Dc,pulse,ac and square wave polarographic techniques new

DC,PULSE,AC AND SQUARE WAVE
POLAROGRAPHIC TECHNIQUES
Presented By : Biji Saro Varghese
M.Sc. I Sem-II
Guided by : Prof. R. M. Jugade
1
RASHTRASANT TUKADOJI MAHARAJ NAGPUR UNIVERSITY
P. G. T. DEPARTMENT OF CHEMISTRY
Date-6/4/18

CONTENTS
• Introduction
• Polarography
• DC polarography
• Pulse polarography
• NPP
• DPP
• AC polarography
• Square wave polarography
2

INTRODUCTION
3
POLAROGRAPHY is an
electromechanical technique of analyzing
solutions that measures the current flowing
between two electrodes in the solution as
well as the gradually increasing applied
voltage to determine respectively the
concentration of a solute and its nature.
Created by: Jaroslav Heyrovsky in 1922
Received Nobel prize in 1959

POLAROGRAPHY
4
 Is a method of analysis based on the
measurement of current of 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), which are useful for their
wide cathodic ranges and renewable surfaces.

In this method, a reference electrode and an
indicator electrode are required.
5
Reference electrode- acts to maintain a
constant potential throughout the
measurement.
• It is non polarizable electrode.
eg: mercury pool or saturated calomel
electrode
Indicator electrode- assumes the potential
impressed upon it from an external source.
• It is polarizable electrode.
eg:Dropping mercury electrode(DME)
Reference
electrode
Indicator
electrode

PRINCIPLE
6
 Change in electrical properties of solution
when subjected to electrolysis depends on
nature and concentration of electroactive
species present in solution.
 When e.m.f is applied between these two
electrodes some current flows through the
circuit and the change in current is
monitored w.r.t scanning e.m.f

POLAROGRAPHIC DATA
7
The current –potential characteristics
is called as POLAROGRAM.
This curve is used to determine the
concentration of electroactive species.
Types of currents represented on
polarogram are
i. Residual current
ii. Diffusion current
iii.Limiting current

Why MERCURY?
8
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.

DC POLAROGRAPHY
9
Typical DC Polarogram

• In DC polarography ,a continuously increasing DC potential is
applied to DME and corresponding current is recorded in a
continuous manner.
• The drop time of mercury is usually between 2-8 seconds.
• Detection limit is up to 10-3 to 10-4 M.
10

Advantages
• This technique was useful to determine the
concentration of almost all organic and
inorganic analyte.
• Detection limit is up to 10-4 M.
• It is sensitive as compared to earlier
classical techniques.
11

• In pulse polarography the potential is applied in the form of
pulses and current is recorded in discrete manner.
• Two different pulse techniques are used
 Normal Pulse Polarography(NPP)
 Differential Pulse Polarography(DPP)
13

NORMAL PULSE
POLAROGRAPHY(NPP)
14

• It is also called as large amplitude pulse polarography.
• In this technique a constant base line potential is applied to
DME and pulse of increasing amplitude are superimposed
on it.
• The pulse is applied only for 50mS at the end of drop time
while current is recorded just for last 17mS at the end of
pulse.
• After each pulse the capillary is mechanically tapped and
mercury drop falls down.
• The resultant polarogram is sigmoidal in nature but it is of
stair case type.
15

DIFFERENTIAL PULSE
POLAROGRAPHY(DPP)
16

• In this technique, a continuously increasing DC potential is
superimposed with pulse of constant amplitude.
• The pulse is applied only at the end of drop time for 50mS.
• The current is recorded twice during each drop .The current
is recorded just before application of pulse and at the end of
the pulse.
• The difference between these two currents is plotted as a
function of baseline potential(E).
• The polarogram obtained consists of a peak instead of a
sigmoidal curve.
• The peak current or the peak height is directly proportional to
concentration of electroactive species.
17

Advantages
• Non faradaic condenser current is completely
eliminated, residual current becomes almost
zero.
• The total current is recorded at the end of the
pulse hence, the only contributor is faradaic
current.
• Detection limit goes down to 10-8 M.
18

time
E
AC POLAROGRAPHY
19
Eac
Edc
ip
ipαc
idc
Ep
Edc

• In this technique, a continuously increasing D.C.
potential (Edc) is superimposed with a constant
amplitude A.C. potential.
• When such combination is applied to DME, two
types of currents are generated at DME.
• The total current is the resultant of D.C. and A.C.
current.
• The A.C. component is electronically rectified and
plotted as a function of applied D.C. potential(Edc).
20

Advantages
• Detection limit of the order of 10-6 to 10-8 M
can be achieved.
• There is no need of purging nitrogen gas to
remove oxygen because a.c. polarography is
insensitive to irreversible process like oxygen
reduction.
21

∆E
Potential
Time
Amplitude (E)
i1
i2
Sample
period
SQUARE WAVE
POLAROGRAPHY

• Large amplitude differential technique.
• The wave form applied to the working electrode is a symmetric
square wave superimposed on a base staircase potential.
• Current is sampled twice during each square-wave cycle.
• One at the end of the forward pulse (i1) and one at the end of the
reverse pulse (i2).This results in square-wave modulation.
• Modulation amplitude is very large.
• Reverse pulses cause the reverse reaction of any product formed
from the forward pulse.
• The net current (i1 – i2) is then plotted versus the base staircase
potential.
• This gives the peak-shaped polarogram
• Peak current is proportional to the concentration of the analyte.
• The net current is larger than the forward or reverse currents.

Advantages
• Higher sensitivity than differential-pulse in
which reverse current is not used.
• Detection limit is up to 10-8 M.
• Reduced analysis time due to higher scan
rates.
24

REFERENCES
• Quantitative analysis, fifth edition by
R.A.Day,Jr. and A.L.Underwood (Prentice-
hall Of India)
• Electroanalytical chemistry, theory and
applications, by R.T.Sane and A.P.Joshi
(Quest Publications)
• Basic concepts of analytical chemistry, third
edition by S.M.Khopkar (New Age
International Publishers)

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