POLAROGRAPHY AND VOLTAMMETRY :
BASIC PRINCIPLES
&
APPLICATIONS
Qualitative
Quantitative
 In one step analysis
Metals
Other inorganics
Organics
Polarography & Voltammetry:
ELECTRO-ANALYTICAL
TECHNIQUES
NATURE OF SAMPLE
Practically any
LOD
10-5 → 10-15 M
g → pg
A MICRO SCALE
“ELECTROLYSIS”
OF
ANALYTE SOLUTION
REQUIREMENTS :
Analyte, be
 AN OXIDIZABLE / REDUCIBLE SPECIES
(Directly or indirectly)
Aqueous
 IN SOLUTION
Non Aqueous
BASIC APPARATUS / MATERIALS REQUIRED
1. Cell : A container to hold the analyte solution
2. Electrodes
( Classical technique: 2 : RE & WE )
( Modern techniques: 3: RE, WE & CE)
3. Voltage Supply (Variable DC/AC) / (Potentiostat)
4. Voltmeter
5. Ammeter
FOR ELECTROLYSIS TO OCCUR:
 E “WE”  >  E eqbm l
i.e.
For “Oxidation” : E “WE” more (+)ve than E eqbm
For “Reduction” : E “WE” more (-)ve than E eqbm
REPRESENTATION OF A SIMPLE SETUP WITH
A TWO ELECTRODE CELL
Voltage supply
V
A
W.E.
R.E
Analyte solution
W.E. = Working Electrode
R.E. = Reference Electrode
V = Voltmeter
A = Ammeter
WHAT DO WE MEASURE IN 2
ELECTRODE CELLS ?
Potential of the cell
BOTH
Current through the circuit
REPRESENTATION OF A MORDEN VOLTAMMETRIC
SETUP: WITH A THREE ELECTRODE CELL
WITH A POTENTIOSTAT
voltmeter
V
Ammeter
A
Electrolysis
cell
(working
electrode)
Potentiometer
Counter
electrode
Reference electrode
WHAT DO WE MEASURE IN
3 - ELECTRODE CELLS ?
Potential of “WE” vs “RE”
BOTH
Current via “WE” & “CE”
CELL CURRENT
POTENTIAL OF “WE”
Quantification
Identification
WE: Dropping Mercury Electrode:
“Polarography”
WE: Other electrodes, mostly solids:
“Voltammetry”
ION TRANSPORT DURING ELECTROLYSIS
 MIGRATION
 DIFFUSION
 CONVECTION
MIGRATION
WE
+
+
+
+
-
-
+
-
-
- -
-
-
Movement of oppositely charged ions towards electrode due
to electrostatic attractions.
DIFFUSION
WE
Diffusion
+
+
+
+
-
+
-
-
- -
-
-
Movement of ions from region of higher concentration (bulk)
to region of lower concentration (near the electrode surface)
Convection
WE
+
+
+
+
+
- +
+
+
+
+
+ -+ - - +
Transport of ions towards electrode due to agitation, vibration
and temperature gradients
THE TWO MAJOR DIVISIONS IN
VOLTAMMETRY
1. Voltammetry under diffusion controlled
mass (ion) transfer
eg: Polarography, LSV, CV, NPP, DPP, etc.
THE TWO MAJOR DIVISIONS IN
VOLTAMMETRY (cont’d)
2. Voltammetry under convection controlled
mass (ion) transfer
a) Movement of electrode in a still solution which promote
„convection‟
eg: RDE, RRDE
b) Movement of solution past the stationary electrode
eg: electrochemical detection for LC where „flow cells‟,
„channel electrode‟ (wall jet electrode) are used.
VOLTAMMETRY UNDER DIFFUSION
CONTROL MASS TRANSFER
 NO MIGRATION
 NO CONVECTION
 ONLY DIFFUSION
How to achieve this condition?
METHODS OF STOPPING OR MINIMIZING
MIGRATION
 Add an excess ( 100 fold or more) an inert
electrolyte to the analyte solution
 This screens the electric field produced by the
electrode
 Therefore no attraction of ions from the bulk
to the electrode
HOW TO STOP CONVECTION?
