17.3 Complexometric (EDTA) Titrations: Conditional Constants, Indicators, Masking

17.3.1 Complex formation, chelates and unidentate titrations ★★

• With a unidentate ligand (NH₃ with Cu²⁺) the steps overlap, no single complex dominates and the titration curve has no sharp break. A chelating (multidentate) ligand binds through several donor atoms, closes 5- or 6-membered rings and forms one complex in one step: large K (chelate effect, mostly entropy), fixed 1:1 stoichiometry, sharp end point Geo 2021 Q1(g) 5m.SN
• Cyanide with Ag⁺ (Liebig): Ag⁺ + 2CN⁻ → [Ag(CN)₂]⁻; once free CN⁻ is used up, Ag⁺ + [Ag(CN)₂]⁻ → Ag[Ag(CN)₂]↓, a permanent faint turbidity = end point. n(CN⁻) = 2n(Ag⁺); keep the solution alkaline (no HCN). Denigès: NH₃ + a little KI, end point = first AgI turbidity, sharper Geo 2023 Q8(a)(i) 10m.SN
• Chloride with Hg²⁺: Hg²⁺ + 2Cl⁻ → HgCl₂ (soluble, barely ionised); excess Hg²⁺ is shown by diphenylcarbazone (violet).

17.3.2 EDTA as an acid and the conditional formation constant ★★★

• EDTA is hexaprotic as H₆Y²⁺: pK_a 0.0, 1.5, 2.0, 2.66 (COOH) and 6.16, 10.24 (NH⁺). Na₂H₂Y·2H₂O is the primary-standard salt. Every metal ion, whatever its charge, forms a 1:1 complex: Mⁿ⁺ + Y⁴⁻ ⇌ MY⁽ⁿ⁻⁴⁾⁺ Geo 2025 Q9(b).SN

• α(Y⁴⁻) = 0.36 at pH 10, 2.3 × 10⁻⁵ at pH 6, 3.8 × 10⁻⁹ at pH 4, 1.9 × 10⁻¹⁸ at pH 1. A sharp titration needs log K′_f ≳ 8, so each metal has a minimum pH: Fe³⁺ about 1.5, Zn²⁺ 4.5, Ca²⁺ 7.5, Mg²⁺ 9.5. This is the basis of selectivity by pH Geo 2022 Q8(a) 10m.SN
• At pH 6 (0.10 M EDTA, [Y⁴⁻] = 2.3 × 10⁻⁶ M): α = 2.3 × 10⁻⁶/0.10 = 2.3 × 10⁻⁵; most free EDTA is H₂Y²⁻ (0.059 M) and HY³⁻ (0.041 M) Geo 2023 Q8(a)(ii) 5m.SN
• FeY⁻ (K_f 1.3 × 10²⁵): K′ = 3.8 × 10⁻⁹ × 1.3 × 10²⁵ = 4.9 × 10¹⁶ at pH 4.00; 1.9 × 10⁻¹⁸ × 1.3 × 10²⁵ = 2.5 × 10⁷ at pH 1.00: still titratable in acid, where Ca, Mg and Zn are not.

17.3.3 EDTA titration curves ★★★ 15-marker

1. Start: pM = −log C_M (pCa 2.00).
2. Before equivalence: free M = untitrated metal (dilution included); the small dissociation of MY is ignored. At 25 mL: [Ca²⁺] = 0.25 mmol/75 mL, pCa 2.48.
3. Equivalence: only MY; [M] = C_EDTA = x, x = √([MY]/K′). pH 10: K′ = 0.36 × 10^10.69 = 1.8 × 10¹⁰, [CaY] = 0.0050 M, [Ca²⁺] = 5.3 × 10⁻⁷ M, pCa 6.28.SN
4. After: pM = log K′ + log([MY]/C_{EDTA, excess}): a plateau set by K′.SN

• The jump grows with K′ (higher pH, larger K_f) and with concentration. An auxiliary ligand (NH₃ in the buffer, tartrate, citrate) keeps metals such as Zn²⁺ or Pb²⁺ from precipitating as hydroxides but lowers the jump before the equivalence point: α_M < 1, K″ = α_Mα_YK_f Geo 2021 Q8(a) 15m.SN