The Winkler Method is a technique used to measure dissolved oxygen in freshwater systems. Dissolved oxygen is used as an indicator of the health of a water body, where higher dissolved oxygen concentrations are correlated with high productivity and little pollution. This test is performed on-site, as delays between sample collection and testing may result in an alteration in oxygen content.
How does the Winkler Method Work?
The Winkler Method uses titration to determine dissolved oxygen in the water sample. A sample bottle is filled completely with water (no air is left to skew the results). The dissolved oxygen in the sample is then "fixed" by adding a series of reagents that form an acid compound that is then titrated with a neutralizing compound that results in a color change. The point of color change is called the "endpoint," which coincides with the dissolved oxygen concentration in the sample. Dissolved oxygen analysis is best done in the field, as the sample will be less altered by atmospheric equilibration.
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.
The Winkler method is the standard technique for the determination of dissolved oxygen in fresh and salt water. This scaleddown procedure requires reduced sample volume, reduced reagent volumes, less expense, and less time.
The relevant chemical reactions occurring throughout the procedure are outlined below: Mn2+ + 2OH– + 1/2 O2 oxygen-manganese complex + H2O (1) oxygen-manganese complex + 4H+ + 2I– I2 + Mn2+ + 2H2O (2) I2 + 2Na2S2O3 Na2S4O6 + 2NaI (3)
Addition of the manganous sulfate and the alkaline-iodide results in the formation of an insoluble oxygen-manganese complex
(1), the precipitate in step 2. The oxygen is stable in this form for several days. Both the