Chem 23 M9

• Complexes or coordination compounds are formed when metal ions react with a ligand.
• Ligands are ions or molecules that have a pair of unshared electrons capable of satisfying the coordination number of the cation.
• The amount of covalent bonds a cation form is dictated by the coordination number, which has a value of typically two, four, and six.
• Titrations based on complex formation, sometimes called complexometric titrations.
• Equilibrium constant for complex formation is called formation constant (Kf ). The formation constant (Kf ) has the same general form as any other equilibrium constant expression.
• For any equilibrium, the larger the value of the equilibrium constant (in this case, Kf ), the more stable the product.
• Metal –ligand complex forms with coordinate covalent bonds
• Most unidentate ligands do not have an easily identifiable end point.
• Polydentate ligands provide sharper end point.
• Polydentate ligands provide sharper end point.

  • Contain two or more atoms that can simultaneously coordinate to a central metal ion, occupying two or more coordination spheres.

  • Chelating agents • Because of the multiple bonding site, polydentate ligands react strongly with cations, thus providing sharper end points
• EDTA is a hexadentate ligand with six binding sites, four negatively charged oxygen atoms in the carboxylate groups and two nitrogen atoms in the tertiary amino groups.
• EDTA is often depicted as H4 Y (the neutral form - tetraprotic) Fully deprotonated EDTA is represented by Y4–.
• All metal–EDTA complexes have a 1:1 stoichiometry
• Based on the conditional formation constant, cadmium EDTA titration is not possible below pH 4.
• Because of the pH-dependency of the formation constant of metal-EDTA complexes, metal ions can be analyzed by preparing a titration set-up at different pH and using appropriate indicators.
• EDTA Titration Techniques could be classified as: Direct titration, Back Titration, Displacement titration and Indirect Titration.
• In direct titrations you simply add an indicator to a buffered solution of the metal ion and titrate with standard EDTA.
• In a back titration an excess of EDTA is added to the sample solution containing an analyte metal ion. The excess EDTA is then titrated with a known concentration of a second metal ion that forms a weaker complex with EDTA, so to avoid displacement of analyte ion. Back titration is useful when the analyte metal ion binds the indicator permanently, or when the metal-EDTA complex is unstable or when the metal ions form precipitates in the without EDTA.
• Displacement titration can be used if the analyte metal ions do not have a good indicator. In this case, a metal ion-EDTA complex is added. The bound metal ions are released when the analyte metal ions react with the complex. The release metal ion, usually Magnesium, is titrated with standard EDTA,
• Indirect Titration is useful for anions which for insoluble precipitate like sulfate. Sulfate can be reacted with known amounts of Barium chloride. The precipitate BaSO4 is filtered, and the remaining solution is titrated with standard EDTA solution.
• The most common method of determining the amount of calcium present in milk is through the use of EDTA titrimetry.
• Calcium reacts with EDTA forming a stable chelate (CaEDTA). This reaction is reversible, but it is favored towards the right side because CaEDTA has a lower free energy than CaCl2 + EDTA.
• When the molar ratio between Ca and EDTA is 1:1, all the calcium will be removed from the solution.