Q1(g). Displacement EDTA titration

Answer in brief

An analyte metal displaces Mg²⁺ from Mg-EDTA; the released Mg²⁺ is titrated with EDTA. Calcium provides a suitable example.

Key points

• Choose an analyte whose EDTA complex is more stable than MgY²⁻ under the selected pH conditions; Y⁴⁻ denotes EDTA.
• Displacement: Ca²⁺ + MgY²⁻ ⇌ CaY²⁻ + Mg²⁺. The larger Ca-EDTA formation constant favours release of Mg²⁺.
• Add sufficient Mg-EDTA, buffer near pH 10 with NH₃/NH₄Cl, and add Eriochrome Black T.
• Titrate released Mg²⁺ with standard EDTA: Mg²⁺ + Y⁴⁻ → MgY²⁻. The colour changes from wine red to blue.
• Stoichiometry is 1 mol Ca²⁺ : 1 mol liberated Mg²⁺ : 1 mol EDTA titrant.