Nov 16, 2004VoltammetryLecture Date: April 28th, 2008
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.
VoltammetryVoltammetry 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
PolarizationSome electrochemical cells have significant currents. Electricity within a cell is carried by ion motionWhen small currents are involved, E = IR holdsR 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 equationThis difference arises from polarization effectsThe difference usually reduces the voltage of a galvanic cell or increases the voltage consumed by an electrolytic cell
Ohmic Potential and the IR DropTo create current in a cell, a driving voltage is needed to overcome the resistance of ions to move towards the anode and cathodeThis force follows Ohms law, and is governed by the resistance of the cell:
More on PolarizationElectrodes in cells are polarized over certain current/voltage rangesIdeal polarized electrode: current does not vary with potential
Overvoltage and Polarization SourcesOvervoltage: 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 currentCharge-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 PolarographyThe first voltammetric technique (first instrument built in 1925)DCP measures current flowing through the dropping mercury electrode (DME) as a function of applied potentialUnder the influence of gravity (or other forces), mercury drops grow from the end of a fine glass capillary until they detachIf 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 observedwww.drhuang.com/.../polar.doc_files/image008.gif
Voltage-Time Signals in VoltammetryA 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 VoltammetryLinear 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 VoltammogramA 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 analysisThe limiting current is proportional to analyte concentration and is used for quantitative analysis Half-wave potentialA + n e- PLimiting currentRemember, E is scanned linearly to higher values as a function of time in linear sweep voltammetry
Hydrodynamic VoltammetryHydrodynamic voltammetry is performed with rapid stirring in a cellElectrogenerated species are rapidly swept away by the flowReactants are carried to electrodes by migration in a field, convection, and diffusion. Mixing takes over and dominates all of theseMost importantly, migration rate becomes independent of applied potential
Hydrodynamic VoltammogramsExample: the hydrodynamic voltammogram of quinone-hydroquinone
Different waves are obtained depending on the starting sample
Both reduction and oxidation waves are seen in a mixtureDiagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3rd Ed. Wiley 1989.Anodic waveCathodic wave
Oxygen Waves in Hydrodynamic VoltammetryOxygen waves occur in many voltammetric experimentsHere, waves from two electrolytes (no sample!) are shown before and after sparging/degassingHeavily used for analysis of O2 in many types of sampleIn some cases, the electrode can be dipped in the sampleIn 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 SensorNamed 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 StudiesSewage TreatmentFermentation ProcessMedicine
The Clark Voltammetric Oxygen SensordissolvedO2
analyte solutionO2 permeable membrane(O2 crosses via diffusion)platinum electrodeelectrolyteO2O2O2O2 + 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)/bid - measured currentF - Faraday's constantPm - permeability of O2A - electrode areaP(O2) - oxygen concentrationb - thickness of the membrane
The Clark Voltammetric Oxygen SensorGeneral design and modern miniaturized versions:
Hydrodynamic Voltammetry as an LC DetectorOne 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 VoltammetryCyclic 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 VoltammetryThe waveform, and the resulting I-E curve:The I-E curve encodes a large amount of information (see next slide)
Cyclic VoltammetryA typical CV for a simple electrochemical systemCV can rapidly generate a new oxidation state on a forward scan and determine its fate on the reverse scanAdvantages of CVControlled ratesCan determine mechanisms and kinetics of redox reactionsP. T. Kissinger and W. H. Heineman, J. Chem. Ed. 1983, 60, 702.
Spectroelectrochemistry (SEC)CV and spectroscopy can be combined by using optically-transparent electrodesThis allows for analysis of the mechanisms involved in complex electrochemical reactionsExample: 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 VoltammetryCyclic voltammetry cell with a hanging mercury drop electrodeFrom www.indiana.edu/~echem/cells.html Sweep generators, potentiostats, cells, and data acquistion/computers make up most systemsBasic voltammetry system suitable for undergraduate laboratory workFrom www.edaq.com/er461.html
Homework Problems and Further ReadingOptional 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.
*Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.*Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.*Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.*Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.*Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.*The E1/2 is pH sensitive in this case*A drawback to the Clark sensor is that oxygen is consumed during the measurement with a rate equal to the diffusion in the sensor. To avoid this, the system must be stirred to get an accurate measurement and avoid stagnant pools of electrolyte. The oxygen consumption increases and so does the stirring sensitivity with increasing sensor size. Clark oxygen sensors can be made very small (e.g. a few m). The oxygen consumption of a microsensor is small, and its measure