Equipment:
- Water trough
- 250 cm³ reagent bottle
- Dropper
- Deionised water
- Manganese (II) sulfate solution
- Alkaline potassium iodide solution
- Concentrated sulfuric acid
- Sodium thiosulfate solution
- Starch indicator
- Pipette and filler
- Burette
- Conical flask
- Retort stand and clamp
- River water sample
Experiment:
- Rinse the reagent bottle with deionised water, with vigorous shaking to avoid air bubbles
- Completely fill bottle underwater with sample, making sure to avoid trapped air bubbles
- Using a dropper placed well below surface of water, add 1 cm³ each of alkaline potassium iodide and manganese (II) sulfate solution
- Stopper bottle. Some solution will overflow at this point
- Invert bottle and allow precipitate to settle out
- Run 1 cm³ sulfuric acid down inside of bottle, stopper and invert to dissolve precipitate. If precipitate does not dissolve, repeat this step.
- Rinse burette, pipette and conical flask with deionised water and rinse burette with thiosulfate and flask with iodine solution
- Fill burette to the 0 mark with thiosulfate and pipette 25cm³ iodine solution into the flask
- Carry out one rough and two accurate titrations, adding starch indicator when solution is pale yellow. Endpoint when colour changes from blue-black to colourless.
- Calculate concentration of iodine solution
Calculating Total Dissolved Oxygen
Lets take the concentration of the thiosulfate as 0.005M and the titre as 12.5cm³
The ratio of oxygen:thiosulfate is 1:4
0.005 x 12.5 = 25 x M
4 1
M = 0.000625 moles/litre
0.000625 x 16 = 0.01g/litre
0.01g x 1000 = 10 p.p.m.
Possible Questions
1. Why is the reagent MnSO4 used?
Dissolved oxygen will not react completely in its absence. The use of MnSO4 results in the formation of Mn(OH)2, which reacts completely with dissolved oxygen.
2. Why is concentrated H2SO4 used?
To enable the Mn(IV) species to release the free iodine needed for the redox reaction.
