The Winkler method for dissolved oxygen in water is based on the rapid oxidation of solid...

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Chemistry

The Winkler method for dissolved oxygen in water is based on therapid oxidation of solid Mn(OH)₂ to Mn(OH)₃in alkaline medium. When acidified, the Mn (III) readily releasesiodine from iodide. A 240.-mL water sample, in a stoppered vessel,was treated with 1.10 mL of a concentrated solution of NaI and NaOHand 1.10 mL of a manganese(II) solution. Oxidation of theMn(OH)₂ was complete in about 1 min. The precipitateswere then dissolved by addition of 2.20 mL of concentratedH₂SO₄, whereupon an amount of iodineequivalent to the Mn(OH)₃ (and hence to the dissolvedO₂) was liberated. A 24.5-mL aliquot (of the 244 mL) wastitrated with 14.2 mL of 0.00861 M thiosulfate. Calculate the massin milligrams of O₂ per milliliter sample. Assume thatthe concentrated reagents are O₂ free and take theirdilutions of the sample into account.

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4.3 Ratings (706 Votes)

Write down the equations taking place Mn2 aq 2 OH aq MnOH2 s 1 2 MnOH2 s O2 aq 2 H2O l 2 MnOH3 s 2 2 MnOH3 s 2 I aq 4 H aq 2 Mn2 aq I2 aq 6 H2O l 3 I2 aq 2 S2O32 aq S4O6 aq 2 I aq 4 Therefore moles I2 titrated 2moles of thiosulfate added as See Answer

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