POLAROGRAPHY AND VOLTAMMETRY :

BASIC PRINCIPLES

APPLICATIONS

&

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

OF

ANALYTE SOLUTION

A MICRO SCALE

“ELECTROLYSIS”

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

WHAT DO WE MEASURE IN 2

ELECTRODE CELLS ?

BOTH

Potential of the cell

Current through the circuit

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)

WHAT DO WE MEASURE IN

3 - ELECTRODE CELLS ?

BOTH

Potential of “WE” vs “RE”

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

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

MASS TRANSPORT

ONLY BY

“DIFFUSION”

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

POLARIZATION OF ELECTRODES

Polarized Electrodes

Non Polarized Electrodes

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

POLAROGRAPHY

WE : Dropping Mercury Electrode (DME)

Capillary id = 0.05 mm

mercury = (20 – 100) cm

CLASSICAL POLAROGRAPHY

E

Time

Potential ramp Polarogram

Polarograms for (a) 5 x 10-4 M Cd2+ in 1 M HCl and (b) 1M HCl alone

A POLAROGRAM

Ilkovic Equation

id(ave) = 708 n D1/2 m2/3 t1/6 C

No Migration

No Convection

Diffusion Only

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.)

LIMITING CURRENT

Its saw toothed shape

SAWTOOTHED SHAPE & GROWTH

OF Hg DROP

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

EXTERNAL CALIBRATION CURVE

C / (g dm-3)

id / A

idu

..

.

. ..

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

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 .

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 (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

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

Time

1

2

3

4

4 > 3 > 2 > 4

1

2

3

4

E vs SCE

ip

Ep

= Scan rate

Normally 5 mV / s – 100 mV / s

Linear sweep voltammetry of butylated hydroxyanisole in 0.12

M H{SO in ethanol / benzene

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)

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

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

SCHEMATIC REPRESENTATION OF

ROTATING DISC ELECTRODE

SCHEMATIC REPRESENTATION OF

FLUID FLOW AT 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)

= 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

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