Upper secondary · 15 min

How hard is this water? An EDTA titration

Drip EDTA into a water sample buffered at pH 10; while Ca²⁺/Mg²⁺ are still free the indicator stays wine red, then snaps to blue at the endpoint.

Goal

Use $n(\mathrm{EDTA}) = n(\ce{Ca^2+} + \ce{Mg^2+})$ to turn a burette volume into a hardness in mol/L, and see how sample hardness moves the endpoint.

Apparatus and reagents

Virtual bench: 25 mL water sample, pH 10 ammonia buffer, Eriochrome Black T, and a burette of EDTA at adjustable concentration.

Procedure

Keep the EDTA at 0.010 mol/L and hardness at 0.010 mol/L. Add EDTA slowly and find the volume where the colour snaps.
Check that this endpoint lands at 25.0 mL: equal concentrations mean equal volumes at equivalence.
Raise the hardness to 0.020 mol/L. Predict the new endpoint before you titrate, then confirm it.
Now double the EDTA concentration and repeat: what happens to the endpoint volume and to the sharpness of the jump?

What to observe

Equal concentrations of EDTA and water hardness give an endpoint at exactly 25.0 mL — one EDTA chelates one metal ion.
Doubling the hardness doubles the endpoint volume; doubling the EDTA concentration halves it — the product $C \times V$ is what counts.
Past the endpoint the solution stays coloured: the free indicator's blue form dominates once no free metal is left.

Explanation

EDTA is a hexadentate claw: one molecule wraps one metal ion with a stability constant so large ( $\log K_\mathrm{CaY} \approx 10.7$ ) that the titration is quantitative. Eriochrome Black T is wine red while bound to Mg²⁺ and pure blue when free, so the endpoint appears exactly when the last metal ion is captured. Hardness is then $c(\ce{Ca^2+}+\ce{Mg^2+}) = c_\mathrm{EDTA}V_\mathrm{EDTA}/V_\mathrm{sample}$ — usually converted to mg/L as $\ce{CaCO3}$ for reporting.

History of the experiment

The chelating agent EDTA was patented in Germany in 1935, and in 1945 Gerold Schwarzenbach at ETH Zurich showed it could titrate metals quantitatively — founding complexometric analysis. Alfred Werner had laid the theoretical basis decades earlier with his 1913 Nobel-winning theory of coordination compounds.

Chemists behind it

Alfred Werner Jöns Jacob Berzelius