1. EDTA Titrations Chapter 13

2. 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.

3. 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.

4. Thermodynamic favorable • The delta H’s for mono and multidentates are generally comparable. • However, the delta S’ s (entropy) favors a reaction with the multidentate. • ΔG° = ΔH° - TΔS°

5. Chelation in Medicine • Chelation therapy was first introduced in Germany in the 1930’s 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.

6. EDTA • Ethylediaminetetraacetic acid • Most widely used chelating agent • Forms 1:1 complexes • Through titration or indirect reactions, EDTA can quanitatively measure nearly all elements.

7. EDTA Structure

8. EDTA Complexes • Equilibrium constant for the reaction of a metal with a ligand is called the formation constant. • M+n     +     Y-4   MYn-4 • Kf = (MYn-4)/(M+n)(Y-4) • Pb+2  +  CaY-2  PbY- +  Ca+2 •     K ~ 108  • The Pb+2 ion replaces the Ca+2 ion because K f for the lead complex is greater than the calcium complex

9. 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 point:[Mn+] = [EDTA] Some free Mn+ generated by MYn-4  Mn+ + EDTA Region 3 Excess EDTA. Virtually all metal in MYn-4 form.

10. 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.

12. 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.

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

14. Sources • I will never tell ! • Textbook, Quantitative Chemical Analysis • http://www.chem.wits.ac.za/chem201/Complexation%20reactions.ppt#52