Chapter 6 electrochemical analysis

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Chapter 6 electrochemical analysis

  1. 1. Chapter 6 Electrochemical Analysis
  2. 2. Anode reaction: Red === Ox + ne - Cathode reaction: Ox + ne - === Red 6.1 Introduction <ul><li>Oxidation – reduction reaction </li></ul>Cell reaction expression Anode  solution,(  Ox )  solution, (  Red )  Cathode (6r-1) (6r-2)
  3. 3. For example : <ul><li>Zn  ZnSO 4 ,( x Mol)   CuSO 4 , ( y Mol)  Cu </li></ul><ul><li>Anode: Zn Zn 2+ + 2e - </li></ul><ul><li>Cathode: Cu 2 + + 2e - Cu </li></ul>(6r-3) (6r-4)
  4. 4. 2. Half-cell Potential <ul><li>For half – cell reaction : </li></ul><ul><li>r A red + n e - p A Ox </li></ul><ul><li>Nernst equation : </li></ul><ul><li>For a Cell: </li></ul><ul><li>E cell = E cathode - E anode </li></ul><ul><li>If, E cell > 0: Primary Cell </li></ul><ul><li>E cell < 0: Electrolyic Cell </li></ul>(6r-5) (6-1) (6-2)
  5. 5. 3.The Types of Electrodes <ul><li>A metal in Equilibrium with its ions </li></ul><ul><li>(Class Ⅰelectrodes) </li></ul><ul><li>Ag + + e - Ag </li></ul>(6r-6) (6-3)
  6. 6. <ul><li>A metal in equilibrium with a saturated solution of a slightly soluble salt (Class Ⅱelectrodes) </li></ul><ul><li>Ag  AgCl  Cl - ,(  =1 ) </li></ul><ul><li>AgCl(s) + e - Ag + Cl – </li></ul><ul><li>Reference electrodes </li></ul><ul><li>Saturated calomel electrode ( SCE ) </li></ul><ul><li>Hg  Hg 2 Cl 2 (s)  Cl - ,(sat’d KCL ) </li></ul><ul><li>Hg 2 Cl 2 (s) + 2e - 2Hg + 2Cl – (sat’d KCL) </li></ul>(6r-7) (6r-8)
  7. 7. <ul><li>A metal in equilibrium with tow slightly soluble salts with a common Anion (Class Ⅲelectrodes) </li></ul><ul><li>Ag  Ag 2 S,CdS  Ag + ,Cd 2+ ,S 2- , </li></ul><ul><li>Ag 2 S (s) 2Ag + +S 2- </li></ul><ul><li>CdS (s) Cd 2+ +S 2- </li></ul>(6r-9) (6r-10)
  8. 8. 4. The departure of potential <ul><li>Liquid-junction potential </li></ul><ul><li>HCl(0.1M)  KCl(salt bridge, xM)  KCl(0.1M) </li></ul><ul><li>When x>3.6 E ljp <1mV </li></ul><ul><li>Polarization </li></ul><ul><li>E fact ≠E Nernst and C surf ≠C bolk </li></ul><ul><li>Over-voltage </li></ul><ul><li>real potential start a reaction > equilibrium potential </li></ul><ul><li>Ohm drop </li></ul><ul><li>E cell = E cathode - E anode + IR </li></ul><ul><li>R: resistance of solution, I: current </li></ul>(6-4)
  9. 9. 6.2 Potentiometry <ul><li>Principle </li></ul>(6-5) (6-6) (6-7) (6-8)
  10. 10. 2. Ion selective Membrane Electrode <ul><li>Structure of ISE </li></ul><ul><li>Types </li></ul>Fig 6-1
  11. 11. (1) The Glass Electrode <ul><li>Ag ︱ Agcl(s) ︱ HCl(  inner ) ︱ glass ︱ H + (unknown solution) </li></ul>(6-9) Fig 6-2
  12. 12. Glass electrode ︱ unknown solution ︱ SCE (6-10) (6-11) (6-12)
  13. 13. Selectivity of Glass electrode <ul><li>H + G - +M + (sol) M + G - + H + (sol) </li></ul>k: selectivity coefficient (6-13) (6-14) (6r-11)
  14. 14. (2) The Response Behavior of ISE <ul><li>Nernst response and Detect limit </li></ul>(6-15) Fig 6-3
  15. 15. <ul><li>Selectivity </li></ul><ul><li>Response time </li></ul>(6-16) Fig 6-4
  16. 16. <ul><li>The Prerequisite of Experiments </li></ul><ul><li>Ion Intensity Buffer </li></ul>3.Quantitative Analysis (6-17) (6-18) (6-19) f _activity coefficient If C ion,T ≈constant, f ≈constant.
  17. 17. <ul><li>pH Buffer </li></ul><ul><li>M Z+ + x OH - M(OH) x (z-x)+ </li></ul><ul><li>H + + OH - H 2 O </li></ul><ul><li>Complex reagent </li></ul><ul><li>M Z+ + n L MLn Z+ </li></ul>(6-20) (6r-12) (6r-13) (6r-14)
  18. 18. (6-21) (6-22) (6-23) (6-24)
  19. 19. (2)Standard calibration Methods standard concentration series If  =1: E = K + s lgC 0 Fig 6-5 -5 -4.500 -4 -3.500 -3 lgc 10 -5 3.16x10 -5 10 -4 3.16x10 -4 10 -3 C 0 / molL -1
  20. 20. (3)Standard Addition Methods (6-25) (6-26) (6-27) (6-28)
  21. 21. assume: f 1 =f 2 ,  1 =  2 , S = 0.0591/n (6-29) (6-30) (6-31)
  22. 22. 6.3 Polarography <ul><li>Introduction </li></ul><ul><li>(1) Electrolytic cell </li></ul><ul><li>Cathode: </li></ul><ul><li>M + + e - ->M </li></ul><ul><li> Hg(l) ∣M + (C) ︱ SCE </li></ul>Wkg: Working Electrode Ref:Reference Electrode(SCE)
  23. 23. (2) Polarization M + (Bulk) -> M + (Cathode) Fig 6-7
  24. 24. 2. The Dropping Mercury Electrode(DME) <ul><li>(1) Structure of DME </li></ul>Fig 6-8
  25. 25. (2)Electrolytic current and current density Fig 6-9
  26. 26. 3. Quantitative Analysis (1) Ilkovic Equation m ____rate of mercury flow D ____diffusion coefficient ____ Average diffusion current (6-32) (6-33)
  27. 27. (2)The factor of affect diffusion current <ul><li>Residual current </li></ul><ul><li>Changing current </li></ul><ul><li>Migrating current </li></ul><ul><li>Maximum phenomenon </li></ul><ul><li>Oxygen interference </li></ul>
  28. 28. 4. Qualitative Analysis <ul><li>Half wave potential </li></ul>(6-34) (6-35)

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