EDTA - Complexometric titrations many metals form complex...

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Unformatted text preview: Complexometric titrations many metals form complex with a ligand: Cu ++ + NH 3 ! Cu(NH 3 ) ++ K f 1 Cu(NH 3 ) 2+ + NH 3 ! Cu (NH 3 ) 2 2+ K f 2 Cu (NH 3 ) 2 2+ + NH 3 ! Cu (NH 3 ) 3 2+ K f 3 Cu (NH 3 ) 3 2+ + NH 3 ! Cu (NH 3 ) 4 2+ K f 4 Zn (NH 3 ) complexes vs lg NH 3 " theoretically could titrate Zn with ammonia stepwise K f very similar difficult to know stoichiometry to ensure all one form, need excess NH 3 titration never gives good equivalence point cant use monodentate ligand to titrate metals Titrations with EDTA use multidentate ligands e.g. EDTA tetraprotic, always forms 1:1 complexes different stabilities depending on metal usually use more soluable sodium form Na 2 H 2 Y (H 4 Y is insoluble) Example EDTA- metal complex Mn-EDTA Co-EDTA EDTA complexes 1:1 complexes can form up to six bonds using lone pairs in each oxygen of --COOH and each N lone pairs donated into empty metal orbitals 1:1 mole ratio simplifies calculations Most metal chelates - very stable - many sites for bonding charge on EDTA-metal depends on metal & pH EDTA - weak acid, ionizes stepwise Fraction of NaEDTA species (pH) specie that reacts with metal depends on pH Fraction of EDTA species (pH)- the last two protons " pH The formation constant vary pH of EDTA solution different forms of EDTA present smaller stability constant complexes need higher pH to give particular form of complex Y 4- present at pH>10 H 4 Y has low solubility in water Na 2 H 2 Y 2H 2 O usually used (i.e. two acid groups neutralized) dissociates to H 2 Y 2- to give a solution of pH 4-5 lg formation constants for various metals numbers large so chelates strong metal chelates very stable - many sites for bonding charge on EDTA-metal complex depends on metal and pH M + + H 2 Y 2-...
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This note was uploaded on 02/25/2012 for the course CHEM 232 taught by Professor Michel, r during the Fall '08 term at UConn.

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EDTA - Complexometric titrations many metals form complex...

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