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Determination of dissolved oxygen by colorimetric titration: the Winkler method


  • Stand above a large crystallizer (caution, the experiment always overflows).
  • Fill a 250 mL Erlenmeyer flask up to the top with the water to be analyzed and insert a magnetic stir bar
  • Add roughly 700 mg of sodium hydroxide ash and 2 g manganese chloride (II). Close quickly without trapping any air.
  • Stir vigorously for 30 minutes. A brown solid should form. The solution is turbid.
  • Open the Erlenmeyer flask and quickly add sulfuric acid 9 mol/L until a pH of below 7 is reached (use pH paper and a glass rod to check). The solution is brown and translucent.
  • Add 3 g of potassium iodide and stir vigorously. The brown color disappears and turns to orange.
  • Withdraw exactly 50 mL of the previous solution with a pipette and titrate with a sodium thiosulfate solution at 0.01 mol/L.
  • When the diiode’s yellow-orange color has almost disappeared, add a few drops of the starch solution and continue the titration until the blue color disappears.
Illustration of a chronometer ≈ 1 hour / sample
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