Winkler titration

The Winkler test is used to determine the concentration of dissolved oxygen in water samples. Dissolved Oxygen, abbreviated D.O., is widely used in water quality studies and routine operation of water reclamation facilities. An excess of manganese(II) salt, iodide (I-) and hydroxide (HO^-) ions is 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 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.

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Analysis

From the above stoichiometric equations, we can find that:

1 mole of O₂ → 2 moles of MnO(OH)₂ → 2 mole of I₂ → 4 mole of S₂O₃^2-

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.

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.

Instrumental methods for measurement of dissolved oxygen have widely supplanted the routine use of the Winkler test, although the test is still used to check instrument calibration.

BOD₅

To determine five-day Biochemical oxygen demand (BOD₅), several dilutions of a sample are analyzed for dissolved oxygen before and after a five-day incubation period at 20 degrees Celsius (68 degrees Fahrenheit) in the dark. In some cases, bacteria are used to provide a source of oxygen to the sample; these bacteria are known as "seed". The difference in DO and the dilution factor are used to calculated BOD₅. The resulting number (usually reported in parts per million or milligrams per liter) is useful in determining the relative organic strength of sewage or other polluted waters.

The BOD₅ test is an example of analysis that determines classes of materials in a sample.

See also

• Oxygen saturation

References

• Lajos Winkler (1888). "Die Bestimmung des in Wasser Gelösten Sauerstoffes". Berichte der Deutschen Chemischen Gesellschaft. 21 (2): 2843–2855. doi:10.1002/cber.188802102122.

• Moran, Joseph M.; Morgan, Michael D., & Wiersma, James H. (1980). Introduction to Environmental Science (2nd ed.). W.H. Freeman and Company, New York, NY ISBN 0-7167-1020-X

• Standard Methods for the Examination of Water and Wastewater - 20th Edition ISBN 0-87553-235-7. This is also available on CD-ROM and online by subscription
• http://www.ecy.wa.gov/programs/wq/plants/management/joysmanual/4oxygen.html

Good overview of technique

• Y.C. Wong & C.T. Wong. New Way Chemistry for Hong Kong A-Level Volume 4, Page 248. ISBN 962-342-535-X

Manganese (III) consistently claimed

• http://ocw.mit.edu/NR/rdonlyres/Earth--Atmospheric--and-Planetary-Sciences/12-097January--IAP--2006/35FAFFC3-135B-4266-93B4-2144F7487004/0/dissolved_oxygen.pdf

NB: Gives unbalanced equation for formation of MnO(OH)₂ Claims manganese (III)

• http://io.uwinnipeg.ca/~simmons/ysesp/ap12pg7.htm
• http://io.uwinnipeg.ca/~simmons/ysesp/ap12pg3.htm
• http://io.uwinnipeg.ca/~simmons/ysesp/ap12pg11.htm

Gives manganese (IV) consistently