Diocesan Boys’ School G12 Chemistry Investigative Study School-Based Assessment Laboratory Long Report Name: Chu Kim Long Matthew Class: G.12D Class Number: 9
Submission Date 19th December, 2013
Title Analysis of the quality of potable water
Objectives To analyze the quality of 3 potable water samples (laboratory, sea water and tap water) from different geographic zones by determining their permanganate index.
Schedule of work Date 14th November, 2013 21st November, 2013 28th November, 2013 12th December, 2013 19th December, 2013
Work Proposal submission Experiment 1 (Lab water sample) Experiment 2 (Tab water sample) Experiment 3 (Sea water sample) Report submission
Materials a) Chemicals Chemicals Laboratory water sample Sea water sample Tap water sample 0.005003 M standard sodium oxalate solution 0.00200 M standard potassium permanganate solution 0.5 M sulphuric acid Deionized or distilled water
Quantities 250.0 cm3 250.0 cm3 250.0 cm3 350.0 cm3 400.0 cm3 300.0 cm3 Accessible
b) Apparatus Apparatus Safety spectacles Measuring cylinder, 10 cm3 Conical flask, 250 cm3 Burette, 50.0 cm3 Pipette, 10.0 cm3 Pipette, 25.0 cm3 Magnetic stirrer-hotplate Stand and clamp Plastic washbottle White tile
Quantities 3 1 2 1 1 2 1 1 set 1 1
Theory The quality of water can be determined by permanganate index, which is a scale. Permanganate index indicates the amount of potassium permanganate solution used to oxidize the oxidizable matter present in a water sample. The lower the permanganate index, it means that the less amount of the permanganate used to oxidize the oxidizible matter in water, and thus it imply that the water sample is polluted or contaminated by oxidizable matter to a smaller extent, and hence the quality of water is higher, and vice versa. The permanganate index can be expressed as KMnO4 in mg dm-3 and in milligrams of oxygen per liter (dm-3) of water. The half equation of permanganate is: MnO4-(aq) + 8H+(aq) + 5eMn2+(aq) + 4H2O(l), which shows that permanganate is undergoing reduction. The half equation of oxygen is: O2(g) + 4H+(aq) + 4e2H2O(l), which shows that oxygen is undergoing reduction. Usually, the amount of permanganate consumed will be converted to the amount of oxygen. According to the above half equations, each permanganate ion use 5 electrons during reduction and each oxygen mole use 4 electrons during reduction. Therefore, one mole of permanganate ions oxidize the same amount of oxidizable matter in water as 1.25 mole of oxygen. Potassium permanganate solution is purple in colour and it is a strong oxidizing agent. According to the above half equation of permanganate, since permanganate ions, which is purple in colour, will be reduced to manganese(II) ions, which is very pale pink in colour, in acidic medium,. Therefore, it acts as an indicator as well.
Experimental Procedures 1. A clean 25.0 cm3 pipette was used to transfer 50.0 cm3 of laboratory water sample into a clean 250 cm3 conical flask. 2. A measuring cylinder was used to add 5 cm3 of 0.5 M sulphuric acid into the laboratory water sample in the conical flask. 3. A clean 10.0 cm3 pipette was used to transfer 10.0 cm3 of 0.00200 M standard potassium permanganate solution into the laboratory water sample in the conical flask.
4. The solution was heated to about 60 °C for 10 minutes on a hotplate. The solution was stirred with a magnetic stirrer in order to make sure the solution is still purple in colour. 5. A clean 25.0 cm3 pipette was used to transfer 25.0 cm3 excess of 0.005003 M standard sodium oxalate solution into the hot solution. 6. The content was shaken well. 7. A clean burette was filled with 0.00200 M standard potassium permanganate solution. 8. The initial reading of the burette was recorded. 9. A measuring cylinder was used to add 15 cm3 of 0.5 M sulphuric acid into the contents in the conical flask. 10. The hot solution was titrated with 0.00200 M standard potassium permanganate solution until a permanent pink colour appeared. 11. The final reading of the burette was recorded. 12. Steps 1-11 were repeated for three times for consistent results. 13. Steps 1-12 were repeated for sea water sample and tap water sample instead of the laboratory water sample.
