EDTA Titrations Chapter 13

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EDTA Titrations Chapter 13
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Metal-Chelate Complexes

• Metal ions are Lewis acids, the corresponding
  Lewis bases are electron donating ligands.
• Monodentate ligand binds to a metal ion through
  only one metal ion.
• Multidentate or chelating ligand attaches to a
  metal ion through more than one atom.

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Chelate Effect

• Chelating is the ability of multidentate ligands
  to form more staple metal complexes than those
  formed by monodentate ligands.
• These reactions happen over the monodentate
  because of favored thermodynamics.

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Thermodynamic favorable

• The delta Hs for mono and multidentates are
  generally comparable.
• However, the delta S s (entropy) favors a
  reaction with the multidentate.
• ?G ?H - T?S

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Chelation in Medicine

• Chelation therapy was first introduced in Germany
  in the 1930s to combat heavy metal poisoning.
• First used for good in the U.S.A. in the late
  forties.
• Utilized in medical treatments to help remove
  lead from the systems of patients.
• EDTA is the chelation agent of choice.

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EDTA

• Ethylediaminetetraacetic acid
• Most widely used chelating agent
• Forms 11 complexes
• Through titration or indirect reactions, EDTA can
  quanitatively measure nearly all elements.

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EDTA Structure
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EDTA Complexes

• Equilibrium constant for the reaction of a metal
  with a ligand is called the formation constant.
• Mn          Y-4   ?   MYn-4
• Kf (MYn-4)/(Mn)(Y-4)
• Pb2    CaY-2 ?  PbY-   Ca2  
•     K 108 
• The Pb2 ion replaces the Ca2 ion because K f
  for the lead complex is greater than the calcium
  complex

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EDTA Titration Curve
Region 1
Excess Mn left after each addition of EDTA.
Conc. of free metal equal to conc. of unreacted
Mn.
Region 2
Equivalence pointMn EDTA Some free Mn
generated by MYn-4 ? Mn EDTA
Region 3
Excess EDTA. Virtually all metal in MYn-4 form.
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Metal Ion Indicators

• Compounds whose color changes when they bind to a
  metal ion.
• The color change singles the end point
• Most indicators can only be used in a certain pH
  range.

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EDTA Titration Techniques

• Direct titration analyte is titrated with
  standard EDTA.
• Back titration a known excess of EDTA is added
  to the analyte.
• Displacement titration For metal ions that do
  not have a satisfactory indicator.
• Indirect titration Anions that precipitate with
  certain metal ions.

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• Masking reagent that protects some component of
  the analyte from reaction with EDTA.
• Demasking releases metal ion from a masking
  agent.

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Sources

• I will never tell !
• Textbook, Quantitative Chemical Analysis
• http//www.chem.wits.ac.za/chem201/Complexation20
  reactions.ppt52