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Dissolved oxygen by the Winkler method

Fix the oxygen in a water sample as a brown precipitate, release iodine with acid and titrate it with thiosulfate. Compare cold, warm and undersaturated samples and see how dissolved oxygen depends on temperature.

Difficulty ●●○ ≈ 30 min

Aim Measure the dissolved oxygen of air-saturated water by the Winkler method, compare it with the Benson–Krause saturation value, and work out the percentage saturation from the titre.
300 mL oxygen bottle · 0.02500 M Na₂S₂O₃ · starch indicator

The Winkler titration for dissolved oxygen

2 Mn(OH)X2+OX2→2 MnO(OH)X2\ce{2Mn(OH)2 + O2 -> 2MnO(OH)2} MnO(OH)X2+3 IX−+4 HX+→MnX2++IX3X−+3 HX2O\ce{MnO(OH)2 + 3I- + 4H+ -> Mn^2+ + I3- + 3H2O} IX3X−+2 SX2OX3X2−→3 IX−+SX4OX6X2−\ce{I3- + 2S2O3^2- -> 3I- + S4O6^2-}
③ Measure 201.3 mL (equal to 200 mL of the original water) into the flask and titrate
Oxygen bottle 05101520253035404550STIR50100150200
Burette reading 0.00mL
I₃⁻ left0.00 µM

Fix, then acidify, then titrate: yellow-brown → pale yellow, add starch → deep blue → colourless.

Computed: O₂ oxidises Mn(OH)₂ to MnO(OH)₂ (2 Mn per O₂); after acidification each MnO(OH)₂ releases one I₂ from the excess I⁻, and I₃⁻ reacts 1 : 2 with S₂O₃²⁻, so one O₂ uses four S₂O₃²⁻ — with 0.02500 M thiosulfate and 200 mL of original water, each millilitre is exactly 1.000 mg/L. The two 1 mL reagent additions push 2 mL of sample out of the bottle, which is why 200 × 300/298 = 201.3 mL is taken. The sample’s oxygen comes from the Benson–Krause equation for the chosen temperature and percentage saturation. The flask colour is computed from the bands of I₃⁻, I₂ and the starch–iodine complex (as in the site’s iodometry bench, with about 3 mmol/L of I⁻); the floc colour in the bottle is only illustrative: white Mn(OH)₂ with no oxygen, browner the more oxygen is fixed.

Procedure

  1. Sample: 20 °C, air-saturated. Press “Add MnSO₄ and alkaline iodide–azide” and then “Add conc. H₂SO₄”, watching the colour of the precipitate and of the solution.
  2. Titrate 201.3 mL (equal to 200 mL of original sample) with 0.02500 M thiosulfate. When the solution has faded to pale yellow, add starch and continue drop by drop until the blue just disappears; record the endpoint reading.
  3. Repeat with air-saturated samples at 5 °C and at 30 °C.
  4. Finally measure a sample at 20 °C that is only 60 % saturated, and one with all its oxygen removed (0 %).

What to notice

  • Air-saturated water at 20 °C: a brown precipitate, yellow-brown after acidification, and an endpoint near 9.09 mL — 9.09 mg/L, matching the Benson–Krause saturation value.
  • The 5 °C sample needs about 12.77 mL and the 30 °C sample only about 7.56 mL: between 5 and 30 °C the oxygen water can hold falls by about 41 %.
  • The 60 % sample needs about 5.46 mL (5.46 mg/L); the deoxygenated sample gives white Mn(OH)₂ on fixing, is almost colourless after acidification, and does not turn blue with starch.

In a real lab

  • When sampling, lead the water to the bottom of the bottle through a tube and let it overflow for a while before stoppering, with no bubble left inside; fix the oxygen on the spot and titrate soon after acidifying, before iodine evaporates or air oxidises the iodide.
  • The alkaline iodide–azide reagent is strongly caustic and sodium azide is highly toxic; add the concentrated sulfuric acid slowly down the neck of the bottle. Wear gloves and eye protection.
  • Thiosulfate solutions slowly decompose and must be standardised against potassium iodate or dichromate before use; old starch solution loses its sensitivity and blurs the endpoint.

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