Transcript Introduction to Electroanalytical Chemistry

Voltammetry
Nov 16, 2004
Lecture Date: March 27th, 2006
Reading Material
● Skoog, Holler and Crouch:
Ch. 25
● Cazes: Chapter 17
● For those using electroanalytical chemistry in their work,
see:
A. J. Bard and L. R. Faulkner, “Electrochemical Methods”, 2nd
Ed., Wiley, 2001.
Voltammetry
 Voltammetry techniques measure current as a
function of applied potential under conditions that
promote polarization of a working electrode
 Polarography:
Invented by J. Heyrovsky (Nobel
Prize 1959). Differs from voltammetry in that it
employs a dropping mercury electrode (DME) to
continuously renew the electrode surface.
 Amperometry:
current proportional to analyte
concentration is monitored at a fixed potential
Polarization
 Some electrochemical cells have significant
currents.
– Electricity within a cell is carried by ion motion
– When small currents are involved, E = IR holds
– R depends on the nature of the solution (next slide)
 When current in a cell is large, the actual potential
usually differs from that calculated at equilibrium
using the Nernst equation
– This difference arises from polarization effects
– The difference usually reduces the voltage of a galvanic
cell or increases the voltage consumed by an electrolytic
cell
Ohmic Potential and the IR Drop
 To create current in a cell, a driving voltage is
needed to overcome the resistance of ions to move
towards the anode and cathode
 This force follows Ohm’s law, and is governed by
the resistance of the cell:
Ecell  Eright  Eleft  IR
IR Drop
Electrodes
More on Polarization
 Electrodes in cells are polarized over certain
current/voltage ranges
 “Ideal” polarized electrode: current does not vary
with potential
Overvoltage and Polarization Sources
 Overvoltage:
the difference between the equilibrium
potential and the actual potential
 Sources of polarization in cells:
– Concentration polarization: rate of transport to
electrode is insufficient to maintain current
– Charge-transfer (kinetic) polarization: magnitude
of current is limited by the rate of the electrode
reaction(s) (the rate of electron transfer between
the reactants and the electrodes)
– Other effects (e.g. adsorption/desorption)
DC Polarography
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The first voltammetric technique
(first instrument built in 1925)
DCP measures current flowing
through the dropping mercury
electrode (DME) as a function of
applied potential
Under the influence of gravity (or
other forces), mercury drops grow
from the end of a fine glass
capillary until they detach
If an electroactive species is
capable of undergoing a redox
process at the DME, then an Sshaped current-potential trace (a
polarographic wave) is usually
observed
www.drhuang.com/.../polar.doc_files/image008.gif
Voltage-Time Signals in Voltammetry
 A variable potential
excitation signal is applied
to the working electrode
 Different voltammetric
techniques use different
waveforms
 Many other waveforms
are available (even FT
techniques are in use)
Linear Sweep Voltammetry
 Linear sweep voltammetry (LSV) is performed by applying a
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linear potential ramp in the same manner as DCP.
However, with LSV the potential scan rate is usually much
faster than with DCP.
When the reduction potential of the analyte is approached,
the current begins to flow.
– The current increases in response to the increasing
potential.
– However, as the reduction proceeds, a diffusion layer is
formed and the rate of the electrode reduction becomes
diffusion limited. At this point the current slowly declines.
The result is the asymmetric peak-shaped I-E curve
The Linear Sweep Voltammogram
 A linear sweep
voltammogram for the
following reduction of “A”
into a product “P” is shown
A + n e-  P
A + n e-  P
Half-wave potential
Limiting current
 The half-wave potential

E1/2 is often used for
qualitative analysis
The limiting current is
proportional to analyte
concentration and is used
for quantitative analysis
Remember, E is scanned
linearly to higher values as
a function of time in linear
sweep voltammetry
Hydrodynamic Voltammetry
 Hydrodynamic voltammetry is performed with rapid
stirring in a cell
– Electrogenerated species are rapidly swept
away by the flow
 Reactants are carried to electrodes by migration in
a field, convection, and diffusion. Mixing takes
over and dominates all of these
– Most importantly, migration rate becomes
independent of applied potential
Hydrodynamic Voltammograms
 Example:
the
hydrodynamic
voltammogram of
quinone-hydroquinone
 Different waves are
obtained depending on
the starting sample
OH
O
+ 2H+ + 2e
O
OH
quinone
hydroquinone
Cathodic wave
 Both reduction and
oxidation waves are
seen in a mixture
Anodic wave
Diagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3 rd Ed. Wiley 1989.
Oxygen Waves in Hydrodynamic Voltammetry
 Oxygen waves occur in
many voltammetric
experiments
– Here, waves from two
electrolytes (no sample!)
are shown before and after
sparging/degassing
 Heavily used for analysis
of O2 in many types of
sample
– In some cases, the
electrode can be dipped in
the sample
– In others, a membrane is
needed to protect the
electrode (Clark sensor)
Diagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3rd Ed. Wiley 1989.
The Clark Voltammetric Oxygen Sensor
 Named after its generally recognized inventor (Leyland
Clark, 1956), originally known as the "Oxygen Membrane
Polarographic Detector“
 It remains one of the most commonly used devices for
measuring oxygen in the gas phase or, more commonly,
dissolved in solution
 The Clark oxygen sensor finds applications in wide areas:
–
–
–
–
Environmental Studies
Sewage Treatment
Fermentation Process
Medicine
The Clark Voltammetric Oxygen Sensor
At the platinum cathode:
O2 + 2H2O + 4e4OHAt the Ag/AgCl anode:
Ag + ClAgCl + e-
O2
O2
dissolved
O2
id - measured current
O2
analyte solution
electrolyte
O2 permeable membrane
(O2 crosses via diffusion)
id = 4 F Pm A P(O2)/b
F - Faraday's constant
Pm - permeability of O2
A - electrode area
P(O2) - oxygen concentration
b - thickness of the membrane
platinum electrode
(-0.6 volts)
The Clark Voltammetric Oxygen Sensor
 General design and modern miniaturized versions:
Hydrodynamic Voltammetry as an LC Detector
 One form of electrochemical LC detector:
Classes of Chemicals Suitable for Electrochemical Detection:
Phenols, Aromatic Amines, Biogenic Amines, Polyamines, Sulfhydryls,
Disulfides, Peroxides, Aromatic Nitro Compounds, Aliphatic Nitro
Compounds, Thioureas, Amino Acids, Sugars, Carbohydrates,
Polyalcohols, Phenothiazines, Oxidase Enzyme Substrates, Sulfites
Cyclic Voltammetry
 Cyclic voltammetry (CV) is similar to linear sweep
voltammetry except that the potential scans run
from the starting potential to the end potential, then
reverse from the end potential back to the starting
potential
 CV is one of the most widely used electroanalytical
methods because of its ability to used to study and
characterize redox systems
Cyclic Voltammetry
 The waveform, and the resulting I-E curve:
Cyclic Voltammetry
 A typical CV for a simple
electrochemical system.
 CV can rapidly generate
a new oxidation state on
a forward scan and
determine its fate on the
reverse scan
 Advantages of CV
– Controlled rates
– Can determine
mechanisms and
kinetics of redox
reactions
Instrumentation for Voltammetry
 Sweep generators, potentiostats, cells, and data
acquistion/computers make up most systems
Basic voltammetry system suitable for undergraduate laboratory work
From www.edaq.com/er461.html
Cyclic voltammetry cell with a
hanging mercury drop electrode
From www.indiana.edu/~echem/cells.html