Complexometric titrations many metals form complex with a ligand: Cu++ + NH3 ⇔ Cu(NH3)++ Cu(NH3)2+ + NH3 ⇔ Cu (NH3)22+ Cu (NH3)22+ + NH3 ⇔ Cu (NH3)32+ Cu (NH3)32+ + NH3 ⇔ Cu (NH3)42+ Kf1 Kf2 Kf3 Kf4 Zn (NH3) complexes vs lg NH3 α .• theoretically could titrate Zn with ammonia • stepwise Kf very similar • difficult to know stoichiometry • to ensure all one form, need excess NH3 • titration never gives good equivalence point • can’t use monodentate ligand to titrate metals Titrations with EDTA • • • • use multidentate ligands . EDTA tetraprotic, always forms 1:1 complexes different stabilities depending on.
