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Winkler test for dissolved oxygen

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.
Table of contents

Materials

Stage 1


Stage 2

Method

Stage 1


Collect and label water samples in 25ml stoppered bottles.(Two samples per location are required for Biological Oxygen Demand testing).
Add 0.1ml of manganese(II) sulphate solution, and mix carefully, without letting in air. Add 0.2ml 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 0.3ml sulphuric 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 0.1ml acid. Fill the burette with thiosulphate solution and adjust to zero (or note the burette reading). Transfer 10ml of the sample to a conical flask, and add a few drops of starch solution. The subsample should turn blue. Titrate the subsample with thiosulphate 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 1ml of thiosulphate titre is equivalent to 0.1mg of oxygen in the 10ml subsample. Thus 1ml of thiosulphate is equivalent to 1mg oxygen per 100ml 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.



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