exp9 cyclic voltam

8/3/2019 Exp9 Cyclic Voltam

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EXPERIMENT NUMBER 9:

CYCLIC VOLTAMMETRY

APAGA

CLIMACOMONTES


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VOLTAMMETRY POTENTIOMETRY COULOMETRY

based on the

measurement of thecurrent that

develops in an

electrochemical cell

under conditions

where

concentration

polarization exists

Measurements

are made atcurrents that

approach zero and

where polarization

is absent

measures are taken

to minimize orcompensate for the

effects of

concentration

polarization

minimal

consumption of

analyte

all of the analyte is

converted to another

state

INTRODUCTION


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INTRODUCTION


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INTRODUCTION:

CYCLIC VOLTAMMETRY


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INTRODUCTION:

CYCLIC VOLTAMMETRY SET UP


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INTRODUCTION:

CYCLIC VOLTAMMETRY

TERMS:

Electrode - the interface at which dissolved substrates may

pick up or lose electron(s)Electrolyte - needed in order to provide electrical

conductivity between the two electrodes

*The presence of supporting electrolyte, such as KNO3, is

required to prevent charged electroactive species frommigrating in the electric field gradient.

Indicating electrode- known as the test or working

electrode; the electrode at which the electrochemical

phenomena (reduction or oxidation) being investigated are

taking place.


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INTRODUCTION:

CYCLIC VOLTAMMETRY

Reference electrode- the electrode whose potential is

constant enough that it can be taken as the reference

standard against which the potentials of the other electrodespresent in the cell can be measured. Ex. Calomel, silver-silver

chloride

Counter or auxiliary electrode -serves as a source or sink forelectrons so that current can be passed from the external

circuit through the cell. In general, neither its true potential

nor current is ever measured or known.


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INTRODUCTION:

CYCLIC VOLTAMMETRY


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INTRODUCTION:

CYCLIC VOLTAMMETRY

the current response of a small stationaryelectrode in an unstirred solution is excited by a

triangular voltage waveform


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INTRODUCTION:

CYCLIC VOLTAMMETRY

Switching potential determine

diffusion controlled process?


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CYCLIC VOLTAMMETRY:

Example

the current response when a

solution that is 6 mM in K3Fe(CN)6 and 1M

in KNO3 is subjected to the cyclic

excitation signal


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CYCLIC VOLTAMMETRY:

Example

the potential is

changed linearly with

time starting from apotential where no

electrode reaction

occurs and moving to

potentials where

reduction or oxidation

of a solute occurs


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CYCLIC VOLTAMMETRY


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OBJECTIVES

To determine the E values of the

[FeIII(CN)6]-3/[FeII(CN)6]-4 couple

To evaluate the effects of scan rate,

concentration of electroactivespecies, and supporting electrolyte


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THEORY


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Motion of the Particles in the Solution

Convection Migration

Diffusion


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Convection

Stirring, temperature

and/or density

gradient

Provides laminar flow

near the electrode

surface

d1


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Migration

Attraction of

electrode and ion

with opposite

charge

Repulsion between

two ions of same

charge

d1v3


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Diffusion

Spontaneous

movement of

electroactive

species to area of

lower concentration

d1v3


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During discharging

process the

electroactive

component nearelectrode is

depleted

d1v3


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Supporting electrolyte

Limits migration

salt, acid, base, buffer solution or a chelatingreagent

same charge with the electroactive species onsurface electrode

Decreases interaction between electrode and

electroactive species Increase in charge leads to higher effect in

decreasing interaction


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Supporting electrolyte

chemically not reactive

does not interfere with diffusion and with the

electrons exchange on the electrode surface

have a different discharge potential (at least

100 200 mV)

have an high ionic conductivity and guarantee

a low electrical resistance


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Double Layer

Consequence of too

high concentration of

supporting electrolyte

Cause capacitivecurrent

non specific

backgroundinterference of the

faradic current


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Diffusion Layer

Thin layer near theelectrode formedduring discharge

Motion only due todiffusion

Electrolyte reaching

electrode [electrolyte] insolution


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IR drop: E=IR

Overvoltage: =E-Eeq


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METHODS AND RATIONALE


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referenceelectrode

auxiliaryelectrode

workingelectrode


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Methods Rationale

A. Generating a CyclicVoltammogram

Fill cell with 1 M KNO3 Supporting electrolyte- added in excess-limit migration-ensure sufficient conductivity- sharpens the peak

Volume of electrolyte soln shouldbe enough for tip of electrode toimmerse

Ensure sufficient amt ofelectrolyte

Purge solution with N2 for 5 min. deoxygenate the sample

Can be reduced and interferew. cathodic wave

Set initial E= 600 mV and scanlimits at 600 mV and -600 mV

excitation signal

-600 mV: switching potential

Initiate scan in the negative

direction. Rate= 100 mV/s

Fe(CN)63 + e Fe(CN)6

4


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Methods Rationale

Switch on working electrode.Initiate potential scan.

