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Determination of dissolved oxygen in a water sample (Winkler Method)

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V6 Determination of dissolved oxygen in a water sample (Winkler Method) Objective 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. ...read more.


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) ...read more.


However, once we withdrawn the pipette out of the flask, the liquid level drops significantly.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 ?? ?? ?? ?? ...read more.

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