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Chapter 23
Voltammetry
Voltammetry and Polarograph
• Electrochemistry techniques based on current (i) measurement as function of voltage (Eappl)
• Voltammetry—Usually when the working electrode is solid, e.g., Pt, Au, GC.
• Polarograph—A special term used for the voltammetry carried out with a (liquid) MURCURY electrode.
• Voltammogram—The plot of the electrode current as a function of potential.
Typical polarographic curves (dependence of current I on the voltage Eapplied to the electrodes; lower curve - the supporting solution ofammonium chloride and hydroxide containing small amounts ofcadmium, zinc and manganese, upper curve - the same after addition ofsmall amount of thallium.
Tl+
“Polarographic curves”--Voltammograms
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Mercury Droplet Electrode
Electrochemical Cell
• Working electrode:place where redox occurs, surface area few mm2 to limit current flow.
• Reference electrode:constant potential reference
• Counter (Auxiliary) electrode: inert material, plays no part in redox but completes circuit
•Supporting electrolyte:alkali metal salt does not react with electrodes but can reduce the effect of migration and lower the resistance of the solution.
2-Electrode vs. 3-Electrode Cell• 2-electrode cell is OK in potentiometry-- very small i• Now in voltammetry, measuring (big) i vs. applied E, but
(1) Potential drops when current is taken from electrode due to solution resistance (iR drop): The actual EWE is smaller than EAppl (vs ERef. E)(2) Large i passes the ref. electrode instability of the reference potential (not constant)
i
EAppl
WE REF. E
R
Appl WE Ref.E
Appl Ref.E WE
Appl Ref.E WE
WE Appl Ref.E
or (vs )
(vs )
E E iR E
E E E iR
E E E iR
E E E iR
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Advantages of 3- over 2-electrode Cell System
Remember: In 3-electrode cell system, electrochemical cell current passes between WE and Counter electrode
3-electrode system
1. Provides great flexibility in location of the reference and the working electrodes and minimizes the effect of solution iR drop.
2. Virtually has no current passes through the reference electrode.
Potentiostat
• Voltage source that drives the cell
• Supplies whatever voltage needed between working and counter electrodes to maintain specific voltage between working and reference electrode
• Very high impedance (so that currentpasses though the reference electrode is minimized)
• Almost all current carried between working and counter electrodes
• Voltage measured between working and reference electrodes
• Analyte dissolved in cell not at electrode surface!
~NOTE~
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Potential Excitations vs Voltammogram/Polarograms
Staircase Voltammetry
Square Wave Voltammetry
Electrodes• Mercury electrodes (liquid)—dropping mercury
electrode, hanging mercury drop electrode, static mercury drop electrode.
• Solid electrodes: mm in diameters, Pt, Au, GC. • Micro(Ultramicro) electrodes: m in diameter: Pt,
Au, Carbon fiber.• Solid/liquid electrode: Mercury film electrodes,
carbon paste electrode.• Chemically modified electrodes• ITO electrode (Transparent glass coated with In-
SnO2
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Dropping Mercury Electrode
(DME)
Hanging MercuryDrop Electrode
(HMDE)
Static Mercury DropElectrode
Electrode material vs. Potential Window
Potential window varies with material/solution due to overpotentials
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Advantages of Mercury Electrodes
• A more reproducible surface—No polish needed “easy” to prepare
• More negative potentials can be attained in aqueous systems—due to overpotentials
• Amalgamation with heavy metals (e.g., lead and cadmium) –preconcentration of metal ions very high sensitivity in stripping voltammetry
n+ n+[Red on Hg] [OX]
M + ne M(Hg) M (+Hg) (Preconcentration Stripping)E E
Disadvantages of Mercury Electrode• Hg easily oxidized, limited use as anode (E <
+0.4 V)
2Hg + 2Cl- Hg2Cl2 + 2e
• non-faradaic residual currents (Impurities and charging current limit detection to >10-5 M
• cumbersome to use (toxic mercury)
• sometimes produce current maxima for unclear reasons maxima suppressor is needed to add
In the rest of the chapter, all discussions are based on solid
electrodes.
X = 0
(Ox + ne Red) (reversible)
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A is reduced to A- at potential E2
(A)
(A-)
i: CurrentF: Faraday constantA: area of electrodeD: Diffusion Coefficient: diffusion layer thickness
Flux
= (Dt)1/2
0 0
* * *
* 1/ 2 *
1/ 2 1/ 2
Cottrell Equation
( ) ( ) , δδ
0 0 ( ) ( )
δ
o oo x o x
o o o oo o
o o o o
c ciD D Dt
nFA x
c c D cD
D c nFAD ci nFA
tD
DDt
t
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Diffusion Layer Thickness
0 20 40 60 80 100 1200
100
200
300
400
500
600
Diff
usio
n la
yer
m)
Time, t (s)
δ DtD = 1x10-5 cm2/s
Sampled Current Voltammetry(Normal Pulse Voltammetry)
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Voltammogram for a Reversible (Nernstian) Reaction
Limiting Current(Quantitative)
Half-wave Potential(Qualitative)
1/ 2'
1/ 2 1/ 2 1/ 2
( )ln ln
( )ol R
o
i i DRT RTE E E E
nF i nF D
il
il/2
(E-Eo’)/V
or id
Hydrodynamic Voltammetry
• Voltammetry in which analyte solution is kept in continuous motion.
• Two ways: Stirring the solution, and rotating the electrode.
Electrode Rotator
Rotating-disk electrode
Flow patterns and regions of interest near the working electrode in hydrodynamic voltammetry
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Linear Scan (Sweep) VoltammetryPeak-shaped i~E profile
Cyclic Voltammetry
Cyclic Voltammograms for a Reversible Reaction
pc pa
opa pc
0'pa pc
3/ 2 1/ 2 * 1/ 2p
o 5
/ 1.0
( ) 57 mV/n at 25 C
(independent of the scan rate)
= ( ) / 2
= constant
(at 25 C, constant = 2.69 10 )
o o
i i
E E
E E E
i n AD c v
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Diffusion Controlled Reversible Process
p
For electrode adsorption process
i v
Electrode: Hg filmDetection: DPV--Differential Pulse Voltammetry.
Stripping Voltammetry
Cd
(Ultra)microelectrodes
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UMERadial Diffusion
Macro-disk ElectrodePlanar Diffusion
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UMEs Applications
• Measure electrochemical behavior without added electrolyte (a small i small iR 0) Bioanalytical Applications.
• Scanning Electrochemical Microscopy imaging & chemical information, fast electron transfer rate measurement.
• ……
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