Question

Discuss the importance of dissolved oxygen in water. How is COD of a polluted water measured $?$

Answer 1

Hint : Oxygen is a diatomic, colourless, odourless, tasteless gaseous element that plays a vital role in the survival of living organisms. It is an essential and inevitable component for life on earth. Most of the living organisms- both terrestrial and aquatic- uptake oxygen for metabolism and energy and give out carbon dioxide.

Complete Step By Step Answer:
We know that oxygen is an important gas required for survival. Animals as well as plants require oxygen. They acquire it from the surrounding atmosphere. But in the case of aquatic organisms like fish, it is nearly impossible for them to get oxygen directly from the atmosphere. They intake oxygen in the form of dissolved oxygen. Oxygen present in water bodies is called dissolved oxygen. It is measured in parts per million. Dissolved oxygen is produced either through photosynthesis by aquatic plants and algae or when atmospheric oxygen dissolves in the surface layers of water bodies.
Chemical oxygen demand (COD) is the amount of oxygen that is consumed to chemically oxidize organic contaminants present in water to inorganic end products. It gives a measure of quality of water and the COD test helps to determine the efficiency of water treatment plants. COD is measured using strong oxidising agents (oxidants) like potassium permanganate, potassium dichromate or potassium iodate. They can completely oxidise the organic compounds to carbon dioxide. An excess amount of oxidising agent is taken and added to the sample. In addition to the oxidant, an oxidation reaction takes place. After the completion of oxidation, the amount of oxidant left in the solution is used to calculate the concentration of organic molecules in the sample. The above reaction must take place in an acidic condition and the amount of oxidant can be measured using volumetric analysis- titrations. The concentration must be in $mg{L^{ - 1}}$ and it is the mass of oxygen consumed per litre of solution.

Note :
Biochemical oxygen demand (BOD) is almost similar to COD but is more precise than COD. It is the amount of dissolved oxygen required to break down organic molecules present in a water sample by using aerobic biological organisms over a certain time span at a certain temperature. More BOD means the water is highly polluted.