Q10(c). Why Al cannot be titrated directly with EDTA

1. Kinetics: [Al(H₂O)₆]³⁺ is small and highly charged (r=53 pm, charge 3+), with a water-exchange rate of only about 1 s⁻¹; Ba²⁺ (r=135 pm, 2+) exchanges water at ≈10⁹ s⁻¹. The Al–EDTA reaction is slow in the cold: the end point would be overshot.
2. Hydrolysis: EDTA binds Al³⁺ efficiently only at pH 4–5 (Y⁴⁻ is present as H₂Y²⁻); at pH 7–10, where metal indicators work, Al³⁺ precipitates as Al(OH)₃ or forms [Al(OH)₄]⁻, which does not react with EDTA quickly.
3. Indicator blocking: Al³⁺ binds the indicators (Eriochrome Black T, xylenol orange) irreversibly so the end-point colour does not return.
4. Stability: log K_f: AlY⁻ 16.1 (large) but BaY²⁻ 7.8 (modest); stability alone is therefore not the problem. BaY²⁻ is complete only in alkaline medium (pH 10–11, EBT or phthalein purple).

Estimation of Al by back titration

Add a measured excess of standard EDTA to the aluminium solution; adjust to pH 3–4 with acetate buffer; boil 5 min to complete complexation; cool, pH 5–6; titrate the unreacted EDTA with standard Zn²⁺ (or Pb²⁺) using xylenol orange (yellow → red-violet) or dithizone:

Al³⁺ + H₂Y²⁻ → AlY⁻ + 2H⁺, H₂Y²⁻(excess) + Zn²⁺ → ZnY²⁻ + 2H⁺

a displacement titration, adding excess ZnY²⁻ (or MgY) to release an equivalent Zn²⁺, is also used, and NaF is then added to release EDTA from the Al complex (demasking).