Q1(g). Chelators in complexometric titrations

Answer in brief

Multidentate ligands form stable metal chelates with definite stoichiometry, enabling quantitative complexometric titration.

Key points

• A chelating agent binds one metal through two or more donor atoms, producing one or more chelate rings.
• EDTA normally uses two N and four O donors and gives 1:1 metal:ligand stoichiometry: Mⁿ⁺ + Y⁴⁻ ⇌ MY⁽ⁿ⁻⁴⁾⁺.
• Large conditional formation constants favour a sharp decrease in unbound metal concentration near equivalence; buffering controls ligand protonation.
• Chelators can also mask interfering ions. A metal indicator changes colour when EDTA displaces it from its weaker metal complex.