EDTA - Complexometric titrations many metals form complex...

Info iconThis preview shows pages 1–6. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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 • can’t 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 ƒ (pH) • specie that reacts with metal depends on pH Fraction of EDTA species ƒ (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-...
View Full Document

Page1 / 13

EDTA - Complexometric titrations many metals form complex...

This preview shows document pages 1 - 6. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online