The EDTA (ethylene diamine tetraacetic acid) method for estimating the hardness of water involves a complexometric titration. In this titration, a standardized solution of EDTA is added to a water sample containing calcium and magnesium ions until the metal ions form stable complexes with the EDTA molecules. The point at which all the metal ions have reacted with the EDTA is indicated by a color change, typically using a metal ion indicator such as Eriochrome Black T. The volume of EDTA solution used to reach this endpoint can then be used to calculate the concentration of calcium and magnesium ions, and consequently, the water hardness.

 

Here's a step-by-step guide on how to perform the EDTA titration for water hardness estimation:

 

Materials and Equipment:

1. Sample of water
2. Standardized EDTA solution
3. Eriochrome Black T indicator solution
4. Buffer solution (pH buffer, usually around pH 10)
5. Erlenmeyer flask
6. Burette
7. Pipette
8. Conical flask
9. Graduated cylinder
10. Stirring rod

 

Procedure:

1. Preparation of the Sample:

   • Measure a known volume (usually 50 or 100 mL) of the water sample using a graduated cylinder and transfer it to a clean conical flask.
   • If the water sample contains any suspended solids, filter it to obtain a clear solution.

2. Addition of Indicator and Buffer:

   • Add a few drops of Eriochrome Black T indicator to the water sample in the conical flask.
   • Add a few milliliters of buffer solution to adjust the pH of the solution to around 10. The buffer helps maintain a constant pH, which is important for accurate results.

3. Titration with EDTA Solution:

   • Fill the burette with the standardized EDTA solution.
   • Slowly add the EDTA solution to the conical flask while stirring continuously. The indicator will initially give the solution a wine-red color.

4. Endpoint Detection:

   • As you approach the endpoint, the color of the solution will change from wine-red to blue. This indicates that all the calcium and magnesium ions have formed complexes with the EDTA molecules.
   • The blue color change is typically quite distinct and easy to observe.

5. Calculations:

   • Note the volume of EDTA solution (in mL) required to reach the endpoint. This volume represents the amount of EDTA needed to react with the calcium and magnesium ions in the water sample.
   • Use the volume of the sample taken and the volume of the EDTA solution to calculate the concentration of calcium and magnesium ions in the water sample.

 

Remember that accurate results depend on careful technique, proper calibration of equipment, and accurate standardization of the EDTA solution. The choice of indicator and pH buffer is also critical to achieving a sharp and distinct color change at the endpoint. Additionally, ensure that you follow established laboratory procedures and safety precautions while performing the titration.

 

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