The correct answer is: D. Phenol-di-sulphuric acid.
Phenol-di-sulphuric acid is a reagent that is used to detect the presence of free ammonia in sewage. When phenol-di-sulphuric acid is added to sewage, it reacts with the free ammonia to form a precipitate. The presence of the precipitate indicates that there is free ammonia in the sewage.
Boiling, adding potassium permanganate, and adding sulphuric acid are not effective methods for detecting the presence of free ammonia in sewage. Boiling will cause the ammonia to evaporate, while adding potassium permanganate or sulphuric acid will react with the ammonia to form other compounds.
Here is a more detailed explanation of each option:
- Option A: Boiling. Boiling will cause the ammonia to evaporate, so it is not an effective method for detecting the presence of free ammonia in sewage.
- Option B: Adding potassium permanganate. Potassium permanganate is a strong oxidizing agent, and it will react with the ammonia to form other compounds. This reaction will not produce a precipitate, so it is not an effective method for detecting the presence of free ammonia in sewage.
- Option C: Adding sulphuric acid. Sulphuric acid is a strong acid, and it will react with the ammonia to form ammonium sulphate. This reaction will not produce a precipitate, so it is not an effective method for detecting the presence of free ammonia in sewage.
- Option D: Phenol-di-sulphuric acid. Phenol-di-sulphuric acid is a reagent that is used to detect the presence of free ammonia in sewage. When phenol-di-sulphuric acid is added to sewage, it reacts with the free ammonia to form a precipitate. The presence of the precipitate indicates that there is free ammonia in the sewage.