For the COD test of sewage, organic matter is oxidised by K₂Cr₂0₇ in the presence of

ANS:A - H₂SO₄

In the Chemical Oxygen Demand (COD) test for sewage, organic matter is oxidized to measure the amount of oxygen required for this process. The oxidation is carried out using a strong oxidizing agent, usually potassium dichromate (K2Cr2O7), in the presence of sulfuric acid (H2SO4). Briefly, the steps of the COD test involve:

1. Sample Preparation: A representative sample of sewage is taken.
2. Digestion: The sewage sample is mixed with a known quantity of potassium dichromate and sulfuric acid. Sulfuric acid serves as a catalyst and creates an acidic environment.
3. Heating: The mixture is heated to facilitate the oxidation of organic matter. During this process, the potassium dichromate is reduced, and the organic compounds in the sewage are oxidized.
4. Color Change: The oxidation reaction results in a color change from orange (due to potassium dichromate) to green. The extent of the color change is proportional to the amount of organic matter present in the sewage.
5. Measurement: The decrease in the concentration of potassium dichromate is measured, indicating the amount of oxygen consumed in oxidizing the organic content of the sewage.