The Winkler test is used to determine the concentration of dissolved oxygen in water samples. Dissolved oxygen (D.O.) is widely used in water quality studies and routine operation of water reclamation facilities to analyze its level of oxygen saturation.
In the test, 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) to form.  This precipitate is then oxidized by the oxygen that is present in the water sample into a brown manganese-containing precipitate with manganese in a more highly oxidized state (either Mn(III) or Mn(IV)).
In the next step, a strong acid (either hydrochloric acid or sulfuric acid) is added to acidify the solution.  The brown precipitate then converts the iodide ion (I) to iodine.  The amount of dissolved oxygen is directly proportional to the titration of iodine with a thiosulfate solution.  Today, the method is effectively used as its colorimetric modification, where the trivalent manganese produced on acidifying the brown suspension is directly reacted with ethylenediaminetetraacetic acid to give a pink color. As manganese is the only common metal giving a color reaction with ethylenediaminetetraacetic acid, it has the added effect of masking other metals as colorless complexes.