Hardness of water (EDTA titration)

A/L Chemistry · G.C.E. Advanced Level · Grades 12–13

Aim

To find the hardness of a water sample by EDTA titration.

Apparatus

• 0.0100 mol dm⁻³ EDTA
• 50.0 cm³ water sample and NH₃/NH₄Cl buffer
• Eriochrome Black T indicator

Method

1. Pipette 50.0 cm³ of water, add buffer and indicator (wine red).
2. Add EDTA while swirling.
3. Stop when the colour turns clear blue.
4. Calculate c(Ca²⁺ + Mg²⁺) and mg CaCO₃ per dm³.

Conclusion

EDTA forms 1:1 complexes with Ca²⁺ and Mg²⁺; hardness = c × 100 090 mg CaCO₃ dm⁻³. Above about 300 the water is very hard.