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Water hardness by EDTA titration

Titrate a tap-water sample with 0.01000 M EDTA at pH 10 with Eriochrome Black T, watch the wine red turn through violet to pure blue, and report the total hardness as CaCO₃. Then use a calcium-only standard to discover why a little magnesium is needed.

Difficulty ●●○ ≈ 20 min

Aim Determine the total hardness of a water sample (mg/L as CaCO₃) and show how the Eriochrome Black T endpoint depends on magnesium.
pH 10 ammonia buffer · Eriochrome Black T · 0.01000 M EDTA

Water hardness by EDTA titration

Flask: 50.00 mL of water + 2 mL of pH 10 buffer + 3 drops of Eriochrome Black T

05101520253035404550STIR50100150200
Burette reading 0.00mL
Colourwine red

The indicator right now

Bound to metal MIn⁻ (wine red) 100 %
Free HIn²⁻ (blue) 0 %

Wine red → violet → pure blue. The endpoint is when the last trace of red disappears and the solution is pure blue.

Computed pM curves and the EBT colour-change range

EBT transition (pMg) 024681012EDTA volume / mL246810pM = −lg[M²⁺]

pMg pCa

Computed: after every drop the competition of Ca²⁺ and Mg²⁺ for EDTA is solved (conditional constants K′ = K_f·α(Y⁴⁻) at pH 10; lg K_f: CaY 10.69, MgY 8.79), then Eriochrome Black T is shared out with MgIn⁻ 1.0 × 10⁷, CaIn⁻ 2.5 × 10⁵ and the indicator’s acid constants (pKa 6.3, 11.55); the flask colour comes from each form’s teaching-fit bands via Beer–Lambert and CIE colorimetry. The sample’s Ca (20–120 mg/L) and Mg (3–30 mg/L) are set by its number. Mg(OH)⁺, activity coefficients and the indicator’s tiny effect on the metal balance are ignored; the drops and the brief colour where they land are illustrative.

Procedure

  1. Choose a sample number. The flask holds 50.00 mL of water, 2 mL of pH 10 NH₃–NH₄Cl buffer and 3 drops of Eriochrome Black T; the solution is wine red.
  2. Make a rough run in 1.00 mL steps to find roughly where the colour turns violet; refill, and near the end add single drops.
  3. Record the volume when the last trace of red disappears and the solution is pure blue; total hardness = c(EDTA)·V ÷ 50.00 mL × 100.09 g/mol.
  4. Reveal the true calcium and magnesium contents and compare.
  5. Switch to the calcium standard and titrate once without and once with Mg–EDTA; compare how sharp and where the two endpoints are.

What to notice

  • The solution stays wine red until close to equivalence: the indicator is bound to magnesium, and EDTA takes the free calcium and magnesium first and the indicator’s magnesium last.
  • The colour goes from violet to pure blue within less than 0.1 mL, where the pMg jump crosses the indicator’s range (about pMg 4.4–6.4).
  • With calcium alone the red starts turning violet long before equivalence and the endpoint drags; a little Mg–EDTA makes it sharp again without changing the volume.

In a real lab

  • The ammonia buffer is pungent — use it with ventilation; disodium EDTA should be dried at 80 °C or standardised against calcium carbonate.
  • Eriochrome Black T solutions decompose on standing; make them fresh or use a solid indicator mixture.
  • Cu²⁺, Fe³⁺ and Al³⁺ in the water block the indicator (no endpoint): mask them with triethanolamine, or a little KCN (highly toxic — only under expert supervision).
  • Near the endpoint the reaction is slowish, especially in cold water: swirl well between drops and warm to about 40 °C if needed.

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