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