The Winkler titration for the determination of dissolved oxygen involves the treatment of the sample with iodide ion (I-) in the presence of manganese ion catalyst (Mn2+) as follows:

O2 + 2Mn2+ + 4OH- => 2MnO2(s) + 2H2O

MnO2(s) + 4H+ + 2I- => Mn2+ + I2 + 2H2O

The liberated iodine is then titrated with a standard thiosulfate solution.

I2 + 2S2O3^2- => 2I- + S4O6^2-

A 50.00 ml sample of water was treated as above and the I2 titrated with 0.01136 M Na2S2O3, of which 8.11 ml were required to reach the end point. Determine the concentration of the dissolved O2 in M (moles/L) and ppm (mg/L).