Data and Calculation Water sample used Laboratory water sample Date of experiment 21st November, 2013 (Thursday) Chemicals Chemicals Sulphuric acid Sodium oxalate solution Potassium permanganate solution
Molarity 0.5M 0.00200M 0.005003M
Observations Before the experiment 1. The laboratory water sample is colourless 2. The 0.005003 M sodium oxalate solution is colourless 3. The 0.5 M sulphuric acid is colourless 4. The 0.00200 M potassium permanganate solution is purple in colour During the experiment 1. After adding 10.0 cm3 0.00200 M potassium permanganate solution and 5 cm3 0.5 M sulphuric acid to the water sample, the reaction mixture turns from colourless to red-orange 2. After heating the reaction mixture for 10 minutes in 60°C, the colour of the reaction mixture turns from red-orange to red
3. After adding 25 cm3 0.005003 M sodium oxalate solution and 15 cm3 0.5 M sulphuric acid to the reaction mixture, the reaction mixture turns from red to colourless 4. At the end-point of titration, the reactions mixture turns from colourless to permanent very pale pink After the experiment 1. The reaction mixture remains permanent very pale pink
Result 3
Final reading of burette (cm ) Initial reading of burette (cm3) Amount of potassium permanganate solution used (cm3)
Trial 22.00 0.00
1st Titration 43.50 22.00
2nd Titration 21.70 0.00
3rd Titration 43.30 21.70
22.00 (rej.)
21.50
21.70
21.60
Average amount of potassium permanganate solution used = = 21.60 cm3 Calculation Ionic half equations: MnO4-(aq) + 8H+(aq) + 5e- Mn2+(aq) + 4H2O(l) (1) 2C2O4 (aq) 2CO2(g) + 2e (2) By (1) x 2 + (2) x 5, Overall equation: 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq) 2Mn2+(aq) + 8H2O(l) +10CO2(g) The number of moles of potassium permanganate solution used in titration =
x 0.00200
= 4.32 x 10-5 mol The mole ratio of C2O42-(aq) to MnO4-(aq) = 5:2 The number of moles of excess sodium oxalate solution = 4.32 x 10-5 x = 1.08 x 10-4 mol Total number of moles of sodium oxalate solution added =
x 0.005003
=1.25075 x 10-4 mol
The number of moles of sodium oxalate solution reacted with excess potassium permanganate solution = 1.25075 x 10-4 - 1.08 x 10-4 = 1.7075 x 10-5 mol The mole ration of MnO4-(aq) to C2O42-(aq) = 2:5 The number of moles of excess potassium permanganate solution = 1.7075 x 10-5 x = 6.83 x 10-6 mol Total number of moles of potassium permanganate solution added =
x 0.00200
= 2 x 10-5 mol The number of moles of potassium permanganate solution reacted in laboratory water sample = 2 x 10-5 - 6.83 x 10-6 = 1.317 x 10-5 mol Permanganate index (in mg dm-3) of the laboratory water sample =
-
= = 41.6172 mg dm-3 ∴ The permanganate index of the laboratory water sample is 41.6172 mg dm-3 Permanganate index (in mg of O2 per litre of water) of the laboratory water sample =
-
= = 10.536 mg of O2 per litre of water ∴ The permanganate index of the laboratory water sample is 10.536 mg of O2 per litre of water Water sampled used Tap Water Sample
Date of Experiment 28th November, 2013 (Thursday) Chemicals Chemicals Sulphuric acid Sodium oxalate solution Potassium permanganate solution
Molarity 0.5M 0.00200M 0.005003M
Observations Before the experiment 1. The tap water sample is colourless 2. The 0.005003 M sodium oxalate solution is colourless 3. The 0.5 M sulphuric acid is colourless 4. The 0.00200 M potassium permanganate solution is purple in colour During the experiment 1. After adding 10.0 cm3 0.00200 M potassium permanganate solution and 5 cm3 0.5 M sulphuric acid to the water sample, the reaction mixture turns from colourless to purple 2. After heating the reaction mixture for 10 minutes in 60°C, the colour of the reaction mixture remains unchanged 3. After adding 25 cm3 0.005003 M sodium oxalate solution and 15 cm3 0.5 M sulphuric acid to the reaction mixture, the reaction mixture turns from purple to colourless 4. At the end-poin1t of titration, the reaction mixture turns from colourless to permanent very pale pink After the experiment 1. The reactions mixture remains permanent very pale pink Result Final reading of burette (cm3) Initial reading of burette (cm3) Amount of potassium permanganate solution used (cm3)
Trial 15.50 0.00
1st Titration 30.50 15.50
2nd Titration 15.10 0.00
3rd Titration 30.30 15.10
15.50 (rej.)