 No Vibration
 No Agitation
In the solution
 No Shaking
 No Temperature gradient
MASS TRANSPORT
ONLY BY
“DIFFUSION”
CONCENTRATION POLARIZATION
Species
concentration at
the electrode
surface
Species
concentration in
the bulk
CONCENTRATION PROFILES
C
t1
t2
t5
t6
t3 t
4
t6>t5>t4>t3>t2>t1
x
x = distance away from electrode surface
C = Concentration
From time t4 onwards surface concentrations are zero
Beyond the time t6, no change in concentration profile with time
i.e. steady state has been reached
AT STEADY STATE:
Rate of
removed of
ions at the
electrode
Rate of
supply of
ions from the
bulk to the
electrode
Cell Current, I  Rate of ion removal
Cell Current, I  Rate of supply of ion
Cell Current, I  Concentration
POLARIZATION OF ELECTRODES
 Polarized Electrodes
 Non Polarized Electrodes
POLARIZED ELECTRODES
Current, I, remains unchanged with changes in the
electrode potential, E.
I
A
B
E
Over the potential range A to B the electrode is polarized
AT STEADY STATE :
“WE”
is polarized
A condition necessary for voltammetry
Note 1: Microelectrodes reaches the state
of polarization very rapidly
2: Current is very small < μA – pA, as a
result at the end of the analysis original
concentration of the solution remains
unchanged
NON POLARIZED ELECTRODES
i
B
A
E
Over the current range A to B, the electrode is non polarized;
what ever the current passing through it, potential remains
unchanged
VOLTAMMETRY NEEDS A NON
POLARIZED ELECTRODE
 Reference electrodes have this property
over a limited current range
 Therefore reference electrode use in
voltammetry
Different Methods of Variations of potential of WE
Normal voltammetry
Square wave voltammetry
Differential Pulse
Polarography
Cycle voltammetry
POLAROGRAPHY
WE : Dropping Mercury Electrode (DME)
Capillary id = 0.05 mm
mercury = (20 – 100) cm
CLASSICAL POLAROGRAPHY
E
Time
Potential ramp
Polarogram
A POLAROGRAM
Polarograms for (a) 5 x 10-4 M Cd2+ in 1 M HCl and (b) 1M HCl alone
Ilkovic Equation
id(ave) = 708 n
1/2
2/3
1/6
D m t C
Diffusion Only
 No Migration
 No Convection
EFFECT OF DISSOLVED OXYGEN
 Prior to apply potential oxygen dissolved in the
test solution must be removed by passing pure
N2 gas through the solution. (N2 purging few
minutes)
 Oxygen if not removed undergo reduction /
oxidation at the two potentials -0.1 V and -0.9 V
vs SCE
INTERFERENCE OF DISSOLVED O2
O2 + 2H+ + 2e
H2O2 + 2H+ + 2e
H2O2
E1/2  - 0.1 V (versus S.C.E.)
H2O E1/2  - 0.9 V (versus S.C.E.)
LIMITING CURRENT
Its saw toothed shape
SAWTOOTHED SHAPE & GROWTH
OF Hg DROP
Current Maximum
Polarograms of 3 mM Pb2+ and 0.25 mM Zn2+ in 2 M
NaOH in the absence of a suppressor and in the presence
of 0.002 wt% Triton X-100
ANALYTICAL UTILITY
Classical Polarography
a)
E1/2 – Identify the Analyte
– In a given matrix an analyte has a characteristic
unique value for E1/2
Note : When matrix change the E1/2 for a given
analyte varies
b)
Id  C
Analytical uses: (cont‟d)
 Measure id for several standards
Concentration of
standards / mg dm-3
C1
Id / A
C2
Id2
C3
Id3
C4
Id4
C5
Id5
Id1
EXTERNAL CALIBRATION CURVE
id / A
idu
.
.
.
.
.
.
C / (g dm-3)
STANDARD ADDITION
Al3+ in 0.2 M sodium acetate, pH 4.7, with 0.6 mM
pontachrome violet SW used as a maximum suppressor.
