ANS:A - boiling

1. Boiling: Boiling is not a specific method for detecting free ammonia in sewage. While boiling may help to evaporate volatile substances, including ammonia, it does not provide a reliable and specific test for the presence of ammonia. Boiling may be used in some cases to reduce the volume of sewage or to sterilize samples, but it is not a recognized method for ammonia detection.
2. Adding potassium permanganate: Potassium permanganate is often used as an oxidizing agent in water treatment processes to remove organic and inorganic contaminants. However, it is not specifically employed for detecting free ammonia in sewage. While potassium permanganate may react with certain organic compounds and reduce their concentrations, it does not react directly with ammonia or provide a reliable test for its presence.
3. Adding sulfuric acid: Adding sulfuric acid alone does not serve as a specific method for detecting free ammonia in sewage. Sulfuric acid may be used in various chemical reactions and processes, but it does not react specifically with ammonia or provide a reliable indicator of its presence.
4. Phenol-di-sulfuric acid: Phenol-di-sulfuric acid, also known as Nessler's reagent, is a specific reagent used to detect the presence of ammonia or its compounds, including free ammonia, in aqueous solutions. When ammonia reacts with Nessler's reagent, it forms a yellow to brown coloration, providing a visual indication of the presence of ammonia. Nessler's reagent is widely used in laboratory settings and wastewater treatment facilities for ammonia detection due to its specificity and reliability.