Method:
-
First we divided ourselves into groups of about three. Then we made a solution of sodium thiosulphate of 0.025 mol dm-3. This solution was then left for about one day before we began the experiment.
- On the day of the experiment, we began testing the pool water, by first checking its pH level. This was done by putting a little of it in a test tube and adding a few drops of universal indicator solution and comparing the colour change with those colours (with corresponding pH on the colour chart.
- Then we tested the water for the presence of chlorine, by putting some of it in a test tube, and acidifying it with dilute nitric acid and adding aqueous silver nitrate.
-
Next, we measured 50 cm3 of pool water using a measuring cylinder and placed it in a conical flask.
- Then we added 1 g of solid potassium iodide to the flask.
- Then we added exactly 4 drops of starch solution to the mixture in the conical flask, and swirled it so as to properly mix the contents.
- After this, we added 0.025 M sodium thiosulphate solution to a burette till the zero mark, and titrated it with the mixture in the conical flask. The titration was done till the end point was reached. This titration was repeated once more (so as to obtain more accurate readings)
-
The experiment was then repeated (from steps 2 to 7) using 100 cm3 and 150 cm3 of pool water.
Diagram:
Data Collection:
Observations:
- After the addition of universal indicator solution to the pool water, the colour changed to light green. When compared to the colours on the chart, the pH was found to be 8.
- The addition of silver nitrate to the acidified pool water produced a white precipitate, indicating the presence of chlorine.
- The addition of starch solution caused the contents in the flask to change dark blue, illustrating the presence of iodine molecules.
- At the end point of the titration, the colour of the contents of the flasks changed from their original colour of dark blue to colourless – indicating that chlorine was no longer present in the mixture (as it was all used up during the reaction with sodium thiosulphate).
Data processing and presentation:
-
Amount of Chlorine for 50cm3 of Pool water:
-
Volume of pool water = 50 ± 0.5 cm3
= 0.05 ± 0.0005 dm3
-
Average Volume of Na2S2O3 = 1.6 ± 0.1 cm3
= 0.0016 ± 0.0001 dm3
-
Concentration of Na2S2O3 = 0.01 M
-
No. of moles of Na2S2O3 = Concentration (mol dm–3) × Volume (dm3)
= 0.01 mol/dm3 × [0.0016 ± 0.0001] dm3
= 0.000016 ± 0.000001 mole
Chemical Equations:
-
2KI (s) + Cl2 (aq) ➔ 2KCl + I2(aq)
-
2Na2S2O3 (aq) + I2 (aq) ➔ 2NaI (aq) + Na2S4O6 (aq)
But the molar ratio between sodium thiosulphate and Iodine is 2:1
Therefore,
-
No. of moles of Iodine = No. of moles of sodium thiosulphate (Na2S2O3)
2
= (0.000016 ± 0.000001)
2
-
No. of moles of Iodine = 0.000008 ± 0.0000005 mole
But the molar ratio between iodine and chlorine is 1:1
Therefore,
- No. of moles of Chlorine = No. of moles of Iodine
= 0.000008 ± 0.0000005 mole
Given Mass = No. of Moles × Molar Mass
-
Mass of Chlorine in 50cm3 of H2O = [0.000008 ± 0.0000005] × 71
= 0.000568 ± 0.0000355 g
= 0.568 ± 0.0355 mg
-
Mass of Chlorine in 1000cm3 of H2O = (0.568 ± 0.0355) ×1000
50
-
Mass of Chlorine in 1 dm3 of H2O = 11.4 ± 0.71 mg
-
Amount of Chlorine for 100cm3 of Pool water:
Volume of pool water = 100 ± 0.5 cm3
= 0.1 ± 0.0005 dm3
Average Volume of Na2S2O3 = 3.15 ± 0.1 cm3
= 0.00315 ± 0.0001 dm3
Concentration of Na2S2O3 = 0.01 M
No. of moles of Na2S2O3 = Concentration (mol dm–3) × Volume (dm3)
= 0.01 mol/dm3 × [0.00315 ± 0.0001] dm3
= 0.0000315 ± 0.000001 mole
Chemical Equations:
-
2KI (s) + Cl2 (aq) ➔ 2KCl + I2(aq)
-
2Na2S2O3 (aq) + I2 (aq) ➔ 2NaI (aq) + Na2S4O6 (aq)
But the molar ratio between sodium thiosulphate and Iodine is 2:1
Therefore,
-
No. of moles of Iodine = No. of moles of sodium thiosulphate (Na2S2O3)
2
= (0.0000315 ± 0.000001)
2
= 0.00001575 ± 0.0000005 mole
≈ 0.0000158 ± 0.0000005 mole
But the molar ratio between iodine and chlorine is 1:1
Therefore,
- No. of moles of Chlorine = No. of moles of Iodine
= 0.0000158 ± 0.0000005 mole
Given Mass = No. of Moles × Molar Mass
-
Mass of Chlorine in 100cm3 of H2O = [0.0000158 ± 0.0000005] × 71
= 0.0011218 ± 0.0000355 g
= 1.1218 ± 0.0355 mg
≈ 1.12 ± 0.036 mg
-
Mass of Chlorine in 1000cm3 of H2O = (1.12 ± 0.036) × 1000
100
-
Mass of Chlorine in 1dm3 of H2O = 11.2 ± 0.36 mg
-
Amount of Chlorine for 150cm3 of Pool water:
-
Volume of pool water = 150 ± 0.5 cm3