15.00
15.10
15.20
Average amount of potassium permanganate solution used = = 15.10 cm3
Calculation Ionic half equations: MnO4-(aq) + 8H+(aq) + 5e- Mn2+(aq) + 4H2O(l) (1) 2C2O4 (aq) 2CO2(g) + 2e (2) By (1) x 2 + (2) x 5, Overall equation: 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq) 2Mn2+(aq) + 8H2O(l) +10CO2(g) The number of moles of potassium permanganate solution used in titration =
x 0.00200
= 3.02 x 10-5 mol The mole ratio of C2O42-(aq) to MnO4-(aq) = 5:2 The number of moles of excess sodium oxalate solution = 2.86 x 10-5 x = 7.55 x 10-5 mol Total number of moles of sodium oxalate solution added =
x 0.005003
=1.25075 x 10-4 mol The number of moles of sodium oxalate solution reacted with excess potassium permanganate solution = 1.25075 x 10-4 - 7.55 x 10-5 = 4.9575 x 10-5 mol The mole ration of MnO4-(aq) to C2O42-(aq) = 2:5 The number of moles of excess potassium permanganate solution = 4.9575 x 10-5 x = 1.983 x 10-5 mol Total number of moles of potassium permanganate solution added =
x 0.00200
= 2 x 10-5 mol The number of moles of potassium permanganate solution reacted in tap water sample = 2 x 10-5 - 1.983 x 10-5
= 1.7 x 10-7 mol Permanganate index (in mg dm-3) of the tap water sample =
-
= = 0.5372 mg dm-3 ∴ The permanganate index of the tap water sample is 0.5372 mg dm-3 Permanganate index (in mg of O2 per litre of water) of the tap water sample =
-
= = 0.136 mg of O2 per litre of water ∴ The permanganate index of the tap water sample is 0.136 mg of O2 per litre of water Water sample used Sea Water Sample Date of Experiment 12th December, 2013 (Thursday) Chemicals Chemicals Sulphuric Acid Sodium oxalate solution Potassium permanganate solution
Molarity 0.5M 0.005003M 0.00200M
Observations Before the experiment 1. The sea water sample is colourless 2. The 0.005003 M sodium oxalate solution is colourless 3. The 0.5 M sulphuric acid is colourless 4. The 0.00200 M potassium permanganate solution is purple in colour During the experiment
1. After adding 10.0 cm3 0.00200 M potassium permanganate solution and 5 cm3 0.5 M sulphuric acid to the water sample, the reaction mixture turns from colourless to purple 2. After heating the reaction mixture for 10 minutes in 60°C, the colour of the reaction mixture turns from purple to red 3. After adding 25 cm3 0.005003 M sodium oxalate solution and 15 cm3 0.5 M sulphuric acid to the reaction mixture, the reaction mixture turns from red to colourless 4. At the end-point of titration, the reaction mixture turns from colourless to permanent very pale pink After the experiment 1. The reactions mixture remains permanent very pale pink
Result Final reading of burette (cm3) Initial reading of burette (cm3) Amount of potassium permanganate solution used (cm3)
Trial 16.30 0.00
1st Titration 31.00 16.30
2nd Titration 15.50 0.00
3rd Titration 31.10 15.50
16.30 (rej.)
