EXPERIMENT 11: DETERMINATION OF DISSOLVED OXYGEN IN A WATER SAMPLE (WINKLER METHOD)

INTRODUCTION
In an alkaline solution, dissolved oxygen will oxidize manganese(II) to the trivalent state.
8OH-(aq) + 4Mn2+(aq) + 2H2O(l) －－＞ 4Mn(OH)3(s)
The analysis is completed by titrating the iodine produced from potassium iodide by manganese(III) hydroxide.
2Mn(OH)3(s) + 2I-(aq) + 6 H+(aq) －－＞ 2Mn2+(aq) + I2(aq) + 6H2O(l)
Sodium thiosulphate is used as the titrant.

Success of the method is critically dependent upon the manner in which the sample is manipulated. At all stages, every method must be made to assure that oxygen is neither introduced to nor lost from the sample. Furthermore, the sample must be free of any solutes that will oxidize iodide or reduce iodine.

Chemicals: Manganese(II) sulphate solution – prepared by dissolving 48 g of MnSO4.4H2O in water to five 100 cm3 solution; alkaline potassium iodide solution—prepared by dissolving 15 g of KI in about 25 cm3 of water, adding 66 cm3 of 50% NaOH, and diluting to 100 cm3; concentrated sulphuriv(VI) acid; 0.0125 M sodium thiosulphate solution; starch solution (freshly prepared).

Apparatus: 250 cm3 volumetric flask, 250 cm3 conical flask, measuring cylinders, titration apparatus, magnetic stirrer

Procedure:
1. Use a 250 cm3 volumetric flask to collect water sample. Fill the flask completely with water without trapping any air bubbles.
2. Add 1 cm3 of manganese(II) sulphate solution to the sample using a pipette. Discharge the solution well below the surface (some overflow will occur).
3. Similarity introduce 1 cm3 of alkaline potassium iodide solution. Be sure that no air becomes entrapped. Invert the bottle to distribute the precipitate uniformly. [Hazard Warning: Care should be taken to avoid exposure to any overflow, as the solution is quite alkaline.]
4. When the precipitate has settled at least 3 cm below the stopper, introduce 1 cm3 of concentrated sulphuric acid well below the surface. Replace the stopper and carefully mix until the precipitate disappears. A magnetic stirrer is helpful here.
5. Allow the mixture to stand for 5 minutes and then withdraw 100cm3 of the acidified sample into a 250 cm3 conical flask.
6. Titrate with 0.0125M sodium thiosulphate until the iodine colour becomes faint. Then add 1 cm3 of starch solution and continue adding the thiosulphate solution until the blue colour disappears.
7. Record the volume of thiosulphate solution used and calculate the dissolved oxygen content in the sample in mg dm-3.
Remarks

1. If the water sample has a low DO value, it is recommended to withdraw 200 cm3 of the acidified sample into a 500 cm3 flask for the titration describe in step 5.
2. This experiment can be further developed into a project to study the extent of water pollution. a) The water sample under investigation is divided into two portions. One portion of the sample is immediately analysed for dissolving oxygen using the Winkler method. The other portion is stored in the dark for five days. b) Repeat the analysis with the water sample that has been stored in the dark for five days. The difference between the two measurements is the five-day biochemical oxygen demand (BOD5), measured in mg dm-3.