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Chapter 25. Voltammetry

Excitation Signal in VoltammetryVoltammetric InstrumentationHydrodynamic VoltammetryCyclic VoltammetryPulse VoltammetryHigh-Frequency and High-Speed

VoltammetryApplication of VoltammetryStripping MethodsVoltammetry with Microelectrodes

Voltammetry

Voltammetry: measurement of current (I) as a function of applied potential (E). Under condition with polarization (η). Negligible consumption of analyte

– Amperometry: measure I at a fixed E– Potentiometry: measure E when I 0, no polarization – Coulometry: measure C, polarization is compensated, all

analyte is consumedPolarography: voltammetry at the dropping mercury electrode (DME)

– DA: Hg (poison), apparatus (cumbersome), better techniquesApplication:

– Oxidation and reduction process– Adsorption processes on surfaces– Electron transfer mechanism

Jaroslav Heyrovsky1890-1967

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Excitation Signals and Instrumentation

WE: E (relative to RE); RE: constant E; CE: Pt wire (current)Supporting electrolyte: a salt added in excess to the analyte solution, like alkali metal salt

– No reaction at the E region

– Reduce effect of migration

– Lower R of the solution

An op amp potentiostat

E follower, high Z, no I

Ii

Io

R Eo = -IiREo = EiEo

Ei

Measure I, I-to-E converter

Voltammetric Working Electrode

Disk electrode: A small flat disk in a rod of an inert materials like Teflon, glass or Kel-F.HMDE: hanging mercury drop electrode

– Large negative E, fresh metallic surface, reversible reaction

UME: microelectrode, r: < 25 µm, wire in glass, tip polishedFlow cell WE: in flowing stream, PEEK (polyethertherketon)Emin: reduction of water (H2), Emax: oxidation of water (O2)

Disk electrode

HMDE

UME Flow electrode

WE

3

Modified Electrode

Chemical modification:– Irreversibly adsorbing substances:

oxidation of electrode (metal or C) surface (O- or –OH) electrodeposition

– Covalent bonding of components : like SAM of thiols with amine or carboxyl group on the other endOrganosilanes or amines

– Coating of polymer filmsDip coating, spin coating

Application:– Electrocatalysis– Smart window: electrode changes color

upon reaction– Analytical sensor

Circuit Model of a Working Electrode

A. Randles circuit:– RΩ, solution resistance– Cd, double layer capacity– Zf, faradaic impedance f dependence

B. Faradaic impedance:– Rs, electron transfer resistance– Cs, pseudocapacitance, mass transfer

C. Faradaic impedance:– Rct, charge transfer resistance– Zw, Warburg impedance

WE

Bulk electrolyteDiffusion

layer

Double layer

Zf

RΩ

CdA

RΩ

Cd

B

Rs Cs

RΩ

Cd

C

Rct

Zw

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Concentration Profile in Unstirred Solution

Concentration distance profile during diffusion controlled reaction

refA

PAappl E

c

cn

EE −−= 0

00 log0592.0

A

Px

cnFADi A∂∂

=

n: #electronF: Faraday constantA: surface area, cm2

D: diffusion coefficient, cm2/s

A planar electrode with potential stepReaction: A + e- P reversible and rapid Mass transfer: 1. Migration: electric field; Supporting electrolyte (100×) 2.Diffusion: concentration gradient 3.Convection: mechanicalPotential vs. surface concentration:

Current:

Hydrodynamic Voltammetry

Flow pattern in a flow stream

Flow patter near an electrode

00AAP ccc −=

refA

PAappl E

c

cn

EE −−= 0

00 log0592.0

10 ~ 100 µm

the analyte solution is kept in continuous motion

– stir the solution,

– flow solution, like in HPLC

convection

A + e- P reversible and rapid

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Voltammograms

Voltammetric wave: an ∫-shaped wave of I-ELimiting current, il: the current plateau observed at the top, ∝ cA

– cA = 0 at electrode surface– maximum mass transfer rate

Current in American way: – Reduction current +– Oxidation current -

Half-wave potential: – E1/2 at i = il/2, ≠ E0

– Relative to E0

– Identification

Linear-sweep voltammogramat slow scan rate

Al kci =

E0 = -0.26 V

vs. SCE

Volumetric Currents

A planar electrode: Nernst diffusion layer δ control

Limiting current: cA0 at the electrode surface = 0.

