9.1 - Colorimetric Experiment to Estimate Free Chlorine in Swimming Pool Water or Bleach (Leaving Cert Chemistry): Revision Notes

9.1 - Colorimetric Experiment to Estimate Free Chlorine in Swimming Pool Water or Bleach

Materials and Apparatus Required

Chemicals

• 2% potassium iodide solution
• 5% ethanoic acid solution
• Sample of swimming pool water or diluted bleach
• Milton Sterilising Fluid (contains sodium hypochlorite)
• Deionised water

Apparatus

• Colorimeter (with 440 nm wavelength philtre)
• Cuvettes
• 50 cm³ volumetric flasks with stoppers
• Burette
• Graduated cylinder (10 cm³)
• Philtre funnel
• 250 cm³ volumetric flask
• Safety glasses

Safety Precautions

• Wear safety glasses throughout the experiment.
• Milton Sterilising Fluid contains sodium hypochlorite, which is corrosive and can cause burns. Avoid contact with skin and eyes.
• Ethanoic acid is corrosive; handle it carefully.
• Potassium iodide can irritate the skin; avoid direct contact.

Method

1. Prepare diluted chlorine solution: Add 2.5 cm³ of Milton Sterilising Fluid to a 250 cm³ volumetric flask and dilute to the mark with deionised water.
2. Prepare standard solutions: Label five 50 cm³ volumetric flasks A, B, C, D, and E.
3. Add 5 cm³ of 5% ethanoic acid solution to each flask.

• To flask A, add 0 cm³ of the diluted chlorine solution.
• To flask B, add 1 cm³ of the diluted chlorine solution.
• To flask C, add 2 cm³ of the diluted chlorine solution.
• To flask D, add 4 cm³ of the diluted chlorine solution.
• To flask E, add 8 cm³ of the diluted chlorine solution.

4. Add 5 cm³ of 2% potassium iodide solution to each flask and dilute to the 50 cm³ mark with deionised water.
5. Stopper and mix thoroughly. Allow 5 minutes for the colour to develop.
6. Set up the colorimeter: Place the 440 nm wavelength philtre in the colorimeter.
7. Zero the instrument using a cuvette filled with deionised water.
8. Measure absorbance: Measure the absorbance of each solution, starting with flask A.
9. Record the absorbance for all five flasks.
10. Test the sample: Prepare the test solution by adding 5 cm³ of 5% ethanoic acid and 5 cm³ of 2% potassium iodide to 50 cm³ of swimming pool water or diluted bleach.
11. Allow the colour to develop and measure its absorbance using the colorimeter.
12. Determine chlorine concentration: Plot a graph of absorbance vs concentration for the standard solutions.
13. Use the absorbance of the test sample to estimate the chlorine concentration from the graph.

Results

Flask	Diluted Milton (cm³)	Absorbance	NaOCl concentration (ppm)
A	0	0.00	0
B	1	0.06	4
C	2	0.12	8
D	4	0.24	16
E	8	0.43	32
F (Sample)	-	0.18	12.6

The free chlorine concentration in the sample is found by multiplying the sodium hypochlorite ($NaOCl$) concentration by 71/74.5 to account for the difference in molar masses.

Example Questions with Answers