(Solved): 9. To determine the BOD of a water sample, the O2 concentration must be measured before and afte ...

9. To determine the BOD of a water sample, the ${\mathrm{O}}_2$ concentration must be measured before and after the five-day incubation process. This determination can be done in a number of ways, including titration and oxygen electrodes. The standard titration procedure used is the Winkler method, in which the oxygen in a small aliquot of the water sample is reacted with ${\mathrm{MnSO}}_4$ in basic solution. This precipitates ${\mathrm{MnO}}_2$, which converts added ${\mathrm{I}}^{-}$ to ${\mathrm{I}}_2$; the latter can be quantitatively determined by titration with standardized sodium thiosulfate. The overall set of equations is $\begin{array}{l}2{\mathrm{Mn}}^{2+}(\mathrm{aq})+4{\mathrm{OH}}^{-}(\mathrm{aq})+{\mathrm{O}}_2(\mathrm{aq})⟶\\ 2{\mathrm{MnO}}_2(\mathrm{s})+2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\\ {\mathrm{MnO}}_2(\mathrm{s})+4{\mathrm{H}}^{+}(\mathrm{aq})+2{\mathrm{I}}^{-}(\mathrm{aq})⟶\\ {\mathrm{Mn}}^{2+}(\mathrm{aq})+{\mathrm{I}}_2(\mathrm{aq})+2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\\ {\mathrm{I}}_2(\mathrm{aq})+2{\mathrm{S}}_2{\mathrm{O}}_3{}^{2-}(\mathrm{aq})⟶\\ {\mathrm{S}}_4{\mathrm{O}}_6{}^{2-}(\mathrm{aq})+2{\mathrm{I}}^{-}(\mathrm{aq})\end{array}$ Two $10.00-\mathrm{mL}$ aliquots of a natural water sample were analyzed using the Winkler method, one before and the other after incubation. They required 10.15 and $2.40\mathrm{mL}$, respectively, of a $0.00100\mathrm{M}$ standard ${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ solution to titrate the ${\mathrm{I}}_2$ of Natural Waters produced. Calculate the $\mathrm{BOD}$ of this natural water sample in milligrams per liter. Would this be considered to be a polluted water sample?