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 photosynthe photosynthesis. sis. Bacteria and eukaytotic eukaytotic organisms (zooplankton, (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 differ differenc encee betwee between n the physic physical al concent concentrat ration ion of oxygen oxygen in the water 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 sample sampless and to estima estimate te the biolog biologica icall activi activity ty in the water sample. sample. An excess 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 whether the oxidised manganese is tetravalent or trivalent. 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, Manganesecontaining 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 alkali-iodide-azide reagent in the same manner. 4. Stopper the bottle with care to be sure no air is introduced. Mix the sample by inverting several times. Check for air bubbles; discard the sample and start over if any are seen. If oxygen is present, a brownish-orange cloud of precipitate or floc will appear. When this floc has settle to the bottom, mix the sample by turning it upside down several times and let it settle again. 5. Add 2 mL of concentrated sulfuric acid via a pipette held just above the surface of the sample. Carefully stopper and invert several times to dissolve the floc. At this point, the sample is "fixed" and can be stored for up to 8 hours if kept in a cool, dark place. As an added precaution, squirt distilled water along the stopper, and cap the bottle with aluminum foil and a rubber band during the storage period.
6. In a glass flask, titrate 20 mL of the sample with sodium thiosulfate to a pale straw color. Titrate by slowly dropping titrant solution from a calibrated pipette into the flask and continually stirring or swirling the sample water. 7. Add 2 mL of starch solution so a blue color forms. 8. Continue slowly titrating until the sample turns clear. As this experiment reaches the endpoint, it will take only one drop of the titrant to eliminate the blue color. Be especially careful that each drop is fully mixed into the sample before adding the next. It is sometimes helpful to hold the flask up to a white sheet of paper to check for absence of the blue color. 9. The concentration of dissolved oxygen in the sample is equivalent to the number of milliliters of titrant used. Each mL of sodium thiosulfate added in steps 6 and 8 equals 1 mg/L dissolved oxygen. Result Analysis
The total number of milliliters of titrant used in steps 6-8 equals the total dissolved oxygen in the sample in mg/L. Oxygen saturation is temperature dependent - gas is more soluble in cold waters, hence cold waters generally have higher dissolved oxygen concentrations. Dissolved oxygen also depends on salinity and elevation, or partial pressure.
