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Chapter 13

Chapter 13. EDTA Titrations. E thylene D iamine T etra A cetic acid. Gramicidin A antibiotic ion channel. The Transition Metals. Industry : Fe , Cu , Ti , Ag ,  Biosystem : transport , storage , catalyst … (1) General Properties ( Sc → Cu )

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Chapter 13

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  1. Chapter 13 EDTA Titrations EthyleneDiamineTetraAcetic acid

  2. Gramicidin A antibiotic ion channel

  3. The Transition Metals Industry : Fe , Cu , Ti , Ag ,  Biosystem : transport , storage , catalyst … (1) General Properties ( Sc → Cu ) a)Great similarities within a period as well as a group ∵ d subshells incomplerely filled. distinctive coloring  formation of paramagnetic compounds  catalytic behavior  tendency to form complex ions. b) difference : m.p : W / Hg Hard / soft : Fe , Ti / Cu , Au , Ag Reactivity & oxides : Cu / Fe ; Fe2O3 / CrO3

  4. The Transition Metals (2)Electron configurations : 4s before 3d (3)Oxidation states most common : +2 , +3 ( +2 ～ +7 ) more than one oxidation states (4)Reduction Potentials ─────→ period reducing ability ↓ ( Zn , Cr ) ∵ Zeff↑  r ↓ ; IE ↑

  5. Coordination Compounds └→ colored & paramagnetic (often) consists of a complex ion • Coordination compounds are neutral species in which a small number of molecules or ions surround a central metal atom or ion. ex. [Co(NH3)5Cl]Cl2 complex ion : [Co(NH3)5Cl]2+

  6. .. .. .. H2O , NH3 , :Cl－ .. .. Coordination Compounds coordinate covalent bond Complex ion = metal cation + ligands e acceptore donor center (one)surrounding (  2 ) transion metal Lewis acidLewis base [ Co(NH3)5Cl ]Cl2 ionic force counter ions central metal ligands complex ion

  7. 20.3 Coordination Compounds (2) Coordination number : The # of donor atoms surrounding the central metal The most common : 4or6 (3) Ligands : A neutral molecule or ion having a line pair that can be used to from a bond to a metal ion. monodentate : H2O , NH3 bidentate : en , ox polydentate : EDTA  Chelating agents

  8. Bonding in complex ions : Isomerism The Localized Electron Model depend on coordination number

  9. [Cr(NH3)5SO4]Br [Cr(NH3)5Br]SO4 Fig20.10 Fig20-11&12 Fig20-16&17

  10. Fig 20-10

  11. Fig 20-11

  12. Fig 20-12

  13. Fig 20-16

  14. Fig 20-17

  15. ● ● ● ● ● ● ● ● ● ● D The Crystal Field Model • Explains the bonding in complex ions soleoy in terms of electrostatic forces. • Two types of electrostatic forces : attraction : ( M＋) & ( ligand ion －or ligand ： ) repulsion : ( ligand ： ) & ( metal e in d orbitals ) • Consider : octahedral complexes

  16. The Crystal Field Model

  17. The Crystal Field Model

  18.        The Crystal Field Model Co3＋, Fe2＋, Fe3＋ eg ─ ─ E Large Deg ─ ─ E Small D t２g— ─ ─ t２g— ─ ─ Strong field (Low spin) Weak field (High spin) (a) (b) CN－> NO2－> en> NH3> H2O> OH－> F－> Cl－> Br－> I－ Spectrochemical series ─→ weak field ligands

  19. The Crystal Field Model Spectrochemical series: a list of ligands arranged in order of their abilities to split the d orbital energies CO > CN－> en > NH3> H2O > F－> OH－> Cl－> Br－> I－ strong-field ligands weak-field ligands Magnetic properties paramagnetic dimagnetic High spin  more paramagnetic

  20. The Crystal Field Model • Color: arise when complexes absorb light in some portion of the visible spectrum. • ex. [Cu(H2O)6]2+ → blue • D = E = hn • ex.[Ti(H2O)6]3+ max absorption at 498 nm

  21. The complex ion Ti(H2O)63+

  22. The Crystal Field Model

  23. The Crystal Field Model

  24. The Crystal Field Model

  25. 13-1 Metal-Chelate Complexes EDTA forms strong 1:1 complexes with most metal ions

  26. Lewis acid: • Lewis base:

  27. Metal-ATP complex

  28. Figure 13-3 Synthetic chelate covalently attached to an antibody carries a metal isotope (M) to deliver lethal doses of radiation to tumor cells.

  29. Figure 13-4 Iron(III)-enterobactin complex.

  30. Chlorophyll is a porphyrin complex

  31. Representation of the myoglobin molecule

  32. Representation of the hemoglobin structure

  33. Useful chelating agents

  34. Box 13-1 Chelation Therapy & Thalassemia • A successful drug for iron excretion

  35. 13-2 EDTA (ethylenediaminetetraacetic acid, a hexadentate) • The most widely used chelating agent in titration • Forms strong 1:1 complexes regardless of the charge on the cation

  36. Complexes: Formation Constant (Kf)stepwise formation constants (Ki)

  37. For EDTA • Multidentate chelating agents form stronger complexes (Kf ) with metal ions than bidentate or monodentate ligands. • Neutral EDTA is a tetrabasic acid • Metal-EDTA complex is unstable at both low & high pH. • At low pH • H+ competes with M n+ • At high pH • OH- competes with EDTA

  38. (4) Auxiliary complexing agents: prevent metal ions from precipitating. • Pb2+ as example: • At pH 10, tartrate is present to prevent Pb(OH)2 • Pb-tartrate complex must be less stable than Pb-EDTA

  39. 13-3 Metal Ion Indicators • Metal ion indicator: a compound whose color changes when it binds to a metal ion. • For an useful indicator, it must bind metal less strongly than EDTA does.  the indicator must release its metal to EDTA • Example: MgIn + EDTA  MgEDTA + In • Indicator is pH dependent. • If metal block the indicator, use back titration.

  40. Most indicators can be used only in certain pH ranges.

  41. Demonstration 13-1 Metal Ion Indicator Color Changes P.294

  42. Demonstration 13-1 Metal Ion Indicator Color Changes COLOR PLATE 8 Titration of Mg2+ by EDTA, Using Eriochrome Black T Indicator (a) Before (left), near (center), and after (right) equivalence point. (b) Same titration with methyl red added as inert dye to alter colors. 

  43. 13-4 EDTA Titration Techniquesare useful for the determination of [metal] • Direct titration • Titrate with EDTA • Buffered to an appropriate pH • Color distinct indicator • Auxiliary complexing agent • Back titration (example at p295) • Excess EDTA, & titrate with metal ion • For analyte • ppt in the absence of EDTA : • Ex: (Al3+-EDTA) at pH 7, indicator Calmagite) back titration with Zn2+ • react slowly with EDTA • block the indicator

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