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Chapter 6 Electrochemical Analysis

Chapter 6 Electrochemical Analysis. 6.1 Introduction. Oxidation – reduction reaction. Anode reaction: Red === Ox + ne - Cathode reaction: Ox + ne - === Red. (6r-1). (6r-2). Cell reaction expression Anode solution,( Ox )solution, ( Red ) Cathode. For example:.

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Chapter 6 Electrochemical Analysis

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  1. Chapter 6Electrochemical Analysis

  2. 6.1 Introduction • Oxidation – reduction reaction Anode reaction: Red === Ox + ne - Cathode reaction: Ox + ne - ===Red (6r-1) (6r-2) Cell reaction expression Anodesolution,(Ox)solution, (Red) Cathode

  3. For example: Zn ZnSO4,(xMol)  CuSO4, (yMol)  Cu Anode: Zn Zn2+ + 2e- Cathode: Cu2 + + 2e- Cu (6r-3) (6r-4)

  4. 2. Half-cell Potential (6r-5) For half – cell reaction : rAred pAOx + ne - Nernst equation: For a Cell: Ecell = Ecathode - Eanode If, Ecell > 0: Primary Cell Ecell < 0: Electrolyic Cell (6-1) (6-2)

  5. 3.TheTypes of Electrodes • A metal in Equilibrium with its ions (Class Ⅰelectrodes) Ag+ + e- Ag (6r-6) (6-3)

  6. A metal in equilibrium with a saturated solution of a slightly soluble salt(Class Ⅱelectrodes) AgAgCl Cl -,(=1) AgCl(s) + e- Ag + Cl – Reference electrodes Saturated calomel electrode (SCE) HgHg2Cl2(s)Cl -,(sat’d KCL) Hg2Cl2(s) + 2e- 2Hg + 2Cl –(sat’d KCL) (6r-7) (6r-8)

  7. A metal in equilibrium with tow slightly soluble salts with a common Anion(Class Ⅲelectrodes) AgAg2S,CdSAg+,Cd2+,S2-, Ag2S(s) 2Ag++S2- CdS(s) Cd2++S2- (6r-9) (6r-10)

  8. 4. The departure of potential • Liquid-junction potential HCl(0.1M) KCl(salt bridge, xM) KCl(0.1M) When x>3.6 Eljp<1mV • Polarization Efact≠ENernstand Csurf ≠Cbolk • Over-voltage real potential start a reaction > equilibrium potential • Ohm drop Ecell= Ecathode- Eanode + IR R: resistance of solution, I: current (6-4)

  9. 6.2 Potentiometry (6-5) • Principle (6-6) (6-7) (6-8)

  10. 2. Ion selective Membrane Electrode • Structure of ISE • Types Fig 6-1

  11. (1) The Glass Electrode Fig 6-2 Ag︱Agcl(s) ︱HCl(inner) ︱glass ︱H+(unknown solution) (6-9)

  12. (6-10) Glass electrode︱unknown solution ︱SCE (6-11) (6-12)

  13. Selectivity of Glass electrode (6r-11) H+G-+M+(sol) M+ G- +H+ (sol) (6-13) k: selectivity coefficient (6-14)

  14. (2) The Response Behavior of ISE (6-15) • Nernst response and Detect limit Fig 6-3

  15. Selectivity • Response time (6-16) Fig 6-4

  16. 3.Quantitative Analysis • The Prerequisite of Experiments • Ion Intensity Buffer (6-17) f_activity coefficient (6-18) If Cion,T≈constant, f ≈constant. (6-19)

  17. (6r-12) (6r-13) • pH Buffer MZ+ + xOH- M(OH)x (z-x)+ H+ + OH- H2O • Complex reagent M Z+ + nL MLnZ+ (6r-14) (6-20)

  18. (6-21) (6-22) (6-23) (6-24)

  19. (2)Standard calibration Methods Fig 6-5 If =1: E = K + s lgC0 standard concentration series

  20. (3)Standard Addition Methods (6-25) (6-26) (6-27) (6-28)

  21. (6-29) assume: f1=f2 , 1=2 , S = 0.0591/n (6-30) (6-31)

  22. 6.3 Polarography • Introduction (1) Electrolytic cell Cathode: M+ + e- →M Hg(l) ∣M+(C)︱SCE Wkg: Working Electrode Ref:Reference Electrode(SCE)

  23. (2) PolarizationM +(Bulk) → M +(Cathode) Fig 6-7

  24. 2. The Dropping Mercury Electrode(DME) (1) Structure of DME Fig 6-8

  25. (2)Electrolytic current and current density Fig 6-9

  26. 3. Quantitative Analysis(1) Ilkovic Equation (6-32) ____Average diffusion current D ____diffusion coefficient m ____rate of mercury flow (6-33)

  27. (2)The factor of affect diffusion current • Residual current • Changing current • Migrating current • Maximum phenomenon • Oxygen interference

  28. 4. Qualitative Analysis Half wave potential (6-34) (6-35)

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