Learn about the estimation of the permanent hardness of water by the EDTA complexometric method through a laboratory experiment or practical.
Aim:
To determine the amount of total hardness present in a given sample of water by the EDTA titration method.
Requirements for a water hardness test:
Glasswares: burette, burette stand, conical flask, volumetric pipette, beaker, volumetric flask, funnel, glass rod, wash bottle, etc.
Reagents: LR grade ethylenediaminetetraacetic acid (C10H16N2O8) solution, standard solution of calcium carbonate (CaCO3), erichrome black-T indicator (C20H12N3O7SNa), buffer solution, etc.
Apparatus: Digital/analytical balance.
Theory:
At pH = 9-10, EDTA forms stable, colorless complexes with Ca2+ and Mg2+ ions present in water. To maintain the pH of the solution between 9 and 10, a buffer solution of NH4Cl + NH4OH is used. As an indicator, Eriochrome Black-T (EBT) is used. The water sample has to be treated with buffer and EBT indicator, which form unstable, wine-red colored complexes with the Ca2+ and Mg2+ present in the water.
Procedure for standardization of EDTA:
- Pour 20 ml of standard hard water into a conical flask using a pipette.
- Then, add 5 ml of buffer solution and 2-4 drops of EBT indicator. The initially blue indicator would become wine-red.
- Titrate it with EDTA solution until the wine-red color turns blue, which is the endpoint. Consider the burette reading to be V2 ml.
- Repeat the titration three times to get accurate results, properly record the readings of the burette and calculate.
Procedure for the determination of total hardness:
- Instead of standard hard water, repeat the above titration procedure using hard water samples. Consider the EDTA burette reading to be V3 ml.
- The procedure for the determination of permanent hardness:
- Measure the 100 ml sample of hard water and pour it into a beaker.
- To remove temporary hardness, boil it to about half the volume and then cool it to ambient temperature.
- Using filter paper, remove insoluble CaCO3 and MgCO3 by filtration.
- Make up the 100 ml volume by adding distilled water.
- Transfer 20 ml of this solution to a conical flask using a pipette.
- Then, repeat the above titration process steps as mentioned. Consider the burette reading to be V4 ml.
Observation table:
|
Sr. No. |
Content in conical flask |
Burette reading |
Volume of titrant used (ml) |
|
|
Initial |
Final |
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1 |
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|
|
|
|
2 |
|
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|
|
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3 |
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|
|
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Mean: |
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Calculations:
Calculation for standardization of EDTA:
M1V1 = M2V2
Where,
M1 is the molarity of standard hard water.
V1 is the volume of standard hard water in a conical flask
M2 is the molarity of EDTA.
V2 is the volume of EDTA consumed.
Calculation for estimation of permanent hardness:
M2V2 = M4V4
Where,
M4 is the permanent hardness of sample water
V4 is the volume of the hard water sample in the conical flask.
Result:
The hardness of the given water sample has been determined to be:
- Total hardness: ______ppm
- Permanent hardness: ____ ppm
- Temporary hardness: _____ppm
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