15.70
15.50
15.60
Average amount of potassium permanganate solution used = = 15.60 cm3 Calculation Ionic half equations: MnO4-(aq) + 8H+(aq) + 5e- Mn2+(aq) + 4H2O(l) (1) 2C2O4 (aq) 2CO2(g) + 2e (2) By (1) x 2 + (2) x 5, Overall equation: 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq) 2Mn2+(aq) + 8H2O(l) +10CO2(g) The number of moles of potassium permanganate solution used in titration =
x 0.00200
= 3.12 x 10-5 mol The mole ratio of C2O42-(aq) to MnO4-(aq) = 5:2 The number of moles of excess sodium oxalate solution = 3.12 x 10-5 x = 7.8 x 10-5 mol Total number of moles of sodium oxalate solution added
=
x 0.005003
=1.25075 x 10-4 mol The number of moles of sodium oxalate solution reacted with excess potassium permanganate solution = 1.25075 x 10-4 - 7.8 x 10-5 = 4.7075 x 10-5 mol The mole ratio of MnO4-(aq) to C2O42-(aq) = 2:5 The number of moles of excess potassium permanganate solution = 1.7075 x 10-5 x = 1.883 x 10-5 mol Total number of moles of potassium permanganate solution added =
x 0.00200 -5
= 2 x 10 mol The number of moles of potassium permanganate solution reacted in sea water sample = 2 x 10-5 - 1.883 x 10-5 = 1.17 x 10-6 mol Permanganate index (in mg dm-3) of the sea water sample =
-
= = 3.6972 mg dm-3 ∴ The permanganate index of the sea water sample is 3.6972 mg dm-3 Permanganate index (in mg of O2 per litre of water) of the sea water sample =
-
= = 0.936 mg of O2 per litre of water ∴ The permanganate index of the sea water sample is 0.936 mg of O2 per litre of water
Diagram
Results and Conclusion The permanganate index of the laboratory water sample in terms of KMnO4 in mg dm-3 and in mg of O2 per litre of water is 41.6172 mg dm-3 and 10.536 mg of O2 per litre of water respectively. The permanganate index of the tap water sample in terms of KMnO4 in mg dm-3 and in mg of O2 per litre of water is 0.5372 mg dm-3 and 0.136 mg of O2 per litre of water respectively. The permanganate index of the sea water sample in terms of KMnO4 in mg dm-3 and in mg of O2 per litre of water is 3.6972 mg dm-3 and 0.936 mg of O2 pre litre of water respectively. Therefore, the permanganate index of the laboratory water sample is higher than that of sea water sample, and the permanganate index of the sea water sample is higher than that of tap water sample. Hence, the quality of laboratory water sample is lower than that of sea water sample, and the quality of sea water sample is lower than that of tap water sample. In other words, the quality of tap water sample is the highest while the quality of laboratory water sample is the lowers.
