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.

Stage 1

   * Manganese(II) sulphate 48% (IRRITANT)
   * Potassium iodide 15% in potassium hydroxide 70% (CORROSIVE)
   * Sample bottles of river or pond water
   * Latex gloves
   * Safety glasses

Stage 2

   * Sulphuric acid 50% (CORROSIVE)
   * Sodium thiosulphate 0.31%
   * Starch solution 0.1%
   * Burette
   * Burette stand
   * 10ml pipette and pipette filler
   * 100ml conical flasks
   * Filter paper
   * Latex gloves
   * Safety glasses

Method

[edit]

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.