= 0.15 ± 0.0005 dm3
-
Average Volume of Na2S2O3 = 4.75 ± 0.1 cm3
= 0.00475 ± 0.0001 dm3
Concentration of Na2S2O3 = 0.01 M
-
No. of moles of Na2S2O3 = Concentration (mol dm–3) × Volume (dm3)
= 0.01 mol/dm3 × [0.00475 ± 0.0001] dm3
= 0.0000475 ± 0.000001 mole
Chemical Equations:
-
2KI (s) + Cl2 (aq) ➔ 2KCl + I2(aq)
-
2Na2S2O3 (aq) + I2 (aq) ➔ 2NaI (aq) + Na2S4O6 (aq)
But the molar ratio between sodium thiosulphate and Iodine is 2:1
Therefore,
-
No. of moles of Iodine = No. of moles of sodium thiosulphate (Na2S2O3)
2
= (0.0000475 ± 0.000001)
2
= 0.00002375 ± 0.0000005 mole
≈ 0.00002380 ± 0.0000005 mole
But the molar ratio between iodine and chlorine is 1:1
Therefore,
- No. of moles of Chlorine = No. of moles of Iodine
= 0.0000238 ± 0.0000005 mole
Given Mass = No. of Moles × Molar Mass
-
Mass of Chlorine in 150cm3 of H2O = [0.0000238 ± 0.0000005] × 71
= 0.0016898 ± 0.0000355 g
= 1.6898 ± 0.0355 mg
≈ 1.69 ± 0.036 mg
-
Mass of Chlorine in 1000cm3 of H2O = (1.69 ± 0.036) × 1000
150
-
Mass of Chlorine in 1dm3 of H2O = 11.26 ± 0.24 mg
≈ 11.3 ± 0.24 mg
- Average Concentration of Chlorine
-
Average Concentration = [11.4 ± 0.71] + [11.2 ± 0.36] + [11.3 ± 0.24]
3
= (33.9 ± 1.31)
3
= 11.3 ± 0.44
≈ 11.3 ± 0.44 mg dm-3
Conclusion:
After performing the experiment, I would like to conclude that the average value for chlorine in water was found to be 11.3 ± 0.44 mg dm-3. The concentration of chlorine in pool water should not exceed 5 mg dm-3. The calculated value is about twice as high as this value. The pH of the pool water was found to be about 8, while the pH of the human eye is about 7.4. This again shows that pool water is unsuitable for us, as it would cause eye irritation.
There fore, I would like to conclude by saying that this pool water is not suitable for swimming purposes, as it contain a dangerous level of chlorine. Unless the appropriate protective gear, such as swimsuits and googles are used, people should avoid swimming in this water. This level could cause some serious effects for us humans, as mentioned in the general background.
Evaluation:
Sources of error:
- While taking the readings of the volume from the titrations, our eyes were not completely in level with the meniscus of the liquid.
- While comparing the colour change due to addition of universal indicator solution, we didn’t use a white tile to clearly see the colour change.
-
Another source of error could definitely be due to the limitation of the burette which is accurate to within ± 0.05 cm3.
Prevention of these errors:
- Eyes should be kept in level with the meniscus of the liquid in the burette in order to note the exact reading of the volume.
- A white tile should be used to clearly see the colour change, which can then be compared with other colours on the colour chart.
- The error due to the limitation of the burette can be reduced by doing even more titration repetitions, like eight or ten of them.
Improvements:
-
The volume readings all had a high uncertainty of 0.5 cm3. This was due to the usage of a measuring cylinder to measure the volume. More accurate readings of the volume could have been obtained by using a pipette instead of a measuring cylinder.
- Universal indicator solution was used to find the pH by comparing the colour change with colours on a pH colour chart. Through this method it was difficult to determine the exact value of the pH, as the colour chart didn’t contain all possible colour changes with corresponding value of pH. A digital pH meter should have been used to measure the pH of the pool water, instead of using universal indicator solution. This would provide dot accurate readings for the pH.
Further Research:
- Some further investigation could be done on the amounts of other chemicals present in pool water, their applications and effects (if any). And also there could be an investigation on the chlorine content in drinking water and tap water, so as to test its suitability for drinking, bathing, and washing of clothes.
Bibliography:
"Chlorine (Cl) - Chemical Properties, Health and Environmental Effects." Lenntech. 28 Dec. 2011. Lenntech Water Treatment & Air Purification Holding. 30 Jan. 2008 <http://www.lenntech.com/Periodic-chart-elements/Cl-en.htm>
Bartier., Malcolm R. "The Swimming Pool Operators and Owners Resource Page 4." The Swimming Pool Operators. 3 Jan. 2003. The Caromal Leisure Group. 30 Jan. 2012 <http://www.caromal.co.uk/Resource%20Page%204.htm>.