Complex Titrations - T3

Titration method based on complex formation due to reaction between metal ions (cation) and complexing agent (ligand)

• Metal Chelate Complexes • Essentially, a Lewis acid-base reaction, in which an electron pair is donated from one chemical to another • Ligand that attaches to a metal ion through more than one ligand atom • Most chelating agents contain N or O • Elements that contain free electron pairs that may be donated to a metal

• Ligand (the donor species or complexing agents) is the groups bound to the central ion in the complex Example of ligand: H2O, NH3, Cl-, Br-, EDTA

A Chelate is produced when ligands are attached to the central ion at two or more coordinating sites to form a five-or six-member heterocyclic ring

• The organic ligands that involved in the coordination • Multidentate ligands • Most widely used chelating agent = EDTA

• Unidentate– A ligand that is attached to a central ion at only one point (1 binding site) E.g. Ammonia • Bidentate– A ligand that is attached to a central ion at two points due to two lone pair electrons (2 binding sites) E.g. Ethylene diamine • Poly/ Multidentate– A ligand that has two or more coordinating sites (more than two binding sites)

• The equilibrium constant for the reaction between a metal ion (Mn+) and a chelating agent (L-P) is known as a formation constant or stability constant or binding constant • It is a measure of the strength of the interaction between the reagents that come together to form the complex • A metal ion, M reacts with a ligand, L and forming a complex, ML in complexation reaction M + L ⇌ ML • Complexation reactions occur in a stepwise fashion followed by additional reactions: ML + L ⇌ ML2 ML2 + L ⇌ ML3 MLn-1 + L ⇌ MLn

• React more completely with cations compared to ligands with a lesser number of donor groups and tend to form 1:1 complexes • These ligands are more satisfacory

• Multidentate complexes are more stable because of the entropy effect dG = dH - TdS where G = free energy H = enthalpy S = entropy • When M-EDTA tetradentate (4 binding sites) complex dissociates, there would be only 2 products = M+ and EDTA ligand. (low entropy). • Whereas when a ML4 complex dissociates there will be 5 products = M + 4L (higher entropy)

Ka increases

• Reg. 1 • Before equivalence point • Mn+ : excess, unreacted • MYn-4 : dissociation - negligible • Reg 2 • At the equivalence point • MYn-4 : dissolves some some free Mn+,[Mn+]=[EDTA] • (MYn-4 ==> Mn+ + EDTA) • Reg 3 • After equivalence point. • EDTA in excess

• We are able to measure the concentration of free ion during titration with EDTA • The titration of a metal ion with EDTA is similar to the titration of a strong acid (M+) with a weak base (EDTA)

• The metal-EDTA complex becomes less stable as pH decreases thus Kf decreases • [Fe3+] = 5.4 x 10-7 at pH 2.0 ===> [Fe3+] =1.4 x 10-12 at pH 8 therefore, more uncomplexed iron at lower pH • In order to get a “complete” titration (Kf ≥106), EDTA requires a certain minimum pH for the titration of each metal ion • End Point becomes less distinct as pH is lowered, limiting the utility of EDTA as a titrant

One type of metal ion (e.g. Fe3+) can be titrated without interference from others (e.g. Ca2+)

• Because EDTA has many forms, when we prepare a solution of EDTA we only know its total concentration, not the concentration of the specific form, such as Y4− • At any pH, a mass balance of EDTA can be performed CEDTA = [H6Y2+] + [H5Y+] + [H4Y] + [H3Y−]+[H2Y2−] + [HY3−] + [Y4-] All 7 forms of EDTA

• (Ethylenediaminetetraacetic acid) • One of the most common chelating agents used for complexometric titrations in analytical chemistry • EDTA has 6 nitrogens & oxygens (4 carboxyl & 2 amine groups) in its structure giving it 6 free electron pairs that it can donate to metal ions • HEXADENTAT • High Kf values • 6 acid-base sites in its structure

• EDTA exists in up to 7 different acid-base forms depending on the solution pH • In practice, EDTA only partially ionizes therefore < 6 coordinate covalent bonds form • In pH <= 12, amine groups remain protonated therefore do not donate electrons • The most basic form (Y4-) is the one which primarily reacts with metal ions. – 4 donated electron pairs

• EDTA forms an octahedral complex with most 2+ metal cations, M2+, in aqueous solution • The main reason that EDTA is used so extensively in the standardization of metal cation solutions is that the formation constant for most metal cation-EDTA complexes is very high, meaning that the equilibrium for the reaction: M2+ + H4Y → MH2Y + 2H+ lies far to the right • Carrying out the reaction in a basic buffer solution removes H+ as it is formed, which also favors the formation of the EDTA-metal cation complex reaction product • For most purposes it can be considered that the formation of the metal cation-EDTA complex goes to completion, and this is chiefly why EDTA is used in titrations / standardizations of this type