Discussion Precautions a) Safety precaution 1. Keep 0.5 M sulphuric acid and 0.005003 M sodium oxalate solution away from the skin and eye because they are irritating. 2. Handle 0.00200 M potassium permanganate solution carefully because it is a strong oxidizing agent. b) Experimental precaution 1. Do not set the magnetic stirrer to spin too fast so as to prevent any spillage of chemicals in the conical flask. Sources of error 1. The temperature is not constant when the experiment is performed, and thus the data collected is not accurate. Since temperature will affect the solubility of oxygen in water, which mean the solubility of oxygen will change with the temperature, so if the temperature is not constant during the experiment, the data obtained will not be accurate. 2. Some of the oxidizable matters present in the water sample may be oxidized by the oxygen in air. Thus the amount of oxidiziable matter in the water sample will decrease. Thus, the calculated permanganate index will be smaller than the actual value. Improvements 1. An oxygen meter can be employed. The data obtained from the oxygen meter can be used as reference and comparison so as to reduce the first error. 2. Carry out the preparation works and titration as soon as possible to reduce the second error. Other relevant information Manganese, Mn, of permanganate ion, MnO4-, has variable oxidation states, which the maximum oxidation state is +7. Hence, permanganate ion has strong oxidizing ability, and thus it is a strong oxidizing agent and it can be used to oxidize the oxidizable matters in the water samples. Answers to questions 1. Where do your samples of water come from? The tap water sample comes from Hanley Villa, located in Ting Kau in Tsuen Wan. The sea water sample comes from Rambler Channel. 2. What measurements would you carry out during the experiments? The initial reading and the final reading of the burette would be recorded so as to measure the volume of potassium permanganate solution used (cm3) in each titration. Then, the average amount of potassium permanganate solution used would be measured. Then, the number of moles of potassium permanganate solution used in titration would be calculated. 3. What variables will you need to keep constant in this investigation? The amount of potassium permanganate solution added, amount of sodium oxalate solution added, amount of sulphuric acid added, amount of water sample added, the time allowed for heating and the temperature of heating the reaction mixture.
4. Will the proposed procedures be feasible and safe? Yes. Since the molarity of sodium oxalate solution and potassium permanganate solution is very diluted, which are 0.005003 M and 0.00200 M respectively. Thus, it is safer to handle them. Also, the temperature of the water bath is 60°C, which is not very high. To conclude, the proposed procedures is safe. On the other hand, the potassium permanganate solution added to the water sample are excess so as to ensure that all the oxidizable matters in the water sample is oxidized and some potassium permanganate solution remains. Then, the sodium oxalate solution added to the reaction mixture are excess so as to ensure all the remaining potassium permanganate solution are reacted. After, the amount of sodium oxalate solution remains is determined by titration with potassium permanganate solution. Moreover, sulphuric acid is added to acidify the potassium permanganate solution to ensure the reaction is carried out in an acidic medium. In conclusion, the proposed procedures is feasible. 5. Briefly describe the method you used in your analysis. The method I used in the analysis is to determine the permanganate index of a water sample by carrying out back titration. Determining of permanganate index is one of the examples of redox titration, which is a kind of volumetric analysis. Permanganate index indicates the amount of potassium permanganate solution used to oxidize the oxidizable matter present in a water sample. The lower the permanganate index, the higher the quality of water, and vice versa. The steps of determining the permanganate index are as follows: Firstly, after obtaining the average amount of potassium permanganate solution used in titration, the number of mole of potassium permanganate solution can be calculated. Secondly, the number of moles of excess sodium oxalate solution can be obtained by using the mole ratio of C2O42-(aq) to MnO4-(aq) = 5:2. Thirdly, the number of moles of sodium oxalate solution reacted with excess potassium permanganate solution can be obtained by total number of moles of sodium oxalate solution subtracting the number of moles of sodium oxalate solution remains. Fourthly, the number of moles of excess potassium permanganate solution can be obtained by using the mole ratio of MnO4-(aq) to C2O42-(aq) = 2:5. Fifthly, the number of moles of potassium permanganate solution reacted in water sample can be obtained by total number of moles of potassium permanganate solution subtracting the number of moles of potassium permanganate remains. Lastly, the permanganate index of water sample can be obtained by mass of potassium permanganate solutions reacted in water sample (mg) dividing volume of water sample (dm3).
References 1. E.Cheng, J.Chow, Y.F. Chow, A.Kai, K.K.Lai, W.H. Wong, HKDSE Chemistry A Modern View Book 7 Analytical Chemistry, Aristo Educational Press Ltd, 2011, page 2. Guideline of the G12 Chemistry Investigative Study - Analysis of the quality of potable water 3. Analytical Chemistry Notes (Part II - Quantitative and Instrumental Analytical methods) by Ms. H.Y. Tang, page 14-16
