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Voltammetry Stirred - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Indicator Electrodes, Potentiometry, Electrochemistry, Electrode Kinetics, Electrogravimetry, Polarography, Square Wave Voltammetry, Wavelength Selection, Types of Spectroscopy, Detectors, Theory of Molecular Absorption, Absorption by Organic Molecules, Applying UV-vis Spectroscopy are major topics of this course. Main points from these slides are: Voltammetry, Oxidation and Reduction Reactions, Electrochemical Cell, Electrolytic Cell, Electrode Kinetics, Mass Transport Or Mass Transfer, Diffusi

Typology: Slides

2012/2013

Uploaded on 08/30/2013

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Voltammetry (stirred)
Stationary electrode
Stirred = mass transport by convection
Vary potential linearly with time
Measure current vs time
Theory
assume Ox + n e- Red
- both Ox and Red are soluble
- reversible reaction (electrochemically)
- potential varies
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Voltammetry (stirred)

assumeTheory• Measure current vs time• Vary potential linearly with time• Stirred = mass transport by convection• Stationary electrode

Ox + n e

-^

Red

- potential varies- reversible reaction (electrochemically)- both Ox and Red are soluble

such that all Ox is reduced at electrodei.e., applied potential is sufficiently cathodiccurrent when [Ox] = 0 at electrode surfaceDefine - Limiting Current as steady state

E

I

transport-limited current limiting current

nFADC bulk

I = -----------------

δ

Nernst controlled current RT (^) [Red]

E = Eo nF - ----- ln --------^ [Ox]

informationGives quantitative

Linear Scan Voltammetry (stirred)

I

E

½ E= Half wave potentialL^ I= Limiting currentC I= Capacitive current

LI
CI
½E

Ox + e

 Red-^

Linear Scan Voltammetry (stirred)

I

E

Half wave potential (E

½ (^) )

is E when I = I L (^) /

LI
CI
½E

Ox + e

 Red-^

Linear Scan Voltammetry (stirred)

I

E

When I = I L/2, then ½E = E[Red] = [Ox] and

LI
CI
½E
RT
L (I – I)
½ E = E

nF - ------ ln ---------- I

Ox + e

 Red-^

Linear Scan Voltammetry (stirred)

I

E

reversible irreversible

irreversiblemost irreversiblemore

Can assign rate constants (k) for irreversible processes

Linear Scan Voltammetry (stirred)

transport, i.e., I• Fairly sensitive due to convective mass• Can measure many metal ions & organics• LSV can be used for quantitative analysis• Better to use rotating electrode than stir bar• Normally use Pt or C (graphite) electrodes

F is large

  • multiple components gives stacked waves– can be difficult to perform data analysisconsidered a drawback• The output signal in the form of a wave is

Linear Scan Voltammetry (stirred)

I

E

of the first specieslarge concentrationin the presence of athe second speciesit is difficult to measureFor a two component system

LI
LI

first speciesset by thethe range issmall sinceL IbecomesThe second

This problem is inherent for techniques that produce waves

Linear Scan Voltammetry (unstirred)

I

E

P E= Peak potentialP I= Peak current

PI
PE

of the curvecontrolled regionLargely Nernstian

from Cottrell Eq.controlled regionPrimarily diffusion

irreversible

reversible

nF3/

P I= 0.452 ------------ A D

bulkC½^

R T½

New term

ν = scan rate

Increase scan rate & I

P increases, however,

C Iis directly proportional to

Ratio I

F C /Iis greatest at slow scan rates

R T

P E= E

n F

Voltammetry (unstirred) - Theory

E Applied Signal for Linear Scan Voltammetry

time

Slope = scan rate in mV/sec

Convention for plotting current & potential

to oxidationCurrent due to reductionCurrent due

potentialsreducing

potentialsoxidizing

cathodicE

anodicI^ cathodicI^

anodicE

Ox + e-  (^) Red^

Red - e (^)  (^) Ox-

VoltammetryCyclic

Convention for plotting current & potential

cathodicE

anodicI^ cathodicI^

anodicE Ox + e  (^) Red-^ Red - e (^)  (^) Ox-

A
B
C
E D
F
H
G
I
J

scanning in one directionin an unstirred solutionis the same as for LSVGoing from A to B to C to D