Take background cyclicvoltammogram of KNO3

For calibration purposes

Turn off working electrode.Clean the cell.

Eliminate layering of analytes-alter electrode surface

Refill cell with 4 mM K3[Fe(CN)6] in1 M KNO3. Repeat procedure.(E range: 600 to -800 mV)

To obtain voltammogram of[FeIII(CN)6]

-3/[FeII(CN)6]-4


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Figure 1. Background cyclic voltammogram of

the supporting electrolyte

Figure 2. Cyclic voltammogram of the

[FeIII(CN)6]-3/[FeII(CN)6]-4 couple


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Important parameters

Epc= -0.47 V

ipc

Epa= -0.10 V

ipa

Fe(CN)63 + e Fe(CN)6

4

Fe(CN)64 Fe(CN)6

3+ e-


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Peak separation for a reversible process:

Ep > 0.0592/n, irreversible

1 electron transferred:

Theoretical: Ep = 0.0592 V

Exptl: Ep = 0.37 V

[% error: 525 %] Formal reduction potential:

Ep = |Epa - Epc| = 2.303 RT/nF = 0.0592/n V

Eo = Epa + Epc

2


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Quantitative information obtained from the

peak heights according to the RandlesSevcik

equation:

wwhere:

ip- peak current amperesn- mols of e-

A- electrode area (cm2)

D- diffusion coefficient (cm2/s)

C- conc. (mol/cm3)

V- sweep/scan rate (V/s)

ip = (2.69 x 105) n3/2 A D1/2 C v1/2

B. Effect of Scan Rate Variation Use 4 mM K3[Fe(CN)6] in 1 M


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B. Effect of Scan Rate Variation Use 4 mM K3[Fe(CN)6] in 1 M

KNO3. Scan rates 50, 80, 100, and

200 mV

scan rate = 50 mV/s

scan rate = 80 mV/s

scan rate = 100 mV/s scan rate = 200 mV/s


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Scan rate (V/s) Epa (V) Epc (V) Ep (V) E0 (V)

0.05 -0.1140 -0.4200 0.306 -0.2670

0.08 -0.0980 -0.4380 0.34 -0.2680

0.100 -0.0900 -0.4520 0.37 -0.2710

0.200 -0.0640 -0.4780 0.414 -0.2710

Irreversibilityis when the rate of electron transfer is sufficiently slow so

that the potential no longer reflects the equilibrium activity of the redox

couple at the electrode surface.

-0.2715

-0.271

-0.2705

-0.27

-0.2695

-0.269

-0.2685

-0.268-0.2675

-0.267

-0.2665

0 0.05 0.1 0.15 0.2 0.25

E0(

V)

Scan rate (V/s)

C. Effect of Electroactive Species Use 2, 6, 8, 10 mM K3[Fe(CN)6] in


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C. Effect of Electroactive Species

Concentration Variation

Use 2, 6, 8, 10 mM K3[Fe(CN)6] in

1 M KNO3. Scan rate = 100 mV.

Record also unknown solution.

Conc. = 2 mM K3[Fe(CN)6] Conc. = 6 mM K3[Fe(CN)6]

Conc. = 8 mM K3[Fe(CN)6]Conc. = 10 mM

K3[Fe(CN)6]

C. Effect of Electroactive Species Use 2, 6, 8, 10 mM K3[Fe(CN)6] in


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C. Effect of Electroactive Species

Concentration Variation

Use 2, 6, 8, 10 mM K3[Fe(CN)6] in

1 M KNO3. Scan rate = 100 mV.

Record also unknown solution.

Unknown conc.


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Concentration of

[FeIII(CN)6]-3 (mM)

Ipc(A)

2 4

4 7

6 11

8 12.2

10 15

unknown 11.5

y = 1.36x + 1.68

R = 0.97718

0

2

4

6

8

10

12

14

16

0 1 2 3 4 5 6 7 8 9 10

Ipc

(

A)

Concentration of [FeIII(CN)6]-3 (mM)

Ipc = 1.36conc + 1.68

Conc. = (11.5 - 1.68) / 1.36

Conc.(unknown) = 7.22 mM


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Unknown conc.

Conc. = 6 mM K3[Fe(CN)6] Conc. = 8 mM K3[Fe(CN)6]

D. Effect of supporting electrolyte Compare 4 mM K3[Fe(CN)6] in 1 M


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pp g y

variation

p 3[ ( )6]

KNO3 and in 1 M Na2SO4

1 M Na2SO4

1 M KNO3

KNO3 K+ + NO3

-Na2SO4 Na

+ + SO42-


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Remember

Do not stir any of the solutions when running acyclic voltammogram. But, always stir thesolution for at least 10 seconds before scanning.

Do not fill the cell any higher than 1 cm below thecover.

Make sure that the working electrode and thereference electrode are approximately at the

same height. Be sure that the surface of the working electrode

is clean.


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Migration attraction of electrode and ion

Convection laminar flowDiffusion concentration based!

SOURCES OF ERROR


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Thank You!

exp9 cyclic voltam

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