nov 16, 2004 voltammetry lecture date: april 28 th, 2008
TRANSCRIPT
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:
IREEE leftrightcell
Electrodes
IR Drop
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
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 S-shaped 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 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
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
Half-wave potential
A + n e- P
Limiting current
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
Both reduction and oxidation waves are seen in a mixture
O
O
quinone hydroquinone
+ 2H+ + 2e
OH
OH
Diagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3 rd Ed. Wiley 1989.
Anodic wave
Cathodic wave
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 3 rd 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
dissolvedO2
analyte solution
O2 permeable membrane(O2 crosses via diffusion)
platinum electrode
electrolyte
O2
O2
O2
O2 + 2H2O + 4e- 4OH-
At the platinum cathode:
At the Ag/AgCl anode:
Ag + Cl- AgCl + e-
(-0.6 volts)
id = 4 F Pm A P(O2)/b
id - measured current
F - Faraday's constant
Pm - permeability of O2
A - electrode area
P(O2) - oxygen concentration
b - thickness of the membrane
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 study and characterize redox systems from macroscopic scales down to nanoelectrodes
Cyclic Voltammetry
The waveform, and the resulting I-E curve:
The I-E curve encodes a large amount of information (see next slide)
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
P. T. Kissinger and W. H. Heineman, J. Chem. Ed. 1983, 60, 702.
Spectroelectrochemistry (SEC)
CV and spectroscopy can be combined by using optically-transparent electrodes
This allows for analysis of the mechanisms involved in complex electrochemical reactions
Example: ferrocene oxidized to ferricinium on a forward CV sweep (ferricincium shows UV peaks at 252 and 285 nm), reduced back to ferrocene (fully reversible)
Y. Dai, G. M. Swain, M. D. Porter, J. Zak, “New horizons in spectroelectrochemical measurements: Optically transparent carbon electrodes,” Anal. Chem., 2008, 80, 14-27.
Instrumentation for Voltammetry
Cyclic voltammetry cell with a hanging mercury drop electrode
From www.indiana.edu/~echem/cells.html
Sweep generators, potentiostats, cells, and data acquistion/computers make up most systems
Basic voltammetry system suitable for undergraduate laboratory workFrom www.edaq.com/er461.html
Homework Problems and Further Reading
Optional Homework Problems:
– 25-1, 25-2, 25-5
Further Reading:– C. Amatore and E. Maisonhaute, “When voltammetry reaches
nanoseconds”, Anal. Chem., 2005, 303A-311A.
– Y. Dai, G. M. Swain, M. D. Porter, J. Zak, “New horizons in spectroelectrochemical measurements: Optically transparent carbon electrodes,” Anal. Chem., 2008, 80, 14-27.