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Determination Of Oxygen Dissolved In Water By Winkler

1063 words - 5 pages

Determination of Oxygen dissolved in water by Winkler’s Method

History
The test was first developed by Lajos Winkler while working on his doctoral dissertation in 1888. The amount of dissolved oxygen is a measure of the biological activity of the water masses. Phytoplankton and macroalgae present in the water mass produce oxygen by way of photosynthesis. Bacteria and eukaytotic organisms (zooplankton, algae, fish) consume this oxygen through respiration. The result of these two mechanisms determines the concentration of dissolved oxygen, which in turn indicates the production of biomass. The difference between the physical concentration of oxygen in the water (or the theoretical ...view middle of the document...

Some sources claim that Mn(OH)3 is the brown precipitate, but hydrated MnO2 may also give the brown colour.
4 Mn(OH)2(s) + O2(aq) + 2 H2O → 4 Mn(OH)3(s)
The second part of the Winkler test reduces acidifies the solution. The precipitate will dissolve back into solution. The acid facilitates the coversion by the brown, Manganese-containing precipitate of the Iodide ion into elemental Iodine.
The Mn(SO4)2 formed by the acid converts the iodide ions into iodine, itself being reduced back to manganese(II) ions in an acidic medium.
Mn(SO4)2 + 2 I-(aq) → Mn2+(aq) + I2(aq) + 2 SO42-(aq)
Thiosulfate solution is used, with a starch indicator, to titrate the iodine.
2 S2O32-(aq) + I2 → S4O62-(aq) + 2 I-(aq)
Analysis
From the above stoichiometric equations, we can find that:
1 mole of O2 → 4 moles of Mn(OH)3 → 2 moles of I2
Therefore, after determining the number of moles of iodine produced, we can work out the number of moles of oxygen molecules present in the original water sample. The oxygen content is usually presented as mg dm-3.
Application
Dissolved oxygen analysis can be used to determine:
the health or cleanliness of a lake or stream,
the amount and type of biomass a freshwater system can support,
the amount of decomposition occurring in the lake or stream.
Limitations
The success of this method is critically dependent upon the manner in which the sample is manipulated. At all stages, steps must be taken to ensure that oxygen is neither introduced to nor lost from the sample. Furthermore, the water sample must be free of any solutes that will oxidize or reduce iodine.
Procedure
1. Carefully fill a 300-mL glass Biological Oxygen Demand (BOD) stoppered bottle brim-full with sample water.
2. Immediately add 2mL of manganese sulfate to the collection bottle by inserting the calibrated pipette just below the surface of the liquid. (If the reagent is added above the sample surface, you will introduce oxygen into the sample.) Squeeze the pipette slowly so no bubbles are introduced via the pipette.
3. Add 2 mL of...

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