Reverse current: cP in the bulk solution = 0.

Half-wave potential, E1/2: i = il/2

)()( 0AA

AAA ccnFAD

xcnFADi −=∂∂

=δ

AAAA

l ckcnFADi ==δ

cm kness,layer thicdiffuion Nernst :/:c

/scm t,coefficiendiffusion :

cm area, surface electrode :A

electron 96485C/mol :Fanalyte electron / :n

3A

2

2

δcmmol

DA

PneA →+

000 )( PPPP

PPP ckcnFADccnFADi ==−=

δδ

refArefP

AA

reflP

AAappl

EEEkk

nEE

Eii

ink

kn

EE

−≈−−=

−−

−−=

002/1

0

log0592.0

log0592.0log0592.0

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Voltammetric I-E

Based on the kinetics of the reaction:– Reversible systems: obey Nernst

equation– Totally irreversible system: either the

cathodic or anodic reaction is too slow as to be negligible

– Partially reversible system: the reaction in one direction is much slower than the other one.

– like organic system, i = kc, E = f(v, c, il)Voltammogram for mixture:

– ∆E ≥ 0.1 VAnodic/Cathodic Voltammogram:

– A: oxidation current –– B: both reaction– C: reduction current +

∆E = 0.1 V

∆E = 0.2 V

Oxygen Wave and SensorsClark electrode

Oxygen wave:– I is proportional to n– Sparging: deaerate the solution with inert

gas, N2, Ne and He– Highly depends on the pH of the solution

Clark electrode: volumetric sensor– Cathodic Pt electrode: O2 + 4H+ + 4e ↔

2H2O– Anodic Ag electrode: Ag + Cl- ↔ AgCl (s) + e– Diffusion across membrane ( ~ 10 µm)– Diffusion cross the thin electrolyte solution (

~ 10 µm) – Steady-state current I is dependent on

electrochemical equilibrium, [O2] 10 ~ 20 s and dm+s < 20 µm

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Enzyme-based Sensors

• Glucose detection: largest selling chemical instruments• A polycarbonates film (glucose permeable, not for protein and

other blood constitutes): diffuse through• An immobilized enzyme layer (glucose oxidase): glucose

reduction H2O2• A cellulose membrane layer for H2O2 diffusion: H2O2 oxidation

O2– Amperometric detection (I ∝ c) or volumetric detection (E ∝ c) of

sucrose, lactose, ethanol and L-Lactate

−− ++→+

+⎯⎯⎯⎯⎯⎯ →⎯+

eOHOOHOH

OOeglu

22

Hacid gluconiccos

2222

22oxidase glucose

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Amperometric Titration

At least one species is electrochemical activeA WE (rotating Pt) + RE: confined to product either a precipitate or a stable complex.

– Ag+ for X-, Pb2+ for SO42-

– Exception: Br2 (BrO3-)

titration of organics Two WEs:

– simple instrument, determination of a single specie

– Karl fisher titration for determining water

OHBrHBrBrO 223 3365 +→++ +−−

Analyte is reduced

produced is reduced Both analyte and products are reduced

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Rotating Electrodes

Rotating electrode: – RDE: rotating disk electrode, affiliate

mass transfer– RRDE: rotating ring disk electrode,

intermediate detection– Levich equation:

RDE RRDE

cvnFADil6/12/1620.0 −= ω

3A

2

2

/:c

/scm , viscositykinematic :v

radians/s locity,angular ve: /scm t,coefficiendiffusion :

analyte electron / :n

cmmol

Dω

O2 reduction

Polarography

WE: DME, diffusion control, no convectionResidue current: current observed in the absence of an electroactive specieDiffusion current: limiting current which is limited by the diffusionA: DL ~ 10-5 M, Faster equilibrium + new electrode surface reproducible current; High η for H2 evolution low E windowDA: new surface large charging current

