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# Aim/Objective: To find out the dissolved oxygen content (in mg dm-3) in a water sample by titration of iodine against thiosulphate solution.

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Introduction

1) Title: V6 Determination of dissolved oxygen in a water sample (Winkler Method) 2) Aim/Objective: To find out the dissolved oxygen content (in mg dm-3) in a water sample by titration of iodine against thiosulphate solution. 3) Results: Trial 1 Final burette reading (cm3) 1.30 3.40 Initial burette reading (cm3) 25.90 28.10 Volume of Na2S2O3 used (cm3) 24.60 24.70 Temperature: 25.5? 4) Calculation/interpretation: Volume of S2O32- added = 24.70cm3 No. of moles of S2O32- added = volume * molarity = 24.70 * 0.001 * 0.0125 = 0.00030875 mol No. of moles of I2 added = 0.00030875 / 2 = 0.000154375 mol No. of moles of Mn(OH)3 added = 0.000154375 * 2 = 0.00030875 mol No. of moles of O2 added = 0.00030875 / 4 = 0.0000771875 mol Mass of O2 = 0.0000771875 * 32 = 0.00247g = 2.47mg Concentration of O2 in sample = 2.47 / (100*0.001) = 24.7mgdm-3 5) Discussion: A number of precautions needed to be carried out in the experiment. ...read more.

Middle

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. ...read more.

Conclusion

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. 6) Conclusion: The dissolved oxygen content in the water sample is 24.7mgdm-3. ?? ?? ?? ?? Yu Wing Yee 6A(30) - 1 - ...read more.

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