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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 e.g. EDTA tetraprotic, always forms 1:1 complexes different stabilities depending on. | Complexometric titrations many metals form complex with a ligand Cu NH3 Cu NH3 Kf1 Cu NH3 2 NH3 cU NH3 22 Kf Cu NH3 22 NH3 Cu nH3 32 Kf3 Cu NH3 32 NH3 Cu NH3 42 Kf4 M L ML _ ML 1 M L ML L ml2 k ML2 2 ML L ML2 L -2 ML3 ML3 3 ML2 L ML _1 L ML k - ML 1 MLn L Zn NH3 complexes vs lg NH3 log NHsJ 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 e.g. EDTA tetraprotic always forms 1 1 complexes different stabilities depending on metal usually use more soluable sodium form Na2H2Y H4Y is insoluble HO2CCH2 CH2CO2H 0.0 hnch2ch2nh p q 1.5 HO2CCH2 ch2co2ii PK3 2.0 h6y2 p 4 2.66 pK5 6.16 pK6 10.24 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 H4Y H H3Y Kai 1.0 X 10 2 LH41J __ _ H H2Y2 HjY- H H2Y2 Ka2 2.2 X 1Q- I Y J ------------------------------------------------------ . . -1 H HY3- H2Y2- H HY3 Kai 6.9 X IO 7 2 __ H2Y J . . - 1 H Y4- HY H Y4 Ka4 5.5 X 10 I ----------------------------------------------------- Fraction of NaEDTA species f pH 0 2 4 6 8 10 12 14 pH specie that reacts with metal depends on .