CLASSICAL SHAPE OF A POLAROGRAM
OF A MIXTURE WITH 3 CATIONS
MODIFIED POLOROGRAPHIC
TECHNIQUES
1. Tast Polarography
2. Normal Pulse Polarography
3. Differential Pulse Polarography
4.Squre Wave Plorography
5.Stripping Analysis
With
HMDE
6.Linear Sweep Voltammetry
7.Cyclic Voltammetry
TAST POLAROGRAPHY
Potential ramp
Polarogramm
V
t
 voltage variations is same as classical polarography
 Current measurement only over the last few ms of the
drop life. (Just before it detached)
 ADVANTAGE: Precision and accuracy improved.
PULSE POLAROGRAPHY
 Normal pulse polarography
 Differential pulse polarography
SPECIAL FEACTURES
 Working electrode potential is not continuously scanned.
 Instead potential is applied in the form of voltage pulses.
 NPP uses voltage pulses with progressively increasing
heights .
POTENTIAL RAMP FOR NORMAL PULSE
POLAROGRAPHY
POLAROGRAM (NPP)
DIFFERENTIAL PULSE POLAROGRAPHY
(DPP)
POLAROGRAM (DPP)
Comparison of direct current (D.C.) and differential pulsed polarography (DPP)
of 1.2 x 10-4 M chlordiazepoxide (the drug Librium) in 3 ml of 0.05 M H2SO4.
LOD (Approx.)
NPP
10-6 mol dm-3
DPP
< 10-6 mol dm-3
LOD depends on the type of analyte too.
SQUARE WAVE POLAROGRAPHY
Waveform for square wave polarography. Typical parameters are pulse potential (E p) = 25
mV, step height (Es) = 10 mV, and pulse period () = 5 ms.
SQUARE WAVE VOLTAMMOGRAM
Square-wave voltammogram for the electro-reduction of a ferric complex (5 x 10-4 M)
in aqueous Oxalate buffer;  = 33.3 ms, Esw = 30 mV and E = 5 mV.
STRIPPING ANALYSIS
CONSISTS OF 3 STEPS
1. Preconcentration step
2. Equilibrium step
3. Stripping step
 Highly sensitive
Stripping voltammogram obtained for the determination of Cu(II) in aqueous solution
LINER SWEEP VOLTAMMETRY & CYCLIC
VOLTAMMETRY AT SOLID ELECTRODES
SOLID ELECTRODES
 Gold
 Platinum
 Silver
 Carbon
 Glassy Carbon (GC)
 Pyrolytic Graphite (PG)
 Carbon Paste Electrode (CPC)
Hanging Mercury Drop Electrode (HMDE) is also
used in Voltammetry
E
4 > 3 > 2 > 4
ip
4
4
3
3
2
2
1
1
Time
Ep
 = Scan rate
Normally 5 mV / s – 100 mV / s
E vs SCE
Linear sweep voltammetry of butylated hydroxyanisole in 0.12
M H{SO in ethanol / benzene
RANDLES-SEVCIK EQUATION
nF
( RT) D
Ip = 0.4463 n F A
1/2 1/2 C
Where,
Ip = peak current (A)
n = # of electrons per molecule / ion
F = Faraday constant
A = area of the electrode (cm2)
T = absolute temperature (K)
D = Diffusion coefficient (cm2 /s)
 = scan rate (mV / s)
C = concentration (mmol / dm3)
CALIBRATION CURVE OF Ip AGAINST
CONCENTRATION
Ip
Concentration of analyte
Non faradic compartment
CYCLIC VOLTAMMETRY
E
time
Potential applied at „WE‟
CYCLIC VOLTAMMOGRAM
APPLICATION OF CV
More diagnostic studies than analytical
applications
eg: Determination of electrochemical reversibility
i.e. Reduction and Oxidation occur reversibly
Electron transfer process is very fast
DIAGNOSTIC TESTS WITH CV FOR
ELECTROCHEMICAL REVERSIBILITY
1.
Ipc = Ipa
2.
The peak peak potentials, Epc and Epa, are
independent of the scan rate 
E0‟ is positioned midway between Epc and
Epa, so Eo‟ = (Epa + Epc) / 2
E0‟ is proportional to 1/2
3.
4.
5.
