Determination of dissolved oxygen - Sample Essay

To find out the oxygen content of a water sample by adding MnSO4 and alkaline KI into the solution. Mn2+ was oxidized into Mn3+. Then Mn3+ was reduced back into Mn2+ and I- was oxidized to I2. The solution was titrated against sodium thiosulphate solution and the oxygen content could be calculated. Result Trial 1 Trial 2 Final burette reading (cm3) 19. 26 38. 42 19. 82 38. 35 Initial burette reading (cm3 0. 10 19. 50 0. 74 19. 82 Volume of Na2S2O3 added (cm3) 19. 16 18. 92 19. 08 18. 53 Data being used for calculation Calculation No of moles of Na2S2O3 used =

2S2O32- + I2 –> S4O62- + 2I- No. of moles of I2 in 100 cm3 solution = = 1. 1825 2Mn(OH)3 + 2I- + 6H+ –>I2 + 2Mn2+ + 6H2O No. of moles of Mn(OH)3 in 100 cm3 solution = 1. 1825 X 2 = 2. 365 4 Mn(OH)2 + O2 + 2 H2O i?? 4 Mn(OH)3 No. of moles of O2 in 100 cm3 solution = 2. 365 Oxygen content = 18. 92 mg dm-3 Discussion Reason of filling the whole volumetric flask. During the experiment, the water sample was allowed to fill with the whole volumetric flask, this is because we have to prevent oxygen from dissolving from the air into the water sample.

Reason of using a magnetic stirrer When conducting the experiment, insoluble manganese (III) hydroxide was formed. It would dissolve in potassium iodide so we have to use a magnetic stirrer to speed up the reaction process. As a magnetic stirrer could stir the solution without spilling, it is very good to be used in this situation. Source of error As the manganese (II) sulphate and potassium iodide solution was added into the flask with over flowing. There will be loss of solution in this process. The chemicals added might be not enough and it will affect the experiment result.

We cannot avoid oxygen in air from dissolving into the solution as the mouth of volumetric flask could still allow oxygen to diffuse in. Only one set of data could be used beside trials. We cannot be sure whether the titration result is accurate. We cannot be sure whether there’s other chemicals inside the water sample which would react with potassium iodide or manganese (II) sulphate and would affect the experiment result. The water sample may not be homogenous so the oxygen content might be different in different part of the sample.

The oxygen content might change on the way to the laboratory as it’s left alone for quite a long time. Difficulties encountered At the first time, we didn’t realize that we have to pipette the manganese (II) sulphate solution and potassium iodide solution into the flask inside the sink therefore the first time that we added chemicals into the flask was far more difficult. Fortunately, when conducting the second time experiment, we put the volumetric flask inside the sink before pipette solutions onto it.

The adding process was much easier. When pipetting solution into the flask, we have to discharge the solution well below the surface. However, once we withdrawn the pipette out of the flask, the liquid level drops significantly. We have to be very careful that the solution level remains near the flask mouth when discharging solutions into the flask. Ways of improvement We may try using 50cm3 solutions during the titration instead of 100cm3 because it could still show a significant change in sodium thiosulphate used.

Also, it could allow us to have few more chances to repeat the titration process using the same flask of solution. As a result, we could obtain the experiment result by taking the average result from titration. It could improve the accuracy of the experiment. Next time, when conducting the experiment, we should examine the water sample immediately after it’s collected to avoid the oxygen content inside it being affected by the surrounding air. Conclusion The oxygen content inside the water sample is 18. 92 mg dm-3.