Water Pollution and DO

Water Pollution

Any human activity that impairs the use of water as a resource may be called water pollution.
Therefore, addition of excess of undesirable substances to water that makes it harmful to man, animal and aquatic life or other wise causes significant departures from the normal activity of various living communities in and around water is called water pollution.


Dissolved Oxygen

DO means Dissolved Oxygen. The optimum DO in natural waters is 4-6 ppm, which is essential for supporting aquatic life. Any decrease in this DO value is an index of pollution. Many aquatic organisms can not survive at lower DO levels in water.

Determination of Dissolved Oxygen

The DO content of a water sample is determined iodometrically by the Modified Winker’s method. The principle involved in this method is that when manganous sulphate is added to the water sample containing alkaline potassium iodide, manganese hydroxide is formed.

This is oxidized to basic manganic oxide by the DO present in the water sample. When the sulphuric acid is added, the basic manganic oxide liberates iodine, which is equivalent the DO originally present in the water sample. The liberated iodine is titrated with standard hypo solution, using starch as a indicator.

Interference due to nitrate can be determined by adding sodium azide to the alkaline potassium iodide solution used above.

MnSO4 + 2KOH >> Mn(OH)2 + K2SO4

2Mn(OH)2 + O2 (from DO) >> 2MnO(OH)2

2MnO(OH)2 + 2 H2SO4 >> Mn(SO4)2 + H2O

Mn(SO4)2 + KI >> MnSO4 + K2SO4 + I2

2 Na2S2O3 + I2 >> Na2S4O6 + 2NaI

The DO is usually expressed as mg/l (ppm).