Index >> Waste Water Microbiology >>Estimation of Dissolved Oxygen

In order for this fixation process to work however, the solution must e at high pH, so another reagent is added to facilitate the above requirement. Potassium iodide is added to function as a dye, and will react with the sulphuric acid added.

Upon addition of the sulphuric acid, the MnO₂ from above is reduced to Mn²⁺, and the iodine from the potassium iodide above is oxidized by the MnO₂ from I- to I₂. This reaction step effectively causes the solution to take on a yellowish brown colour proportional to the number of I₂ molecules present which in turn is proportional to the original amount of O₂ molecules in the water. This iodine is titrated by sodium thiosulphate solution using starch as an indicator.

2S₂O₃^2- + I₂ = S₄O₆^2- + 2I^-

Thus 4 molecules of the sodium thiosulphate are required to change the colour resulting from one molecule of O₂ in the original water. Thus we get a very accurate estimate of the number of O₂ molecules in the original solution.

The dissolved oxygen in any water body diminishes due to various reasons which include:

Heterotrophic microbial activity in water.
Dissolved oxygen used up by aquatic microorganisms, plants and animals.
Addition of sewage or other organic wastes.
Decomposition of dead organic matter.