Presentation on theme: "Voltammetry Nov 16, 2004 Lecture Date: April 28th, 2008."— Presentation transcript:
1VoltammetryNov 16, 2004Lecture Date: April 28th, 2008
2Reading 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.
3VoltammetryVoltammetry techniques measure current as a function of applied potential under conditions that promote polarization of a working electrodePolarography: 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
4Some electrochemical cells have significant currents. 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 cellCurrents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.
5Ohmic 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 cathodeThis force follows Ohm’s law, and is governed by the resistance of the cell:Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.IR DropElectrodes
6Electrodes in cells are polarized over certain current/voltage ranges More on PolarizationElectrodes in cells are polarized over certain current/voltage ranges“Ideal” polarized electrode: current does not vary with potentialCurrents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.
7Overvoltage and Polarization Sources Overvoltage: the difference between the equilibrium potential and the actual potentialSources 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)Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.
8DC 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 observedCurrents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.
9Voltage-Time Signals in Voltammetry A variable potential excitation signal is applied to the working electrodeDifferent voltammetric techniques use different waveformsMany other waveforms are available (even FT techniques are in use)
10Linear 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
11The Linear Sweep Voltammogram A linear sweep voltammogram for the following reduction of “A” into a product “P” is shownA + n e- PThe half-wave potential E1/2 is often used for qualitative analysisThe limiting current is proportional to analyte concentration and is used for quantitative analysisA + n e- PHalf-wave potentialLimiting currentRemember, E is scanned linearly to higher values as a function of time in linear sweep voltammetry
12Hydrodynamic Voltammetry Hydrodynamic 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
13Hydrodynamic Voltammograms Example: the hydrodynamic voltammogram of quinone-hydroquinoneDifferent waves are obtained depending on the starting sampleBoth reduction and oxidation waves are seen in a mixtureCathodic waveThe E1/2 is pH sensitive in this caseAnodic waveDiagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3rd Ed. Wiley 1989.
14Oxygen Waves in Hydrodynamic Voltammetry Oxygen 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.
15The 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 solutionThe Clark oxygen sensor finds applications in wide areas:Environmental StudiesSewage TreatmentFermentation ProcessMedicine
16The Clark Voltammetric Oxygen Sensor At the platinum cathode:O2 + 2H2O + 4e OH-At the Ag/AgCl anode:O2Ag + Cl AgCl + e-O2dissolvedO2id = 4 F Pm A P(O2)/bid - measured currentF - Faraday's constantPm - permeability of O2A - electrode areaP(O2) - oxygen concentrationb - thickness of the membraneO2A 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 µm). The oxygen consumption of a microsensor is small, and its measurements are not affected by stagnant media (so stirring is not needed).analyte solutionelectrolyteO2 permeable membrane(O2 crosses via diffusion)platinum electrode(-0.6 volts)
17The Clark Voltammetric Oxygen Sensor General design and modern miniaturized versions:
18Hydrodynamic 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
19Cyclic 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 potentialCV 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
20Cyclic VoltammetryThe waveform, and the resulting I-E curve:The I-E curve encodes a large amount of information (see next slide)
21A typical CV for a simple electrochemical system 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.
22Spectroelectrochemistry (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,
23Instrumentation for Voltammetry Sweep generators, potentiostats, cells, and data acquistion/computers make up most systemsCyclic voltammetry cell with ahanging mercury drop electrodeFromBasic voltammetry system suitable for undergraduate laboratory workFrom
24Homework Problems and Further Reading Optional Homework Problems:25-1, 25-2, 25-5Further 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,