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Chapter 13

• EDTA Titrations
• Its a Complex World Out There

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Terms

• Ligand - Each of the atoms or groups attached
  to the central (usually the metal) atom of a
  co-ordination complex. (OED)
• Lewis Acids. Electron pair acceptors (Metal
  Ions)
• Lewis Bases. Electron pair donators the
  ligands

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Terms

• Monodentate one atom per ligand molecule.
• Multidentate more than one ligand atom per
  ligand molecule. (Chelating ligand)
• Applied to a group (ligand) that loops round a
  central metal ion to be attached at two or more
  points, and also to the co-ordination compound so
  formed. Hence as n., a chelate compound.
• Chelate - Having chelæ or prehensile claws.
• Most Transition metals bind six ligand atoms.

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Examples of Ligands and complexes.

• Cyanide
• Ethylenediamine
• EDTA (ethylenediaminetetraacetic acid)

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Example of Complexing ligands in nature and
Medicine

• Nonactin - an ionophore
• Ferrioxamine B

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EDTA Acid Base Properties

• EDTA is hexaprotic.
• pKas are 0.0, 1.5, 2.0, 2.66, 6.16 and 10.24
• First four are for carboxylate protons
• The neutral compound is tetraprotic.
• A common salt is Na2H2Y.2H2O
• The Y4- form is the one that binds

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EDTA

• The fraction in that form is

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Fractional Form

• Hn PK Format

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Conditional Formation Constants
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Conditional Formation Constants

• Since these are a function of pH the control of
  pH allow us to determine the strength of the
  metal ligand complex.
• This can be seen in the EDTA titration of Ca2

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Most Metal Ions will form Hydroxides

• Auxiliary Complexing agents are used.
• Ammonia for zinc for example.

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Indicators

• Same function as with acid base indicators but
  with excess EDTA you remove a metal from the
  indicator thus changing its color.

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Techniques

• Direct Titration
• Back Titration (Al ion with excess EDTA) Titrate
  excess EDTA with Zn
• Displacement Titration Metal ion is place in a
  solution with excess CaEDTA. The Ca displaced is
  then titrated with standard EDTA.
• Indirect Titration SO4 _- Add Excess Ba
  filter and boil solid with excess EDTA. Amount
  of excess EDTA determined by titration with Mg

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Techniques

• Masking
• Other ligands are added and block an interfering
  metal ion from reacting with the EDTA
• These systems can get very complex but and work
  quite well - Follow cook book procedures. You
  would want to validate you procedure.