When filling small amount of solution into the volumetric flask using dropper, the dropper should be immersed well below the water surface, to ensure that the added solution could react with the water sample. If the solution was added onto the top of the solution, overflow will occur, and the content overflowed would be the added solution. Thus, less number of moles of the added solution would react with the water sample. Just like in the two equations:
8OH-(aq) + 4Mn2+(aq) + O2(aq) + 2H2O(l) → 4Mn(OH)3(s)
2Mn(OH)3(s) + 2I-(aq) + 6H+(aq) → 2Mn2+(aq) + I2(aq) + 6H2O(l)
Less number of moles of Mn2+ ions and I- ions would react than the calculated amount. So a smaller amount of sodium thiosulphate solution would be used in titration, giving smaller dissolved oxygen content in the sample. On the same basis, after adding alkaline potassium iodide solution, time was required to allow the precipitate to sink to the bottom, ideally 3cm3 below the stopper.
Besides, a smaller magnetic stirrer was chosen. Since a bigger one would lead to a larger amount of overflow, more freshly produced iodine would be lost. In this case, the amount of iodine in each 100cm3 of the acidified sample would become smaller. So, a smaller amount of sodium thiosulphate solution would be required in titration, which would lead to a smaller amount of dissolved oxygen content in the sample.
In the experiment, it was assumed that the water sample did not contain any solutes which will oxidize or reduce iodine. However, if it did contain these solutes, the amount of iodine in the solution to be titrated would be smaller. As a result, less sodium thiosulphate solution would be used, and the calculated dissolved oxygen content would be smaller.
On the other hand, iodine was freshly produced by the in situ generation but not using solid iodine for a number of reasons. For example, solid iodine is not very soluble in water but volatile and reacts readily with reducing agents in the environment. These factors would give a smaller amount of iodine in the sample than predicted. Also, solid iodine used in the laboratory contains impurities, which will react with the other solution. Under these circumstances, less sodium thiosulphate solution would be required to titrate against the solution in titration, giving a smaller calculated dissolved oxygen content.
Although iodine was freshly produced in the experiment, it could not be assured that no aqueous iodine had lost. No matter how short the titration took place, a little bit of iodine would still be lost in the form of gas. Thus, less sodium thiosulphate solution would be used to titrate with the water sample, leading to a smaller dissolved oxygen content.
Judging form the above errors, it could be said that the calculated dissolved oxygen content in the sample was smaller than the actual one.
To improve the results of the experiment, firstly, the time for titration should be carried out as short as possible. This is to minimize the lost of iodine to the atmosphere. Also, the water sample should be well insulated from the atmosphere. For instance, when carrying out the experiment, the volumetric flask should be stoppered frequently.
- Conclusion:
The dissolved oxygen content in the water sample is 24.7mgdm-3.
