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Chemistry 4631

Chemistry 4631. Instrumental Analysis Lecture 23. Chem 4631. Chem 4631. Voltammetry. Voltammetry – measurement of current as a function of applied potential using working electrode. Historically – 1 st voltammetry was polarography developed by Heyrovsky in 1921. Chem 4631. Voltammetry.

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Chemistry 4631

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  1. Chemistry 4631 Instrumental Analysis Lecture 23 Chem 4631

  2. Chem 4631

  3. Voltammetry Voltammetry – measurement of current as a function of applied potential using working electrode. Historically – 1st voltammetry was polarography developed by Heyrovsky in 1921. Chem 4631

  4. Voltammetry Excitation signal A variable potential excitation signal is applied to an electrochemical cell. The excitation signal has a distinctive waveform, from linear to triangular. Chem 4631

  5. Chem 4631

  6. Voltammetry Voltammetric Instrumentation Electrochemical cell consist of three electrodes immersed in a solution of supporting electrolyte. Working electrode – potential varies linearly with time, usually very small Reference electrode – potential is constant with time – SCE or Ag/AgCl common Counter (auxiliary) electrode – conducts current from source through solution – Pt wire common Chem 4631

  7. Voltammetry Chem 4631

  8. Voltammetry Working Electrodes Come in a variety of shapes, compositions, and sizes. Working electrodes have a potential window. This window determines the range of potentials the working electrode can be used. The window is limited on the positive side by the oxidation of water to O2(g) and on the negative side by the reduction of water to H2(g). Chem 4631

  9. Chem 4631

  10. Chem 4631

  11. Voltammetry Working Electrodes Chem 4631

  12. Voltammetry Working Electrodes Working electrode material: Metals: Ag, Au, Pt, Ni, SS, etc… Carbon Mercury Chem 4631

  13. Voltammetry Working Electrodes Carbon More complex than the metal electrodes. The sp2 carbon varies in conductivity, stability, hardness and porosity. Chem 4631

  14. Voltammetry Carbon Microstructure Single-crystal graphite is inherently anisotropic. Chem 4631

  15. Voltammetry Carbon Microstructure Electrochemical properties for the two planes vary greatly. Chem 4631

  16. Voltammetry Carbon Common carbon electrodes: 1) HOPG-highly ordered pyrolytic graphite Made by pyrolysis of light hydrocarbons at ~ 800oC then treated at high temperature and pressure annealed at 3000oC. HOPG has weak interplanar bonding so can be renewed with adhesive tape. Chem 4631

  17. Voltammetry Carbon Common carbon electrodes: 2) Polycrystalline graphite electrodes a. Carbon paste electrodes (CPE) Control porosity of graphite material by filling pores with inert liquid carbon paste is graphite mixed with Nujol (heavy hydrocarbon) or hexadecane. Paste is then packed into shallow well. CPE’s are easily modifies (i.e., steric acids, zeolites, complexing agents). Chem 4631

  18. Voltammetry Carbon Common carbon electrodes: 2) Polycrystalline graphite electrodes b. Spectroscopic Graphite Electrodes Rods with low metal impurity used in emission spectroscopy. Can immerse in molten wax (ceresin or paraffin) then place in a vacuum, polish end of rod. Chem 4631

  19. Voltammetry Carbon Common carbon electrodes: 2) Polycrystalline graphite electrodes c. Graphite Composite Electrodes Mix graphite powder with suitable filler that hardens. Chem 4631

  20. Voltammetry Carbon Common carbon electrodes: 2) Polycrystalline graphite electrodes d. Glassy Carbon High MW carbanaceous polymer (polyacylonitrile, phenol/formaldehyde resin). Polymer is heated to 400-800oC then heated under pressure at 1000- 3000oC. Hard and highly disordered. Chem 4631

  21. Voltammetry Carbon Common carbon electrodes: 2) Polycrystalline graphite electrodes e. Carbon Fibers Made from polyacylontirile (PAN) drawn into filaments during curing. Chem 4631

  22. Voltammetry Working Electrodes Chem 4631

  23. Voltammetry Working Electrodes Modified Electrodes • Irreversible adsorption • Covalent bonding • Coatings (i.e. polymers) • Enzymes • Self-assembled monolayers Chem 4631

  24. Voltammetry Voltammograms When a linear-scan voltammogram is taken in a moving solution, the experiment is called hydrodynamic voltammetry. The resulting curve is a voltammetric wave with a sigmoidal shape. Working electrode can also be rotated. Polarography Voltammetry using a dropping Hg electrode. Chem 4631

  25. Voltammetry Voltammograms Chem 4631

  26. Voltammetry Chem 4631

  27. Voltammetry Concentration Profiles For the following equation A + ne-  P Eo = -0.260 V Eappl = EoA – 0.0592/n log cpo/cAo – Eref Eappl – potential between WE and Ref cpo and cAo – molar concentration of P and A at the electrode surface. Chem 4631

  28. Voltammetry Profiles in Unstirred Solution Chem 4631

  29. Voltammetry Profiles in Unstirred Solution Chem 4631

  30. Voltammetry Cyclic Voltammetry In CV, current response in unstirred solution is excited by triangular waveform. Potential varies linearly, scan direction is reversed. CV important for studying mechanism and kinetics (rates). Chem 4631

  31. Voltammetry Cyclic Voltammetry Chem 4631

  32. Cyclic Voltammetry Chem 4631

  33. Voltammetry Chem 4631

  34. Voltammetry Chem 4631

  35. Voltammetry Chem 4631

  36. Voltammetry Chem 4631

  37. Voltammetry Chem 4631

  38. Voltammetry Chem 4631

  39. Voltammetry Chem 4631

  40. Voltammetry UMEs Application Carbon-fiber electrodes for detection of catecholamine and indoleamine neurotransmitters and metabolites. Chem 4631

  41. Voltammetry Chem 4631

  42. Assignment • Read Chapter 23 • Homework Chapter 23: 2, 4, 7, 8, and 11 • Homework Chapter 22 Due Today • Homework Chapter 23 Due 3/26/14 • Read Chapter 25 • Homework Chapter 25: 1-8 and 10 • Homework Chapter 25 due 04/02/14

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