Definitions

dissolved oxygen

Winkler test for dissolved oxygen

The Winkler test is used to determine the level of dissolved oxygen in water samples. This metric is then used 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 precipitate 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 .

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.

Sample method

In the first step, Manganese(II) sulfate (at 48% of the total volume) is added to an environmental water sample. Next, Potassium iodide (15% in potassium hydroxide 70%) 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, Manganese-containing 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.

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 biological 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

References

  • Lajos Winkler (1888). "Die Bestimmung des in Wasser Gelösten Sauerstoffes". Berichte der Deutschen Chemischen Gesellschaft 21 2843–2855.
  • 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
  • http://www.metrohm.com/basics/encyc/titration/winkler.html

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

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