Determination of DO & BOD in a water sample
Objective
Using volumetric analysis, the amount of dissolved oxygen (DO) in a water sample can be found. On standing another sample for 5 days, the difference between these two values is defined as biochemical oxygen demand in 5 days (BOD5). The aquatic life needs certain amount of DO for survival, whereas the value of BOD reflects the water quality.
Principle
In alkaline solution, dissolved oxygen will oxidize manganese(II) into manganese(III):
4Mn2+(aq) + 8OH-(aq) + O2(aq) + 2H2O(l) → 4Mn(OH)3(s)
The amount of DO can be found by titrating the iodine produced from potassium iodide by manganese(III) with sodium thiosulphate:
2Mn(OH)3(s) + 2I-(aq) + 6H+(aq) → 2Mn2+(aq) + I2(aq) + 6H2O(l)
I2(aq) + 2S2O32-(aq) → 2I-(aq) + S4O62-(aq)
Chemicals
water sample, MnSO4, conc.H2SO4, alkaline KI, standard Na2S2O3, starch solution
Apparatus
titration apparatus, pipette, 250cm3 volumetric flask x2, beaker, droppers, magnetic stirrer, white tile
Procedure
1.> Collect a water sample with two 250cm3 volumetric flasks. Remember to fill the flasks completely with water without trapping any air bubbles.
2.> To one of the volumetric flask, add about 1 cm3 of MnSO4 solution well below the surface with a dropper.
3.> Similarly, introduce about 1 cm3 of alkaline KI solution to the same flask. Be sure that no air becomes entrapped and then invert the stopped
flask to distribute the precipitate uniformly.
4.> When the precipitates has settled at least 2 cm below the stopper, introduce about 1 cm3 of conc.H2SO4 well below the surface. Stop the flask and
using a magnetic stirrer to mix the solution until the precipitate dissolves.
5.> Measure 200 cm3 of the acidified sample into a 500 cm3 conical flask, titrate with standard Na2S2O3 solution. Until the solution becomes pale
yellow, add a few drops of starch solution and complete the titration.
6.> While another volumetric flask containing the sample is stopped, without adding any chemicals, and keep in a cool, dark place (avoid the
photosynthesis of phytoplankton) for 5 days.
7.> After 5 days, investigates the sample by repeating the steps 1-5. The difference between 2 DO is defined as BOD5.