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.
