Titration can be used to determine the concentration of dissolved oxygen in samples of river water. The procedure for the experiment is given. step 1 Use five $$\(50 \mathrm{~cm}^{3}\)$$ graduated syringes, A, B, C, D and E, to collect five separate $$\(30.0 \mathrm{~cm}^{3}\)$$ samples of river water. step 2 In the laboratory, carefully add $$\(5.0 \mathrm{~cm}^{3}\)$$ of $$\(0.220 \mathrm{moldm}^{-3}\)$$ manganese(II) sulfate, $$\(\mathrm{MnSO}_{4}(\mathrm{aq})\)$$, into syringe $$\(\mathbf{A}\)$$ and mix well. step 3 Add $$\(5.0 \mathrm{~cm}^{3}\)$$ of alkaline aqueous potassium iodide into syringe $$\(\mathbf{A}\)$$ and mix well. step 4 Add $$\(10.0 \mathrm{~cm}^{3}\)$$ of dilute sulfuric acid into syringe $$\(\mathbf{A}\)$$ and mix well. step 5 Transfer the contents of syringe $$\(\mathbf{A}\)$$ into a $$\(150 \mathrm{~cm}^{3}\)$$ conical flask. Rinse syringe $$\(\mathbf{A}\)$$ using $$\(10 \mathrm{~cm}^{3}\)$$ of distilled water and add washings to the conical flask. step 6 Carry out one accurate titration of all the contents in the conical flask with $$\(0.00200 \mathrm{moldm}^{-3}\)$$ aqueous sodium thiosulfate, $$\(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})\)$$, using starch indicator. Repeat steps 2-6 for the samples in syringes B-E. Suggest why this method is unsuitable for samples of tap water that have been purified by chlorination and so contain $$\(\mathrm{Cl}_{2}(\mathrm{aq})\)$$. ............................................................................................................................................... . .........................................................................................................................................

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