Voltammetry

4/30/2012 Voltammetry6, 2004 Lecture Date: April 28th, 2008 Reading Material ● Skoog, Holler and Crouch: Ch. 25 ● Cazes: Chapter 17 ● For those using electroanalyticalchemistry in their work, see: A. J. Bard and L. R. Faulkner, “ElectrochemicalMethods”, 2nd Ed., Wiley,2001. 1 4/30/2012 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 – Whensmall 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 polarizationeffects – The difference usually reduces the voltage of a galvanic cell or increases the voltage consumed by an electrolytic cell 2 4/30/2012 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: Ecell = Eright − Eleft − IR IR Drop Electrodes More on Polarization Electrodes in cells are polarized over certain current/voltage ranges “Ideal” polarized electrode: current does not vary with potential 3 4/30/2012 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 throughthe dropping mercury electrode (DME) as a function of applied potential  Under the influence of gravity (or otherforces), mercury drops grow from the end of a fine glass capillary until they detach  If an electroactive species is www.drhuang.com/.../polar.doc_files/image008.gif capableof undergoing a redox process at the DME, then an S-shapedcurrent-potentialtrace (a polarographic wave) is usually observed 4 4/30/2012 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 linearpotential ramp in the same manner as DCP.  However, with LSV the potential scan rate is usually much faster than with DCP.  Whenthe 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 diffusionlimited. At this point the current slowly declines.  The result is the asymmetric peak-shaped I-E curve 5 ... - tailieumienphi.vn