To test Chemical Oxygen Demand (C.O.D.) of sewage, organic matter is oxidized by potassium dichromate in the presence of
- AHydrochloric acid
- BSulphuric acidCorrect
- CNitric acid
- DCitric acid
Solution
Concept
COD test
Step-by-step solution
1. In the COD test the sample is refluxed with potassium dichromate (strong oxidant) in a sulphuric acid medium. 2. Silver sulphate acts as a catalyst and mercuric sulphate removes chloride interference.
Explanation
COD is the oxygen equivalent of the organic matter that can be oxidised by a strong chemical oxidant. The oxidation takes place in a strongly acidic medium of sulphuric acid.
Shortcut
COD = K₂Cr₂O₇ + H₂SO₄ (+ Ag₂SO₄ catalyst).
Common mistake
Choosing hydrochloric acid.
Correct answer: B) Sulphuric acid