A 1-litre water sample is tested for Biochemical Oxygen Demand (BOD). If the sample is classified as 'heavily polluted' (BOD threshold approximately 10 mg/L of O2), what is the maximum volume of 0.01 M Na2S2O3 solution required to titrate the iodine liberated in the Winkler BOD test? (Use:

Question

A 1-litre water sample is tested for Biochemical Oxygen Demand (BOD). If the sample is classified as 'heavily polluted' (BOD threshold approximately 10 mg/L of O2), what is the maximum volume of 0.01 M Na2S2O3 solution required to titrate the iodine liberated in the Winkler BOD test? (Use: 1 mol O2 requires 4 mol Na2S2O3)

Accepted Answer

120 ml. The Winkler method for BOD involves O2 reacting with Mn2+ to form MnO2, which then liberates I2 from KI; I2 is titrated with Na2S2O3 in a 4:1 molar ratio (O2:Na2S2O3). For a heavily polluted sample at the ~10 mg/L BOD threshold, the volume of 0.01 M Na2S2O3 required is approximately 120 mL.

Solution

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