polarography and voltammetry : basic principles applications
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Polarography & Voltammetry:ELECTRO-ANALYTICAL
TECHNIQUES
Qualitative
Quantitative
In one step analysis
Metals
Other inorganics
Organics
NATURE OF SAMPLE
Practically any
LOD
10-5 → 10-15 M
g → pg
REQUIREMENTS :
Analyte, be
AN OXIDIZABLE / REDUCIBLE SPECIES
(Directly or indirectly)
IN SOLUTION
Aqueous
Non Aqueous
BASIC APPARATUS / MATERIALS REQUIRED
1. Cell : A container to hold the analyte solution
2. Electrodes
3. Voltage Supply (Variable DC/AC) / (Potentiostat)
4. Voltmeter
5. Ammeter
( Classical technique: 2 : RE & WE )
( Modern techniques: 3: RE, WE & CE)
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
AV
Analyte solution
R.EW.E.
W.E. = Working Electrode
R.E. = Reference Electrode
V = Voltmeter
A = Ammeter
REPRESENTATION OF A MORDEN VOLTAMMETRIC
SETUP: WITH A THREE ELECTRODE CELL
WITH A POTENTIOSTAT
A
V
Potentiometer
voltmeter
Ammeter Electrolysis
cell
Counter
electrode
Reference electrode
(working
electrode)
MIGRATION
Movement of oppositely charged ions towards electrode due
to electrostatic attractions.
+++
WE
+
+
- -- ---
- -
Diffusion
+++
WE
+
+
- -- ---
- -
DIFFUSION
Movement of ions from region of higher concentration (bulk)
to region of lower concentration (near the electrode surface)
Convection
Transport of ions towards electrode due to agitation, vibration
and temperature gradients
+++
WE
+
+
- ++ ---
+ +++
+
+
-
-
-
-
THE TWO MAJOR DIVISIONS IN
VOLTAMMETRY
1. Voltammetry under diffusion controlled
mass (ion) transfer
eg: Polarography, LSV, CV, NPP, DPP, etc.
2. Voltammetry under convection controlled
mass (ion) transfer
a) Movement of electrode in a still solution which promote
„convection‟
b) Movement of solution past the stationary electrode
eg: electrochemical detection for LC where „flow cells‟,
„channel electrode‟ (wall jet electrode) are used.
THE TWO MAJOR DIVISIONS IN
VOLTAMMETRY (cont’d)
eg: RDE, RRDE
VOLTAMMETRY UNDER DIFFUSION
CONTROL MASS TRANSFER
NO MIGRATION
NO CONVECTION
How to achieve this condition?
ONLY DIFFUSION
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
Add an excess ( 100 fold or more) an inert
electrolyte to the analyte solution
Therefore no attraction of ions from the bulk to the electrode
HOW TO STOP CONVECTION?
No Vibration
No Agitation
No Shaking
No Temperature gradient
In the solution
CONCENTRATION POLARIZATION
Species
concentration at
the electrode
surface
Species
concentration in
the bulk
CONCENTRATION PROFILES
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
t6>t5>t4>t3>t2>t1
t1t2
t3 t4
t5t6
C
x
AT STEADY STATE:
Rate of
removed of
ions at the
electrode
Rate of
supply of
ions from the
bulk to the
electrode
Rate of ion removal
Rate ofsupply of ion
Concentration
Cell Current, I
Cell Current, I
Cell Current, I
POLARIZED ELECTRODES
Current, I, remains unchanged with changes in the
electrode potential, E.
AB
Over the potential range A to B the electrode is polarized
I
E
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
A
Bi
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
Normal voltammetryDifferential Pulse
Polarography
Square wave voltammetry Cycle voltammetry
Different Methods of Variations of potential of WE
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 H2O2E1/2 - 0.1 V (versus S.C.E.)
H2O2 + 2H+ + 2e H2O E1/2 - 0.9 V (versus S.C.E.)
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
Current Maximum
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
Id / A
C1 Id1
C2 Id2
C3 Id3
C4 Id4
C5 Id5
STANDARD ADDITION
Al3+ in 0.2 M sodium acetate, pH 4.7, with 0.6 mM
pontachrome violet SW used as a maximum suppressor.
MODIFIED POLOROGRAPHIC
TECHNIQUES
1. Tast Polarography
2. Normal Pulse Polarography
3. Differential Pulse Polarography
4.Squre Wave Plorography
5.Stripping Analysis
6.Linear Sweep Voltammetry
7.Cyclic Voltammetry
With
HMDE
TAST POLAROGRAPHY
PolarogrammPotential ramp
t
V
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 .
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.
SQUARE WAVE POLAROGRAPHY
Waveform for square wave polarography. Typical parameters are pulse potential (Ep) = 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
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
RANDLES-SEVCIK EQUATION
Ip = 0.4463 n F A( ) D1/2 1/2 CnF
RT
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)
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
3. E0‟ is positioned midway between Epc and
Epa, so Eo‟ = (Epa + Epc) / 2
4. E0‟ is proportional to 1/2
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 (n-
C4H9)4N+PF6
- supporting electrolyte
ELECTROCHEMICAL DETECTION LIMITS
FOR SEVERAL POLAROGRAPHIC
METHODS
TECHNIQUE LOWER DETECTION
LIMITS (mol dm-3)
Classical polarography 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
VOLTAMMETRY UNDER CONVECTION
CONTROL
Rate of convection is made faster
Diffusion also occurs
No migration
Convection >> Diffusion >> Migration
HYDRODYNAMIC
VOLTAMMETRY
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)
= Angular frequency of RDE
= kinematic viscosity of the solution (cm3s-1)
C = concentration (mmol / dm3)
LEVICH EQUVATION
FOR RDE
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)
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 FLOW CELL
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.
A CHANNEL ELECTRODE
RELATIONSHIP BETWEEN THE LIMITING CURRENT
AND VARIOUS CONVECTIVE PARAMETERS FOR A
NUMBER OF ELECTRODE TYPE
System Equation
Rotated disc
electrode
Ilim = 0.620 nFAD2/3-1/61/2C
Flow cell with a
tubular electrode
Ilim = 5.43 nFD2/3x2/3V1/3C
Flat channel
electrode Ilim = 1.165 nFD2/3( )wX2/3C
Wall-jet
electrode
Ilim = 1.59knFAD2/3-5/12a1/2r3/4Vf3/4C
Vf
H2/d
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
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