DO by Winkler Test Method

The "Winkler test" is used to determine the concentration of dissolved oxygen in water samples.

Dissolved Oxygen, abbreviated D.O., is widely used in water quality studies and routine operation of water reclamation facilities.

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 precipitates 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.

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.
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)
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.

BOD5 : 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 calculate 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.