Chapter 23 Voltammetry - University of Southern …sites.usm.edu/electrochem/Analytical...

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1 Chapter 23 Voltammetry Voltammetry and Polarograph Electrochemistry techniques based on current (i) measurement as function of voltage (E appl ) 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 E applied to the electrodes; lower curve - the supporting solution of ammonium chloride and hydroxide containing small amounts of cadmium, zinc and manganese, upper curve - the same after addition of small amount of thallium. Tl + “Polarographic curves”-- Voltammograms

Transcript of Chapter 23 Voltammetry - University of Southern …sites.usm.edu/electrochem/Analytical...

Page 1: Chapter 23 Voltammetry - University of Southern …sites.usm.edu/electrochem/Analytical Chemistry/Lecture...1 Chapter 23 Voltammetry Voltammetry and Polarograph • Electrochemistry

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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.

• ……