The separation between Epc and Epa is
59 mV/n for an n-electron couple
C60 (Buckminsterfullerene)
(b) Cyclic voltammogram and (c) differential
pulse polarogram of 0.8 M C60 in acetonitrile /
toluene solution at -10 oC with (nC4H9)4N+PF6- supporting electrolyte
ELECTROCHEMICAL DETECTION LIMITS
FOR SEVERAL POLAROGRAPHIC
METHODS
TECHNIQUE
Classical polarography
LOWER DETECTION
LIMITS (mol dm-3)
5 x 10-3
Sampled DC polarography
1 x 10-5
Normal pulse polarography
10-7 - 10-8
Differential pulse polarography
10-8 – 5 x 10-8
Square-wave polarography
1 x 10-8
Anodic stripping voltammetry
10-10 - 10-11
HYDRODYNAMIC
VOLTAMMETRY
VOLTAMMETRY UNDER CONVECTION
CONTROL
 Rate of convection is made faster
 Diffusion also occurs
 No migration
Convection >> Diffusion >> Migration
SCHEMATIC REPRESENTATION OF
ROTATING DISC ELECTRODE
SCHEMATIC REPRESENTATION OF
FLUID FLOW AT RDE
LEVICH EQUVATION
FOR RDE
Ilim = 0.620 n F A D2/3 -1/6 1/2 C
Where,
Ilim = limiting current (A)
n = # of electrons per molecule / ion
F = Faraday constant
A = area of the electrode (cm2)
D = Diffusion Constant (cm2s-1)
 = kinematic viscosity of the solution (cm3s-1)
 = Angular frequency of RDE
C = concentration (mmol / dm3)
Voltammograms at a gold RDE, of current density i as a function of potential E (vs. SCE)
and rotation speed f, obtained for a solution of ferrocyanide and ferricyanide ( both at 10
mmol dm-3) in 0.5 mol dm-3 KCl): (a) 20; (b) 15; (c) 10; (d) 5 Hz.
Schematic representation of a rotated ring-disc
electrode, defining the radii r1 (the radius of disc),
and r2 and r3 (the inner and outer radii of the ring,
respectively)
A FLOW CELL
Schematic representation of a typical flow cell used for electroanalytical measurements.
Note the way counter electrode (CE) is positioned downstream, i.e. the product from the
CE flow away from the working electrode
A CHANNEL ELECTRODE
Schematic representation of a typical channel electrode system used for electroanalytical
measurements. The counter electrode is positioned downstream in order to stop the
products from the CE flowing over the working electrode (WE). The reference electrode
is positioned over the WE.
RELATIONSHIP BETWEEN THE LIMITING CURRENT
AND VARIOUS CONVECTIVE PARAMETERS FOR A
NUMBER OF ELECTRODE TYPE
System
Equation
Rotated disc
Ilim = 0.620 nFAD2/3-1/61/2C
electrode
Flow cell with a Ilim = 5.43 nFD2/3x2/3V1/3C
tubular electrode
Flat channel
electrode
Ilim = 1.165 nFD2/3
(
Vf
)wX
2/3C
H2/d
Wall-jet
electrode
Ilim = 1.59knFAD2/3-5/12a1/2r3/4Vf3/4C
REFERENCES
1) Quantitative chemical analysis, Daniel C. Harris,
W.H. Freeman & Co.
2) Fundamentals of electroanalytical chemistry, Paul
M.S. Monk (Wiley)
3) Analytical Electrochemistry, Joseph Wang.
4) Electrochemical Methods, Fundamentals &
Applications, Allen J. Bard & Larry R. Falkner (Wiley)
5) Laboratory Techniques in Electroanalytical
Chemistry edited by Peter T. Kissinger & William R.
Heinemen
6) Morden Techniques in Electroanalysis, Peter
Vangsek
7) Electroanalysis: Theory and Application in Aqueous
and Non-Aqueous Media and in Automated Chemical
Control (Technqs and Instrumentation in Analytical)
E.A.M.F. Dahmen
SUPPLIERS OF ELECTROCHEMICAL
INSTRUMENTATION
1)EG & G INSTRUMENTS
Princeton Applied Research, USA
2) Bioanalytical Systems , USA
3) Chi Instruments , Electro chemical work station
France
4) Metrohm Electro Chemical Instruments
Switzerland