Voltammetry - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Voltammetry - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! 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 docsity.com 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 docsity.com 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) docsity.com 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) docsity.com 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) docsity.com 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 docsity.com 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 potential E1/2 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 docsity.com 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 docsity.com 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 docsity.com The Clark Voltammetric Oxygen Sensor dissolved O2 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 docsity.com The Clark Voltammetric Oxygen Sensor General design and modern miniaturized versions: docsity.com Cyclic Voltammetry  The waveform, and the resulting I-E curve:  The I-E curve contains a large amount of analytical information (see next slide) docsity.com 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. docsity.com 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. docsity.com 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. docsity.com 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) docsity.com 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. docsity.com 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. docsity.com SECM Instrumentation  Princeton Applied Research/Ametek VersaSCAN: docsity.com