The concentration of dissolved oxygen in a sample of water can be measured using the following method. Manganese(II) hydroxide, Mn(OH)₂, is oxidised by the oxygen dissolved in a sample of water to form manganese(III) hydroxide, Mn(OH)₃. O₂(aq) + 4Mn(OH)₂(s) + 2H₂O(l) → 4Mn(OH)₃(s) The manganese(III) hydroxide then reacts with iodide ions to produce aqueous iodine. 2Mn(OH)₃(s) + 6HCl(aq) + 2KI(aq) → 2MnCl₂(aq) + 2KCl(aq) + I₂(aq) + 6H₂O(l) The amount of iodine produced is proportional to the amount of dissolved oxygen. 25.0cm³ of the solution containing aqueous iodine is transferred into a conical flask and titrated against 1.00 × 10⁻³moldm⁻³ sodium thiosulfate, Na₂S₂O₃. I₂(aq) + 2Na₂S₂O₃(aq) → Na₂S₄O₆(aq) + 2NaI(aq) The concentration of oxygen in water at different temperatures is shown in Table 1.2. The concentration value is missing for 25°C. Table 1.2 | temperature/°C | concentration of oxygen × 10⁻⁴/mol dm⁻³ | |---|---| | 0 | 4.58 | | 5 | 3.97 | | 10 | 3.20 | | 15 | 3.13 | | 20 | 2.82 | | 25 | | | 30 | 2.33 | | 35 | 2.15 | | 40 | 2.05 |