Assignment:
The biochemical oxygen demand (BOD) is the capacity of organic and biological matter in natural water to consume oxygen. It is determined experimentally by measuring the oxygen concentration in a sealed sample before and after a 5-day incubation period. The standard titration procedure reacts the oxygen in the sample with manganese (II) sulfate in basic solution. Manganese (IV) oxide is precipitated, which oxidizes iodide ion to diatomic iodine. The concentration of iodine is determined by titration with a standard solution of sodium thiosulfate. The reactions are:
2 Mn2+(aq) + 4 OH-(aq) + O2(aq) -> 2 MnO2(s) + 2 H2O(l)
MnO2(s) + 4 H+(aq) + 2 I-(aq) -> Mn2+(aq) + I2(aq) + 2 H2O(l)
I2(aq) + 2 S2O32-(aq) -> S4O62-(aq) + 2 I-(aq)
On the first day and the fifth day, 10.00 mL aliquots of the water sample were subjected to the above series of reactions. Of the 1.00 x 10-3 M solution of Na2S2O3, 10.15 mL were required for the titration on day 1 and 2.40 mL were required on day 5 to titrate the I2 produced. Calculate the BOD of this natural water sample in milligrams per liter.