9.3 - Estimation of Total Hardness using Ethylenediaminetetraacetic Acid Revision Notes for Leaving Cert Chemistry

9.3 - Estimation of Total Hardness using Ethylenediaminetetraacetic Acid

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Experiment Summary

In this experiment, the total hardness of a water sample is determined by titration with a standard solution of EDTA.

Total hardness refers to the concentration of calcium ( $Ca²⁺$ ) and magnesium ( $Mg²⁺$ ) ions in the water, which react with EDTA to form stable complexes.

The endpoint of the titration is detected using the Eriochrome Black T indicator, which changes from wine-red to blue when all the $Ca²⁺$ and $Mg²⁺$ ions have reacted with EDTA.

Materials and Apparatus Required

Chemicals

Water sample (hard water)
0.01 M EDTA solution
Buffer solution (pH 10)
Eriochrome Black T indicator
Deionised water

Apparatus

Pipette (25 cm³) and pipette filler
Burette (50 cm³)
Conical flask (250 cm³)
Philtre funnel
Beakers (250 cm³)
White tile or philtre paper
Graduated cylinder (10 cm³)
Retort stand, boss head, and clamp
Safety glasses

Safety Precautions

Wear safety glasses during the experiment.
EDTA is harmful; avoid contact with skin and eyes.
The buffer solution (pH 10) is corrosive. Avoid inhaling vapours and handle them with care.
Eriochrome Black T is an irritant and should not come into contact with the skin.

Method

Preparation of Equipment: Wash the pipette, burette, and conical flask with deionised water.
Rinse the burette with EDTA solution and the pipette with the water sample.
Filling the Burette: Use a philtre funnel to fill the burette with the 0.01 M EDTA solution.
Remove the funnel and adjust the level of the EDTA to the zero mark.
Preparing the Sample: Pipette 50 cm³ of the hard water sample into a 250 cm³ conical flask.
Buffering the Sample: Add 2–3 cm³ of pH 10 buffer solution to the flask to maintain the pH.
Adding the Indicator: Add 0.03 g of Eriochrome Black T indicator to the flask.
Swirl the flask until a deep wine-red colour appears.
Titration: Perform a rough titration to find the approximate endpoint, followed by several accurate titrations until two concordant results (within 0.1 cm³) are obtained.
The endpoint is reached when the solution turns from wine-red to blue.
Calculations: Calculate the total hardness of the water sample in terms of ppm of CaCO₃.

Results

Measurement	Value
Volume of hard water sample	50 cm³
Molarity of EDTA solution	0.01 M
Rough titre	22.4 cm³
Second titre	22.2 cm³
Third titre	22.1 cm³
Average of accurate titres	22.15 cm³
Total hardness (mol/l Ca²⁺)	0.00443 M
Total hardness (g/l CaCO₃)	0.443 g/l
Total hardness (ppm CaCO₃)	443 ppm

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Sample Calculation:

Moles of EDTA used:

\text{Moles of EDTA} = \frac{\text{volume of EDTA} \times \text{molarity of EDTA}}{1000}

= \frac{22.15 \times 0.01}{1000} = \highlight[0.0002215 \, \text{moles}]

Moles of metal ions ( $Ca²⁺ $$+ $$Mg²⁺$ ) in water (1:1 ratio with EDTA):

= \highlight[0.0002215 \, \text{moles}]

Concentration of $Ca²⁺$ and $Mg²⁺$ in water sample:

= \frac{0.0002215}{0.050} = \highlight[0.00443 \, \text{mol/l}]

Total hardness (ppm $CaCO₃$ ):

= 0.00443 \, \text{mol/l} \times 100 \, \text{g/mol CaCO₃} = \highlight[443 \, \text{ppm CaCO₃}]

Example Questions with Answers

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Q1: Why is the buffer solution used in this experiment?

The buffer solution ensures that the pH of the solution is maintained at 10, which is necessary for the EDTA to effectively form stable complexes with $Ca²⁺$ and $Mg²⁺$ ions.

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Q2: What would happen if metal ions other than $Ca²⁺$ or $Mg²⁺$ were present in the water sample?

The presence of other metal ions, such as $Fe³⁺$ or $Al³⁺$ , would cause the total hardness value to be overestimated, as these ions would also react with EDTA.

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Q3: How is the endpoint of the titration detected?

The endpoint is detected when the colour of the solution changes from wine-red (indicator complexed with metal ions) to blue (free indicator), indicating that all metal ions have reacted with EDTA.

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Q4: How can the permanent hardness of the water sample be determined?

Permanent hardness can be determined by boiling the water sample to precipitate temporary hardness-causing hydrogen carbonates.

The remaining hardness is then titrated with EDTA to determine the permanent hardness.

9.3 - Estimation of Total Hardness using Ethylenediaminetetraacetic Acid (Leaving Cert Chemistry): Revision Notes