called a reverse scan).
• Generally cycle time ranges from 1ms or less to
100s. In this example the cycle time is 40s.
CV of Ferricyanide

14. • fig. overleaf shows the current response when a
solution that is 6mM in K3Fe(Cn)6 and 1M in KNO3
is subjected to the cyclic excitation signal.
• At the initial potential of +0.8v, a tiny anodic (-ve)
current is observed which immediately decreases to
zero as the scan is continued.
• No current is observed between a potential of +0.7v
& +0.4v because no reducible or oxidisable species is
present in this potential range.
• When the potential becomes somewhat less negative,
a cathodic current (point B) develops.
CV of Ferricyanide
TriangularWaveform

15. • A rapid increase in the current occurs in the region of
B to D as the surface concentration of Fe(Cn)6
-3
becomes smaller and similar.
• With a further increase in the applied potential i.e. as
the time increases, the depleted layer of electroactive
species around the electrode grows.
• The width of the depleted layer through which the
electroactive species must diffuse increases with the
square root of time.
• the decrease in current (D to F) causes the recorded
polarogram to exhibit a peak (D). Eventually the
current becomes nearly constant (F) because a
relatively constant; thickness diffusion layer is
achieved. CV of Ferricyanide

16. • At point F, the scan direction is switched. The
current however continues to be cathodic.
• once the potential becomes positive enough so that
reduction of Fe(Cn)6
-3 can no longer occur. the
current goes to zero (H to I to J) and then becomes
anodic.
• This anodic current then reaches peaks (J) and
then decreases (K) as the accumulated Fe(Cn)6
-4 is
used up by the anodic reaction.
• important parameters in a cyclic voltammagram
are the Cathodic peak potential Epc, the anodic
peak potential. Epa, the cathodic peak current ipc
and the anode peak current ipa.
CV of Ferricyanide

17. CV for Insecticidal Parathion
o 3 peaks are observed.
• The first cathode peak (A)
ϕ NO2 + 4e- + 4H+ → ϕ NHOH + H2O
• The anodic peak (B)
ϕ NHOH → ϕ NO + 2H+ + 2e-
• The cathodic peak at (C)
ϕ NO + 2e- + 2H+ → ϕ NHOH
Parathion Hydroxylamine
derivative
Nitroso
derivative

18. Applications of CV
• CV has become an important tool for the study of mechanism and
rate of oxidation or reduction processes, particularly in organic and
metalorganic system.
• often the voltammogram will reveal the presence of intermediates
in oxidation or reduction reaction. Usually used for fabrication of
micro electrodes used with this technique.
• Although (CV) is useful for quantitative analysis, the primary use of
cyclic voltammetry is a tool for fundamental and diagnostic studies
that provide qualitative information about electrochemical
processes under various conditions.