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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
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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