EDTA Complexometric Titration of Water Hardness

Measuring calcium and magnesium content in water

Titrations Advanced (University) 45 min ~$15.00

Objective

Determine the total hardness of a water sample by titrating dissolved Ca2+ and Mg2+ ions with EDTA using Eriochrome Black T indicator.

Background

EDTA (ethylenediaminetetraacetic acid) forms stable 1:1 complexes with metal ions. In the presence of Eriochrome Black T indicator at pH 10, the solution is wine-red when metal ions are present and turns blue when all metal ions are complexed by EDTA, marking the endpoint.

Safety Warnings

  • Ammonia buffer has a strong odor — use in a ventilated area
  • Wear safety goggles
  • EDTA is generally safe but avoid ingestion

Required PPE

goggles lab_coat

Materials

  • Water sample (250 mL)
    Tap water or well water
  • EDTA solution (100 mL)
    0.01M, disodium salt
  • Eriochrome Black T indicator (5 mL)
    Solid or solution
  • Ammonia buffer (pH 10) (20 mL)
    NH₃/NH₄Cl buffer
  • Distilled water (200 mL)

Equipment

50 mL burette Burette stand and clamp 250 mL Erlenmeyer flask 25 mL pipette White tile

Procedure

1

Pipette exactly 25.0 mL of the water sample into the Erlenmeyer flask.

2 min
2

Add 2 mL of ammonia buffer (pH 10) to the flask.

1 min Ammonia odor — work in ventilated area
3

Add a small amount of Eriochrome Black T indicator. The solution should turn wine-red.

1 min
4

Fill the burette with 0.01M EDTA solution. Record the initial volume.

3 min
5

Titrate by adding EDTA slowly while swirling. The wine-red color will gradually shift.

10 min
6

Near the endpoint, add EDTA drop by drop until the color changes sharply from wine-red to blue.

5 min
7

Record the final volume. Repeat for concordant results.

12 min
8

Calculate total hardness in mg/L CaCO₃ equivalent using the formula: Hardness = (V(EDTA) x C(EDTA) x 100.09 x 1000) / V(sample).

5 min

Expected Results

Typical tap water has 100-250 mg/L total hardness (CaCO₃ equivalent). The color change should be a clear transition from wine-red to blue. Soft water requires less EDTA; hard water requires more.

Cleanup

Dispose of EDTA solutions down the drain (biodegradable at these concentrations). Rinse all glassware.

Details

Category
Titrations
Difficulty
Advanced (University)
Duration
45 min
Est. Cost
$15.00
Steps
8
Materials
5

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Frequently Asked Questions

What is the objective of EDTA Complexometric Titration of Water Hardness?
Determine the total hardness of a water sample by titrating dissolved Ca2+ and Mg2+ ions with EDTA using Eriochrome Black T indicator.
How difficult is EDTA Complexometric Titration of Water Hardness?
This experiment is rated as Advanced (University). It takes approximately 45 minutes to complete.
What safety precautions are needed for EDTA Complexometric Titration of Water Hardness?
Key safety precautions include: Ammonia buffer has a strong odor — use in a ventilated area; Wear safety goggles; EDTA is generally safe but avoid ingestion.
What materials are needed for EDTA Complexometric Titration of Water Hardness?
The main materials required are: Water sample, EDTA solution, Eriochrome Black T indicator, Ammonia buffer (pH 10), Distilled water.
What results should I expect from EDTA Complexometric Titration of Water Hardness?
Typical tap water has 100-250 mg/L total hardness (CaCO₃ equivalent). The color change should be a clear transition from wine-red to blue. Soft water requires less EDTA; hard water requires more.