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 concentration if there were no living organisms) and the actual concentration of oxygen is called the biological demand in oxygen.
Principle: The Winkler test is used to determine the level of dissolved oxygen in water samples and to estimate the biological activity in the water sample. An excess of Manganese(II) salt, iodide (I-) and hydroxide (OH-) ions are added to a water sample causing a white precipitate of Mn(OH)2 to form. This precipitate is then oxidized by the dissolved oxygen in the water sample into a brown Manganese precipitate. In the next step, a strong acid (either hydrochloric acid or sulphuric acid) is added to acidify the solution. The brown precipitates then convert the iodide ion (I-) to Iodine. The amount of dissolved oxygen is directly proportional to the titration of Iodine with a thiosulphate solution.
Method First Manganese(II) sulfate is added to an environmental water sample. Next, Potassium iodide is added to create a pinkish-brown precipitate. In the alkaline solution, dissolved oxygen will oxidize manganese(II) ions to the tetravalent state. 2 Mn(OH)2(s) + O2(aq) → 2 MnO(OH)2(s)
MnO(OH)2 appears as a brown precipitate. There is some confusion about