The Winkler test is used to determine the level of dissolved oxygen in fresh water samples. What follows is a set of instructions on how to perform the test.

Materials

Stage 1

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

Stage 2

• 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

Stage 1

Collect and label water samples in 25 ml stoppered bottles. (Two samples per location are required for Biological Oxygen Demand testing.)
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 pinky-brown precipitate should appear.
At this point the sample may be stored for later analysis in the laboratory.

Stage 2

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 acid. Fill the burette with thiosulfate solution and adjust to zero (or 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.

Notes

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.

For Biological Oxygen Demand (BOD) two samples are collected at each site, an A and a B sample. Stage 1 is carried out only on the A sample. The A sample can then be refrigerated, while the B sample is stored at room temperature in the dark for five days. After five days sample B has its stage 1 treatment, then both samples are put through stage 2 together.