Winkler titration

The Winkler test is used to determine the level of dissolved oxygen in water samples. 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, and at the same time bacteria and all other organisms (zooplankton, algae, fish) consume the oxygen by respiration. The result of these two mechanisms determines the concentration of dissolved oxygen, which indicates the growth 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

This is the sample method once used to perform the test. Note that this method is no longer performed as described below, although the principle remains the same. A major difference is that current testing typically uses a 300 mL sample.

Step 1

Materials

Manganese(II) sulfate 48%
Potassium iodide 15% in potassium hydroxide 70%
Sample of fresh water
Latex gloves
Safety glasses

Method Collect and label each water sample in a 25 mL stoppered volumetric flask, making sure no air bubbles are trapped within the sample. (Two samples per location are required for biological oxygen demand testing.)

Immediately after sampling, add 0.1 mL of manganese(II) sulfate solution, and mix carefully without letting in air. Add 0.2 mL of alkaline potassium iodide, and again mix without letting in air. A pinkish-brown precipitate should appear.

At this point the sample may be stored for later analysis in the laboratory.

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 confusion about whether the oxidised manganese is tetravalent or trivalent. Some sources claim that Mn(OH)₃ is the brown precipitate, but hydrated MnO₂ is more likely to give the brown colour.

Step 2

Materials

Sulfuric acid 50%
Sodium thiosulfate 0.31%
Starch solution 0.1%
Burette
Burette stand
10 ml pipette and pipette filler
100 ml conical flasks
Filter paper
Latex gloves
Safety glasses

Method Add 0.3 mL sulfuric acid to each sample, and mix. Allow the sample to stand for two minutes. If the precipitate does not dissolve into iodine solution, add a further 0.1 mL sulfuric acid.

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 SO₄^2-(aq)

Fill the burette with thiosulfate solution and note the burette reading. Transfer 10 mL of the sample to a conical flask, and add a few drops of starch solution. The subsample should turn blue. Titrate the subsample with thiosulfate until it turns clear. (You may find the endpoint easier to see if the conical flask is stood on a sheet of filter paper.) Record and repeat the titration.

2 S₂O₃^2-(aq) + I₂ → S₄O₆^2-(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.

In the above method, each milliliter of thiosulfate titer is equivalent to 0.1 mg of oxygen in the 10 mL subsample. Thus 1 mL of thiosulfate is equivalent to 1 mg oxygen per 100 mL fresh water.

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.

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)

Gives manganese (IV) consistently