Polarogram

0.5 mM Cd2+ in 1 M HCl

1 M HCl

ctmnDid6/13/22/1

max 708)( =

s ,t:/:c

mg/s capillary, the througHg of flow of rate:m /scm t,coefficiendiffusion :

analyte electron / :n

3A

2

imetcmmol

D

The ripples are caused by the constant forming and dropping of the mercury electrode

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Cyclic Voltammetry

CV: forward scan, switching potential, reverse scanApplication of CV:

– Study of redox reaction– Detection of reaction

intermediates– Observation of follow-up

reactionsReaction:

– A: H2O oxidation O2– B-H: reduction– B-D: cA

0 0– D-F: cA

0 = 0, δ ↑– F-H: reduction– H-K: oxidation

6.0 mM Fe(CN)63-

(-)(+)

(reduction)

(oxidation)

E (vs SCE)(-)(+)

(reduction)Irreversible or rapid removal of Red

Reversible

CV- Fundamental Studies

Peak potential: Epc and Epa– Reversible: ∆Ep = 0.0592 /n– Irreversible: ∆Ep > 0.0592 /n

Peak current:

Qualitative information in organic and inorganic chemistry

– first choice– reaction intermediate

nEEE pcpap

0592.0=−=∆

V/s rate,scan :/:c

cm area, surface electrode:A

/scm t,coefficiendiffusion :

analyte electron / :n

3

2

2

vcmmol

D

NHOHeHNOC

eHNONHOHB

OHNHOHHeNOA

φφ

φφ

φφ

→++

++→

+→++

−+

−+

+−

22:

22:

44: 22

Parathion in 0.5 M acetate buffer in 50% ethanol, pH = 5

cvADnip2/12/12/3510686.2 ×=

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CV of Modified electrode

Reversible surface redox couple no mass transfer effect symmetrical peaks + same peak height 0≈−= pcpap EEE∆

Digital Simulation of CV

Digital simulation: DigiSim, DigiElk– Fast implicit finite difference methods– 1st or 2nd order homogeneous chemical reaction– Generate dynamic concentration profiles– The exact current may be offset as the nonfaradaic current is

not easily simulated

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Differential Pulse Polarography

DPP: increasing sensitivity– Lower DL: ~ 10-7 to 10-8 M (2

~ 3 order lower than CV)– Enhancing faradic current:

diffusion current (id) + Nernst contribution due to ∆E, several times larger than id, ∆t is small enough

– Decrease in nonfaradic current: charging current decays exponentially with time, is small at the late lifetime of the drop, ∆t is large enough

– Trace heavy metal detection

0.36 ppm tatrecylineHCl in 0.1 M acetate buffer, pH=4

∆t

Square-wave Polarography

SWP: increasing sensitivity – Great speed: step < 10 ms, signal average is

possible– Lower DL: ~ 10-7 to 10-8 M– Enhancing faradic current + Decrease in

nonfaradic current – ∆I = If – Ir, the current difference is plotted

50 mV = 2ESW

SWP generation

10 mV

forward

reverse

difference

Guanine, adenine, thymine

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Stripping Methods

Stripping methods: – Anodic stripping methods: C A

for metal– Cathodic stripping methods: A C

for halidesElectrodeposition step:

– Stirring the solution: mass transfer– Only a fraction of analyte is

deposited: accumulation process– Depends on c, stir rate, deposition

time, electrode surface and potential

– t < 1 min. for c ~ 10-7 M– t > 30 min. for c ~ 10-9 M, (higher

sensitivity)– HMDE or noble metal (Pt, Au, Ag

and C)

Cd

Anodic stripping methods

Microelectrodes

Microelectrode: r ~ 1 to 20 µm– r >> δ, normal electrode, short time– δ >> r, UME, long time, steady state

Advantage: – Small current (I ~ pA to nA) small IR

drop no RE– Capacitor charging current (Inf ∝ A)

Inf ↓ faster scan– Faradaic current (If ∝ A/r) bigger

contribution from If lower DL– Rate of mass transport increases

steady state is established within µs faster kinetic study, higher S/N ration

– Little disturbance to the system under study

– Small sample volume– Small current system with low

dielectric constants, like toluene

50 µm

Dtr

nFADci A πδδ

=⎟⎠

⎞⎜⎝

⎛+= ,110

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Homework

25-2 (a, b, c, e), 25-5