nov 16, 2004 voltammetry lecture date: april 10 th, 2013
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Nov 16, 2004Voltammetry
Lecture Date: April 10th, 2013
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: a current proportional to analyte concentration is monitored at a fixed potential
– In other words, voltammetry at a constant potential
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
Current in Electrochemical Cells
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
Polarization
Electrodes in cells are polarized over certain current/voltage ranges
– Electrodes are purposely kept small (mm2 to um2) in voltammetry to promote polarization
“Ideal” polarized electrode: current does not vary with potential
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(needed when current
is significant)
Overvoltage and Polarization Sources
Overvoltage (overpotential) the difference between the equilibrium potential and the actual potential; it develops because of polarization
– Net result is you must means must apply greater potential before redox chemistry occurs
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)
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)
Instrumentation for Voltammetry
Block diagram of a typical 3-electrode voltammeter:
PotentiostatWaveform generator
Current-to-voltage
converterComputer
Eapplied
Cell
See Fig. 29.13 in Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3 rd Ed. Wiley 1989.
Counter electrode
Working electrode
Reference electrode(i = 0)
← e-
e- →
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
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
– n can also be fitted
The limiting current is proportional to analyte concentration and is used for quantitative analysis
Half-wave potentialE1/2
A + n e- P
Limiting current
Remember, E is scanned linearly to higher values as a function of time in linear
sweep voltammetry
Nernst Plot
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 processes.
– 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
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 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 contains a large amount of analytical information (see next slide)
Cyclic Voltammetry
CV for a simple system: hexacyanoferrate(III) and (II) ions
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.
Electrochemical Stripping Voltammetry
A two step process:(1) The analyte is deposited (accumulated) on the
working electrode from solution.
(2) The analyte is then stripped off of the electrode with observation of current by a voltammetric method.
The aim is to concentrate the analyte to obtain lower LOD and LOQ.
Anodic stripping: the working electrode behaves as a cathode during the deposition step, then behaves as an anode during the stripping step.
– Cathodic stripping (less common) is the opposite process.
See pages 748 of the text for more about electrochemical stripping techniques.
Electrochemical Stripping Voltammetry
-1.0 V
-0.6 V
-0.1 V
See pages 748 of the text for a similar figure.
Cd => Cd2+ + 2e-
Cu => Cu2+ + 2e-
The currents observed for Cd and Cu are proportional to the concentration of each metal in solution.
Electrochemical Stripping Voltammetry: Elemental Analysis
Elemental detection using a bismuth-modified carbon paste electrode
Three toxic elements (Cd, Pb, Tl) are easily detected at 200 ppb in this example.
Svancara, et al., Electroanalysis 18, 2006, 177-185.
Electrochemical Stripping Voltammetry: Molecular Analysis
An early example of stripping voltametry (polarography) using a hanging mercury drop electrode on the drug diazepam:
R. Kaldova, Analytica Chimica Acta, 162 (1984) 197—205.
Electrochemical Stripping Voltammetry: Molecular Analysis
Detection of the insecticide methyl parathion using stripping square-wave voltammetry with an electrode made from tetrasulfonated phtalocyanine (p-NiTSPc) electrodeposited on a carbon surface with a Nafion® sulfonated tetrafluoroethylene copolymer coating
M. Sbai, et al. Sensors and Actuators B 124 (2007) 368–375.
irreversible reduction (a, Epa -0.61 V)
reversible reduction-oxidation (b, Epa = -0.08 V, c, Epc = 0.0 V)
(a)(b)
(c)
CV and 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.
More Spectroelectrochemistry
A typical system (Gamry Interface 1000 and Agilent/Varian Cary 50 UV-Vis)
SECM and SECM-AFM
Scanning electrochemical microscopy (SECM) uses nanometer sized tips (electrodes) to probe surface phenomena
– Analyses are run in constant height mode or constant current mode
– Can be combined with AFM
The figures compare steady-state voltammograms of 1 mM ferrocenemethanol and 0.2 M NaCl obtained using a bulk system and using a SECM with a 36 nm polished Pt tip
Sun and Mirkin, Anal. Chem. 2006, 78, 6526-6534.
Bulk
SECM
SECM: Applications to Metal Corrosion SECM can be used to identify precursor sites for corrosion
in passive oxide films that protect metals
The metal substrate is biased with a voltage and the SECM tip detects the product of a reaction, providing an image of the reactive site.
Allows imaging of surface reactivity
Basame and White, Langmuir 1999, 15, 819-825.
SECM Instrumentation
Princeton Applied Research/Ametek VersaSCAN:
Reading Material
● Skoog, Holler and Crouch: Ch. 25
● Cazes: Chapter 17
● Optional 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.
– A. J. Bard and L. R. Faulkner, “Electrochemical Methods”, 2nd Ed., Wiley, 2001.
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