Section 4: Electroanalytical Chemistry

Topics covered:
 
 

1.	Review topics   - balancing redox reactions   - reduction potentials (Eo), electrochemical cells, cell potentials   - Nernst equation Figure: 16-4
2.	Qualitative description of the redox event - what happens on a microscopic scale in an electrochemical cell   - the double layer near the electrode   - redox event includes            (a) mass transfer from bulk to electrode           (b) chemical reaction(s), if necessary           (c) physical state change from solution to attachment to electrode           (d) electron transfer   - any of these could be rate-determining, leading to concentration (a), reaction (b), adsorption/desorption/crystallisation (c), or charge transfer (d) polarisation   - results in overvoltage phenomenon, magnitude of which is affected by           - size of electrode           - temperature           - nature of chemical species (gas phase, soft vs. hard metal, etc.) Figure: 20-3
3.	Potentiometry - measurement of potential with no current.   - parts of the experiment that lead to observed voltage:           (a) reference electrodes - SHE, SCE, Ag/AgCl           (b) junction (e.g., salt bridge)           (c) indicator electrode - piece of metal, membrane electrode (including gas-permeable), field-effect transistor   - determination of concentration using potentiometry - use variations of the Nernst Equation:   general form of the Nernst Equation: (L = constant, including Eo and Ej and any other incidental voltages that are constant for the cell in question) for cation  Xn+ (aq)  +  ne-  --->  X (s) (assuming T = 298 K) for anion  A (s, l, g)  +  ne-  --->  An- (aq) (assuming P = 1 atm, T = 298 K) for a pH meter: (assuming P = 1 atm, T = 298 K)   - to avoid solving for L explicitly, use method of standard addition Figures: 17-2, 19-1, 19-2, 19-4, 19-5, 19-9, 19-16 Example: 19-1
4.	Calculating new standard electrode potentials (especially low-solubility salts) - using the Eo for a known reaction and an equilibrium constant (Ksp)
5.	Coulometry - using electrons as a stoichiometric unit   (a) Electrogravimetry - reduce to solid species, weigh electrode           - determination of separation ranges (potentials) - can the materials be quantitatively separated?   (b) Potentiostatic Coulometry  - constant potential applied, current measured, decays as analyte depleted           -  (note: this is a destructive technique)    total charge (total number of moles of e- used):           - can also be used for titrations (electrogeneration of H+ or OH-)   (c) Voltammetry - plotted as current vs. potential           - polarography = use of dropping mercury electrode           - linear scan voltammetry (hydrodynamic voltammetry) = amount of analyte related to limiting current (Il)   under conditions of concentration polarisation (cA = analyte concentration) Il = kcA                        - for reproducible k: stir solution, supporting electrolyte 50-100 x more concentrated than analyte                        - used in titrations, LC detectors (for ionic compounds), dissolved O2 content           - cyclic voltammetry - normally qualitative                         - peak-to-peak width (voltage difference) = 0.0592/n   (d) Stripping Methods - high voltage reduces all analytes (to pre-concentrate analytes)           - decrease voltage, current will spike as analytes re-oxidise,           - voltage of re-oxidation gives concentration Figures: 20-5, 20-9, 20-10, 20-15

Last updated, March 20, 2006
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