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

Question

The Winkler method for dissolved oxygen in water is based on the rapid oxidation of solid to in alkaline medium. When acidified, the (III) readily releases iodine from iodide. A 230.-mL water sample, in a stoppered vessel, was treated with 1.00 mL of a concentrated solution of and and 1.00 mL of a manganese(II) solution. Oxidation of the was complete in about 1 min. The precipitates were then dissolved by addition of 2.00 mL of concentrated , whereupon an amount of iodine equivalent to the (and hence to the dissolved ) was liberated. A 24.0-mL aliquot (of the 234 mL) was titrated with 14.2 mL of 0.00870 M thiosulfate. Calculate the mass in milligrams of per milliliter sample. Assume that the concentrated reagents are free and take their dilutions of the sample into account.

Explanation / Answer

moles of S2O3^2- used = 0.00870 M x 14.2 ml = 0.12354 mmol

moles of I2 formed = 0.12354/2 = 0.06177 mmol

moles of Mn2+ present = 0.06177 mmol

moles Mn2+ in original solution = 0.06177 x 234/24 = 0.60225 mmol

mass in mg = 0.60225 mmol x 54.94 g/mol = 33.10 mg per ml of sample

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