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.
Final burette reading (cm3)
Initial burette reading (cm3
Volume of Na2S2O3 added (cm3)
Data being used for 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 4 Mn(OH)3
No. of moles of O2 in 100 cm3 solution = 2.365
Oxygen content = 18.92 mg dm-3
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.